ASTRONOMY. The movement of the stars and planets has fascinated humans for thousands of years. For the vast majority of ancient astronomers, the stars seemed to be equally distant from Earth in what was an Earth-centered (or "geocentric") cosmos. The ancient Greeks observed that the stars revolved westward around the north celestial pole every twenty-three hours and fifty-six minutes, thus constituting a kind of objective clock. Most envisaged these revolving stars to be located on a sphere, composing an incorruptible, celestial orb that could easily be contrasted with the world of change, generation, and corruption on Earth (to which irregular meteorological phenomena such as comets were also understood to belong). Against the backdrop of this outer sphere, the Sun was seen to move along a path termed the ecliptic, while the planets moved within eight degrees of this path. These usually moved eastward, though occasionally they moved in the opposite direction, thus exhibiting retrograde motion. Many Greek astronomers suggested that the Sun could be placed on the equator of an inner sphere that revolved once a year, hence constituting a second sphere in addition to the stellar orb. Following the theories of the fourth-century-b.c.e. scholar Eudoxus, astronomers and natural philosophers became increasingly committed to the idea that the motions of each planet could be accounted for by means of their own specific, homocentric spheres.
DEFENDING AN EARTH-CENTERED UNIVERSE
The retrograde motion of the planets offended against the apparent simplicity of heavenly motions, as well as the dictum that their orbits were circular. From the third century b.c.e., astronomers began to conceive of planets rotating in small circles (epicycles) around a point that moved on a carrier orbit (or deferent). This accounted for the apparent motions of the planets, including their retrograde motion, although others followed Aristotle in positing the existence of homocentric spheres. After Ptolemy's composition of the Almagest in the second century c.e., the movements of the planets could be accurately represented by means of techniques involving the use of epicycles, deferents, eccentrics (whereby planetary motion is conceived as circular with respect to a point displaced from Earth), and equants (a device that posits a constant angular rate of rotation with respect to a point displaced from Earth). These approaches, and arguments over the reality of the celestial orbs, continued to be refined in the following centuries by first Muslim and then Christian astronomers. The geocentric cosmos was readily appropriated by Christians, many of whom believed that God existed outside the stellar sphere and (as evidenced by Dante's fourteenth-century poem Divine Comedy ) that angels turned the epicycles and spheres in the intervening spaces.
Astronomy had a primarily practical function, and by the thirteenth century it played a central role in determining the dates of religious festivals. Ptolemy's mathematically sophisticated Almagest, with various commentaries, formed the basis for advanced astronomy in university curricula from this period onward. In the late fifteenth century, the arrival of the printing press coincided with the transmission to western Europe of a number of important Greek mathematical and astronomical manuscripts. These became a new focal point for doing astronomy, as ancient texts could be seen and assessed in their original form (as opposed to being translated into Latin from Syriac or Arabic). Georg von Peuerbach (1423–1461) and his pupil Regiomontanus produced important works that formed the basis of astronomical training for the next century.
Astronomy was equally significant for providing data from which astrological predictions could be made (judicial astrology). The effects that heavenly actions had on earthly events, and the manner in which they were carried out, constituted a problem for most human societies. Astrology was a major part of both daily life and intellectual culture, and was deeply implicated in politics, medicine, and agriculture. However, its use was increasingly questioned by elites in the late sixteenth century; both Catholics and Protestants argued that the notion that the relations between stars and planets influenced or governed human behavior detracted from the primary Christian belief in free will. Nevertheless, this argument was often expressed as a condemnation of bad astrology, the implication being that astrology was a credible art or science that could be reformed. However, although it was still taken seriously by Johannes Kepler at the beginning of the seventeenth century, it was almost universally despised by social elites a hundred years later.
In Aristotle's De Caelo, heavenly bodies were treated as physical objects and thus as a branch of natural philosophy, or physics. Astronomy thus enjoyed a somewhat bifurcated position within the university system, taught partly in tandem with medicine and astrology, and partly as a higher-level, more technically difficult mathematical enterprise. This mirrored the scholarly distinction between the treatment of the physical reality of the supposedly "crystalline" spheres and such ideas as the epicycles, the actual existence of which was debatable. When Nicolaus Copernicus (1473–1543) published his heliocentric (Sun-centered) De Revolutionibus Orbium Coelestium (On the revolutions of the heavenly orbs) of 1543, a number of astronomers believed that his system could be treated as another convenient set of devices for explaining the celestial phenomena without any commitment to the reality of the general cosmology and physics that was prominent in the first part of the book. Indeed, the Lutheran theologian Andreas Osiander (1498–1552) reassured readers in an anonymous preface to the work that Copernicus had merely devised "hypotheses" that facilitated the calculation of celestial positions.
THE COPERNICAN REVOLUTION
However, Copernicus had indeed upset the standard disciplinary division in universities, and he implied that a mathematical astronomer was entitled to speak about the physical nature of things—the traditional preserve of the more prestigious role of the natural philosopher. Not only did he offer the first modern heliocentric system (although he appealed to the authority of ancient heliocentric systems such as that of Pythagoras), but he asserted that the demonstrations in his work could only be understood by mathematically competent astronomers. He condemned the uncertainty of the calendar, and also of the inability of astronomers to determine the precise motions of the heavenly bodies. He lambasted the inconsistent use of homocentric circles and epicycles, and argued that the systems produced by recent astronomers lacked aesthetic credibility. Instead, with all the different techniques and philosophies in play, they had produced a sort of astronomical "monster."
Although he retained epicycles, Copernicus dispensed with the equant and showed that retrograde motion could be explained by means of a heliocentric system. Indeed, contra Osiander, he could hardly have been more assertive about the truth of his Sun-centered cosmos. He proffered tentative physical explanations of why objects on a revolving earth would not be ejected into the heavens, and argued that the stellar sphere had to be an enormous distance further away than that accepted by traditional astronomers. This meant that his inability to detect stellar parallax (the feature by which the apparent position of a star would vary depending on the time of year, since an observer on Earth would view it from a different position) would not be an argument against his system. Ominously, he also suggested that biblical passages that appeared to support geocentricity were the result of the author "accommodating" his discourse to the capacities of ordinary people. Momentous as it now seems, Copernicus's system had only a handful of adherents in the sixteenth century, although Erasmus Reinhold's Prutenic Tables of 1551, which were based on methods pioneered by Copernicus, were used to produce the Gregorian calendar (decreed by Pope Gregory XIII) in 1582.
In the last three decades of the sixteenth century, the Danish astronomer Tycho Brahe (1546–1601) was single-handedly responsible for two major initiatives in astronomy. First, he devised a "geoheliocentric" system that, for the half-century before Copenicus's system was broadly accepted, provided the major alternative to a geocentric cosmos. In this, the Sun again revolved around the Earth, with the five planets (not including the Moon) revolving around the Sun; since the Martian and solar orbits intersected, this system could not accommodate the reality of the spheres. Tycho rejected a heliocentric system, partly because he could not abide the massive distances required by Copernicus's system, partly because he could not find stellar parallax, and partly because he was strongly opposed to heliocentrism on scriptural grounds.
Most important, Tycho made a series of naked-eye observations, conducted from 1574 as part of a monumental observation program at his observatory (Uraniborg) on the island of Hven. He designed and made new instruments that were as much as ten times more accurate than those of his predecessors. He also conceived of a means of using each of his devices to cross-check results obtained from other instruments in his collection. His instruments were less cumbersome than previous examples and he introduced new techniques for more precisely dividing them into minutes and seconds of a degree. He turned himself and his workplace—which had its own printing press—into the hub of an extensive network of correspondents, and he used this and personal contacts to train a number of the best astronomers of the next generation. Two of his measurements, linked to exceptional celestial events in the 1570s, demonstrated the precision of his observations. First, he determined the extreme distance from Earth of the terrifying supernova of 1572, and second, he showed for the first time that the comet of 1577 existed beyond the lunar sphere and hence was technically part of the heavens. Independent of any implications of De Revolutionibus, both seemed to suggest that the celestial sphere could no longer be considered immutable.
As Tycho's work came to a close, Johannes Kepler (1571–1630) burst on to the stage with his heliocentric Mysterium Cosmographicum (The secret of the universe) of 1596. Kepler was trained at Tübingen by the pro-Copernican Michael Mästlin (1550–1631), and throughout his adult life remained committed to a heliocentric system. In the Mysterium he famously attempted to show that the distances between the planets could be represented by nested regular solids, although this ultimately failed to fit the astronomical data produced by Tycho and his colleagues. At the end of 1600 Kepler traveled to Prague to work with Tycho, who had only recently been appointed imperial mathematician to Rudolf II. When Tycho died within the year, Kepler gained access to his data concerning the orbit of Mars. Tycho had observed the Martian orbit with astonishing accuracy, finding a discrepancy between the observed orbit and an ideal circular trajectory of eight feet that could not be explained by instrument error, and Kepler sought strenuously over the next few years to provide a harmonious and geometrically satisfying orbit to fit these observations. After many different shapes had been tried, he decided that Mars and the other planets traveled in ellipses, one of whose foci was the Sun, and that each planet swept out equal areas in equal times (considering the area to be drawn out by a line linking the planet to the Sun).
Kepler published his first two laws of planetary motion in his Astronomia Nova (New astronomy) of 1609, although they are merely a handful of many mathematical relations that he posed for planetary orbits. In this work he also appealed to the magnetic philosophy devised by the English physicist William Gilbert in his De Magnete (On the magnet) of 1600, in order to explain the physical causes of heavenly motion. He suggested that the planets, which all traveled in the same direction and (virtually) in the same plane, were controlled by a motive force emanating from the Sun. His third and final law, first enunciated in his Harmonice Mundi (Harmony of the world) of 1619, is more complicated and states that the square of the mean orbital period of a planet is proportional to the cube of its mean distance from the Sun. Kepler combined a Platonic-Pythagorean concern for the reality of harmonies and ratios with a magnetic physics, but from a disciplinary point of view he was explicitly asserting the right of an astronomer such as himself to produce a "celestial physics" that gave the true (non-Aristotelian) causes of heavenly motion. In a work on astrology of 1601, he also attempted to uncover the physical causes underlying the influence that planetary conjunctions had on earthly activities.
THE SCIENTIFIC REVOLUTION
In 1609, the same year that Kepler published his first two laws, Galileo Galilei (1564–1642) used a combination of lenses to look at the heavens, having heard of a similar invention that had been introduced the year before. Within weeks, he had deduced that the Moon had mountains and valleys (and was thus not perfectly smooth), had noted that the Milky Way was actually composed of numerous stars, and proposed that Jupiter possessed four satellites that revolved around it as the Moon revolved around Earth. For his extraordinary discoveries, which appeared in Sidereus Nuncius (Starry messenger) of 1610, he was rewarded with the position of court philosopher to his onetime pupil, Cosimo de' Medici (Cosimo II), grand duke of Tuscany. Galileo was now a court intellectual to rival Johannes Kepler, who had succeeded Tycho Brahe as imperial mathematician to Rudolf II. Given the courtly locations of both Kepler and Galileo, twentieth- and twenty-first-century historians have pointed to the prevalence of nonuniversity locations as the most important settings for innovative astronomy. Kepler's friendly stance toward Galileo (he wrote a book praising Galileo's discoveries within months of the work appearing) ensured that there was minimal animosity between them, although this may have been helped by Galileo's apparent complete ignorance of Kepler's own discoveries.
In the next two and a half decades, adherence to the Copernican system met with determined opposition and, notoriously, René Descartes felt obliged to suppress his not sufficiently anti-Copernican Le monde (The world) in 1633 soon after Galileo's pro-Copernican Dialogue had been condemned by the Roman Inquisition in July. However, the vast majority of astronomers and natural philosophers were avowed Copernicans by the middle of the seventeenth century, and even those, such as the Jesuits, who were slow to accept the Copernican worldview, made substantial contributions to astronomy. Indeed, Galileo's successes can be overstated as contributions to the demise of the notion of a perfect celestial sphere, for previous events and observations had begun to shake confidence in heavenly incorrigibility. Of all his discoveries, only his sightings of the phases of Venus provided overt support for Copernicus's system, although Tycho's system could also account for them. Nevertheless, Galileo's visually striking discoveries—and their implications—were easily understood by a new audience of scholars and gentlemen, and they had a dramatic impact on writers and poets such as John Donne (1572–1631).
It is important to note, as Allan Chapman has properly argued, that the great theoretical advances in cosmology achieved by Kepler, Isaac Newton, and others were facilitated and indeed made possible by improvements in angular astronomical measurement and not by superior visual acuity. Tycho's advances in instrument design and observational accuracy were made possible by the cadre of excellent craftsmen he had at his disposal, and similarly, John Flamsteed, the first British astronomer royal, whose observations were to prove crucial for Newton's enunciation of universal gravitation, had innovative and highly skilled instrument makers working with him. Only with patient astronomy of this sort could precise measurements be made of celestial magnitudes such as distance and size.
In the 1660s telescopic sights were added to quadrants and a zenith sector in the attempt by the French to measure the length of a degree of meridian in France. In Restoration England a number of episodes occurred that acted as a spur to creating an alliance between accurate measurement and theoretical innovation. The Royal Society of London was founded in 1660, followed by the Royal Greenwich Observatory—founded to aid navigation and the determination of longitude by improving astronomy—in 1675. The Observatory was badly stocked with instruments at first, but gradually, with some private support and with the help of occasionally brilliant suggestions from Robert Hooke (1635–1703) for automating observations, Flamsteed was able to build up a stock of the best instruments then available. By the 1690s he had a degree clock that allowed star positions to be measured with extraordinary accuracy, and the ten-to-twelveseconds error of his mural arc (that is, a large quadrant set on a wall) was a sixfold improvement on the accuracy of Tycho's instruments. Flamsteed used telescopic sights on his instruments but also a filar micrometer, that is, a system of thin wires, minutely movable by means of a carefully graduated screw, that could be placed inside a telescope to finesse its accuracy.
Isaac Newton (1642–1727) developed an early interest in astronomy and became famous initially because of his development of a reflecting telescope in the late 1660s. Flamsteed's data was crucial for Newton in the winter of 1684–1685, when the latter was trying to determine what mutual influence Jupiter and Saturn might have on each other, and again in late 1685, when Newton wanted three items of data (accurate to a minute) on the path of the Great Comet of 1680–1681. This data would constitute crucial evidence for the cosmological system that Newton published in his momentous Principia Mathematica (The mathematical principles of natural philosophy) of 1687, for he could now analyze observed deviations from perfect elliptical orbits by means of his concept of universal gravitation. Perhaps of equal significance were the observations Flamsteed put his way in 1694–1695 when Newton had another go at the Moon. This ultimately unprofitable endeavor was part of an effort to solve the (insoluble) three-body problem of the mutual interactions of Sun, Moon, and Earth, all of which Newton later described as the most difficult science he ever did. The pair fell out irreconcilably soon after this, and Newton behaved abominably toward the astronomer royal, practically stealing Flamsteed's laboriously crafted star catalog by claiming it as the property of the state. Not the least of Newton's actions was to downgrade and even efface (in his Principia ) the contributions made by Flamsteed, who had generously provided the observations that allowed Newton to corroborate and then rework his supreme theory. Whatever his personal dealings with others, Newton's theory provided the basic theory of the heavens that we now take to be true, and his achievements included the recognition that some comets travel in periodic elliptical orbits.
By the early eighteenth century, the London instrument-making trade was widely held to produce the highest quality instruments; Pierre-Louis Moreau de Maupertuis (1698–1759), for example, took a zenith sector and clock constructed by the outstanding London instrument maker George Graham (1673–1751) to Lapland in 1736–1737. It was this expedition that went furthest in determining the shape of Earth, confirming Newton's calculation that it was an oblate spheroid (flattened at the poles). In England, Graham and others made instruments for the astronomers royal who followed Flamsteed, namely Edmond Halley (in 1720) and James Bradley (in 1742). Bradley, who discovered stellar aberration in 1727 and who confirmed Newton's analysis of the extent of the nutation of Earth's axis in the 1740s, combined access to the best instruments of the day with an obsession for accuracy. By the middle of the eighteenth century, measurements were confined to the meridian, and, among other activities, experiments were being undertaken to better ascertain longitude and latitude—an activity seen by the British, the French, and many other naval powers as essential for improving navigation. With massively expensive instrumentation that only large institutions could afford, astronomy had changed beyond all recognition from the medieval period. Religious and other value systems no longer placed barriers on believing in and publishing particular accounts of the cosmos, and all serious intellectuals were heliocentrists.
See also Aristotelianism ; Astrology ; Brahe, Tycho ; Calendar ; Copernicus, Nicolaus ; Cosmology ; Descartes, René ; Galileo Galilei ; Kepler, Johannes ; Newton, Isaac ; Scientific Instruments ; Scientific Revolution .
Copernicus, Nicolaus. On the Revolutions of the Heavenly Spheres. Translated by A. M. Duncan. Newton Abbott, U.K., and New York, 1976.
Descartes, René. The World and Other Writings. Translated and edited by Stephen Gaukroger. Cambridge, U.K., and New York, 1998.
Galilei, Galileo. Sidereus Nuncius; or, The Sidereal Messenger. Translated by Albert Van Helden. Chicago and London, 1989.
Kepler, Johannes. Mysterium Cosmographicum: The Secret of the Universe. Translated by A. M. Duncan. New York, 1981.
——. New Astronomy. Translated by William H. Donahue. Cambridge, U.K., and New York, 1992.
Chapman, Allan. Dividing the Circle: The Development of Critical Angular Measurement in Astronomy, 1500–1850. 2nd ed. Chichester, U.K., and New York, 1995.
Donahue, William H. The Dissolution of the Celestial Spheres. New York, 1981.
Dreyer, J. L. E. A History of Astronomy from Thales to Kepler. Rev. ed., with foreword by W. H. Stahl. New York, 1953.
Jardine, Nicholas. The Birth of History and Philosophy of Science: Kepler's A Defence of Tycho against Ursus, with Essays on its Provenance and Significance. Cambridge, U.K., and New York, 1984.
King, Henry C. The History of the Telescope. New York, 1979.
Kuhn, Thomas S. The Copernican Revolution: Planetary Astronomy in the Development of Western Thought. Cambridge, Mass., and London, 1957.
Newman, William R., and Anthony Grafton, eds. Secrets of Nature: Astrology and Alchemy in Early Modern Europe. Cambridge, Mass., and London, 2001.
Schechner Genuth, Sara. Comets, Popular Culture, and the Birth of Modern Cosmology. Princeton, 1997.
Stephenson, Bruce. Kepler's Physical Astronomy. Princeton, 1994.
Thoren, Victor E., with contributions by John R. Christianson. The Lord of Uraniborg: A Biography of Tycho Brahe. Cambridge, U.K., and New York, 1990.
In the Bible
Although the Bible contains no explicit mention of the science of astronomy, it nevertheless has many references to topics such as the laws of the heavens (Jer. 31:34 ; 33:25; Job 38:33) and the movements of the sun and the moon (Josh. 10:13; Ps. 19:6–7; Job 31:26; Eccles. 1:5–6).
The Israelites did not study the stars as did the Babylonians, Egyptians, and Greeks. They may have refrained from too close observation of the celestial bodies out of a fear of idolatry – "When you look up to the sky and behold the sun, and the moon, and the stars, the whole heavenly host, you must not be lured into bowing down to them or serving them…" (Deut. 4: 19). Nevertheless, some basic knowledge of astronomy was essential to fix the dates of festivals and holidays.
the stars and the planets
The firmament or heavenly vault, the abode of the two "great lights" and the stars, was stretched between the waters above and the waters beneath (Gen. 1:14–18), and was rigid and strong "as a molten mirror" (Job 37:18). The stars of the heaven are as numerous "as the sands on the seashore" (Gen. 22:17); they are also frequently called "the host of heaven." The planets (mazzalot; ii Kings 23:5) are, according to most biblical interpreters, in the twelve regions of the firmament which are later referred to as the signs of the *Zodiac. Other constellations, the five planets, the sun and the moon, and various individual stars are referred to in the Bible (e.g., cf. Job 38:31–32).
the sun and the moon
The sun and the moon are frequently mentioned: the sun is referred to as shemesh (Ex. 22:2; Deut. 24:15), ḥammah (Isa. 24:23; Job 30:28), and ḥarsah (Judg. 14:18). The usual term for the moon (yare'aḥ) was also used to designate the lunar cycle (e.g., Ex. 2:2; Deut. 21:13; i Kings 6:37). The moon is also called levanah (Isa. 24:23; Song 6:10), and the full moon is called kese(h) (Ps. 81:4; Prov. 7:20). The word ḥodesh ("month") originally meant "the renewal of the moon," and described the day of the new moon (i Sam. 20:24, 34; Ezek. 26:1) and the length of its cycle (Gen. 29:14).
the planets saturn and venus
It is generally agreed that Chiun (Amos. 5:26) refers to Saturn (called kaiwānu, kai[a]mānu in Assyrian, and kaivana in Syrian). Lucifer (Heilel), the "son of the morning" is, according to most interpreters, the planet Venus that is visible at dawn (Isa. 14:12). In Arabic, Venus is called al-Zuhara ("the bright one.").
the fixed stars
In the Bible kesil is mentioned four times (Isa. 13:10; Amos 5:8; Job. 9:9; 38:31). Views on its interpretation vary, but it is sometimes taken to represent Orion, which was considered to be one of the giant angels (Gen. 6:4). The Targum Jonathan rendered kesil, the "giant" (nefila; Job 9:9; 38:31); and in Isaiah 13:10 reference is made to "the stars of the heavens and their titans (kesileihem)." Kimah is, according to several interpreters, the constellation of the Pleiades. Other commentators identify it as Aldebaran, Arcturus, or Sirius. Ash (or ayish; Job 38:32) is mentioned with kesil and kimah (Job 9:9), and R. Judah b. Ezekiel claimed that it is the star called Yuta, "the lamb's tail," in Aramaic (Ber. 58b), which is probably Aldebaran. In the Vulgate, ash is translated as the Hyades, while the Septuagint gives it as "the Evening Star," i.e., Venus. Ḥadrei-Teiman (Job 9:9) is thought to represent the twinkling stars of the Southern firmament – the ship Argo, the Southern Cross, Centaurus and others – which could be observed in the land of Israel in the time of Job but cannot now, because of the precession of the equinoxes (that is, the slow westward movement of the earth's axis which makes the position of the stars change continuously), and thus the zodiac seems to change its position in relation to the horizon over hundreds of years. Mezarim in Job 37:9 is possibly a nickname for mazzalot, though according to some modern interpreters the mezarim are the Great Bear and the Little Bear.
In the Apocrypha
In the Book of Enoch several chapters are devoted to the courses of the heavenly bodies, to the fixing of the length of day and night in the different months, to the moon's course during the month, and to the difference between the solar and lunar years. These astronomical ideas, often inaccurate, were interspersed with legends about angels and spirits. Thus, the angels elevate Enoch through the various spheres of the heavens, and at the fourth he perceives the sun and the moon and a multitude of stars.
In the Talmud and Midrash
It is difficult to discuss fully the knowledge of astronomy in the talmudic period on the basis of the limited material in the Talmud and Midrashim. The knowledge of astronomy possessed by the tannaim and amoraim was not committed to paper, and only was recorded after its compilation by the geonim. The talmudic sages viewed astronomy – the computing of seasons and planets – and knowledge of the month order and the *calendar (intercalation) as important adjuncts to the study of the Torah. They attributed these studies to the ancients of the Bible, and interpreted the verse "And of the children of Issachar that had understanding of the times, to know what Israel ought to do" (i Chron. 12:33) as meaning that the children of Issachar knew how to compute the cycles of the planets in order to learn how Israel would determine the months and leap years. The study of this science was even considered an obligation for the talented person (Shab. 75a). Many of the tannaim and amoraim were experts in astronomy as, for example, *Johanan b. Zakkai (Suk. 28a), *Gamalielii, and Joshua b. Hananiah. The last named knew of the existence of a comet which appeared once every seventy years and led mariners astray (Hor. 10a). This was probably Halley's Comet. Among the Babylonian amoraim, *Samuel was important in the field of astronomy. He claimed that he could calculate and adjust the festival calendar of the Diaspora, without recourse to an eyewitness' report of the new moon in Israel (rh 20b), and he even made intercalary calculations covering a period of years. The first generations of the amoraim were acquainted with a *baraita called "Secrets of Intercalation," in which were written precepts for the sanctification and intercalation of the month (rh 20b). In general, this knowledge was rarely committed to paper, being "secrets of the Torah not to be passed on to all and sundry" (Ket. 112a).
In the eyes of the talmudic sages the earth was the center of creation, with heaven as a hemisphere spread over it. The Midrash conceived the heavens as being made up of several spheres or vaults – the sun, moon, stars, and planets being fixed in the second one (Ḥag. 12b). Nevertheless, a knowledge of the order of the celestial bodies, their path and distances from the earth, existed alongside of the above mythological picture. At the horizon, the heaven and earth "kiss each other," and the earth's diameter from east to west is equivalent to the height of the heavens above the earth (Tam. 32a). The earth is usually described as a disk encircled by water. In the Midrash it is pictured as standing on twelve columns, for the tribes of Israel, or seven columns, for the pillars of wisdom. The columns rest upon water, the water upon mountains, the mountains upon the wind, the wind upon the storm, and the storm is dependent on the arm of the Almighty. Yet with all this there existed a clear recognition of the earth as a sphere (tj, Av. Zar. 3:1, 42C; Num. R. 13; 14).
motions of the celestial bodies
In one baraita (Pes. 94b) there are differing opinions regarding the circles of rotation and the planets. "The Jewish sages say 'The sun moves by day beneath the firmament, and by night above the firmament'; the learned of the nations say, 'The sun moves by day beneath the firmament and by night beneath the earth.'" This baraita is most important, as it is evidence of a serious interest in celestial mechanics, of an early knowledge of scientific concepts, and of an objective approach to the solution of astronomical problems. The daily changes in the positions of sunrise and sunset in the annual cycle of the sun were well known. These phenomena are explained by the existence of 365 windows in the firmament – 182 in the east, where the sun rises; and 182 in the west, where it sets; and one in the center of the firmament, the place of its first entrance at the time of the Creation (tj, rh 2:5, 58a; Ex. R. 15:22). The distance traversed in 30 days by the sun, is traversed by the moon in two and one-half days. The sun is called the "Greater Light" and the moon, the "Lesser Light" because the solar year is longer than the lunar year by 11 days (Ex. R. ibid.). As for the courses of the planets, it is said (Gen. R. 10:4) " there is a planet that finishes its cycle in 12 years – that is Jupiter; and there is a planet which finishes its cycle in 30 years – that is Saturn; except for Venus and Mars that do not finish their cycles for 480 years." The figures given for Jupiter and Saturn are correct, according to the geocentric system of the motion of the planets, but the figures relating to Venus and Mars were wholly inaccurate and they seem to have been regarded as doubtful in quite early times.
A concept of the solar motions is found in the baraita (Ber. 59b), which is explained by a great cycle of 28 years, at the end of which the sun returns to its original position relative to the stars and planets. The aggadah even accurately works out the time of the start of both the solar and lunar cycles (Targ. Jon., Gen. 1:16). The great cycle of the moon is also mentioned, being 21 years (Pd–re 7); there is also a possible hint of a cycle of 19 years (Targ. Jon., Gen. 1:14). This length of time is the basis for calendar calculation, having been fixed at a much later period, and it remains valid up to the present day. The monthly changes in the shape of the moon are also well described (Ex. R. 15:26), and it is clear that various writers on this problem were not too far from the truth.
the four seasons
(Tekufot). The change of season and the comparison of day and night are fairly well described: "there are four seasons of the year, from the Nisan season to the Tammuz season the day borrows from the night, and from the Tammuz season to the Tishri season the day repays the night; from the Tishri season to the Tevet season the night borrows from the day, and from the Tevet season to the Nisan season the night repays the day; during the Nisan season and Tishri season, neither one owes anything to the other" (Mid. Ps. 19:3). Samuel gives reasonably accurate figures regarding the periods between the seasons (Er. 56a), but when he discusses the fixing of the dates of the seasons, he allows imaginary bases to be included.
the planets, the zodiac, stars, and comets
The names of the planets – Saturn, Jupiter, the Sun, Mars, Venus (or Kokhevet), Mercury (or Kokhav Hammah), and the Moon – are referred to collectively in an acrostic as שצ״מ הנכ״ל. The 12 signs of the Zodiac and their relation to the months of the year are Aries (Nisan), Taurus (Iyyar), Gemini (Sivan), Cancer (Tammuz), Leo (Av), Virgo (Elul), Libra (Tishri), Scorpio (Marheshvan), Sagittarius (Kislev), Capricorn (Tevet), Aquarius (Shevat), and Pisces (Adar). From the astrological viewpoint, the 12 signs of the Zodiac have different influences on the "four winds of heaven," and sometimes there is a symbolic connection with the 12 tribes of Israel (Yal., Ex. 418; Yal., i Kings 185). In addition to the stars mentioned in the Bible, there is also a reference to the Milky Way (Ber. 58b). The meteors mentioned in the Mishnah (Ber. 9:2) are comets (Ber. 58b), and Samuel admitted that he did not know their nature. The "Baraita of Samuel," which was traditionally written by the amora Samuel, is ascribed by some to the ninth century (see below).
Astronomy in the Middle Ages
The principal contributions of medieval Jewry to astronomy were the calculation of the Hebrew *calendar; the translation of Arabic works and the diffusion of knowledge from the Arabic world; and the compilation of astronomical tables for scientific and navigational purposes. *Ptolemy, the Alexandrian astronomer of the second century c.e., compiled the Almagest (Syntaxis Mathematica), a long work in 13 books systematizing the structure of the universe and Greek astronomy. The Almagest dominated astronomical and astrological thought for 14 centuries, becoming the authority on astronomy and the major source for astronomical commentaries and translations in the medieval period. The Jews were of major importance to scholastic Europe and the beginning of the Renaissance, in that they provided a link between the Arabic translations, commentaries, and compilations of the Almagest and the Christian astronomers, mostly by means of their own translations and commentaries in Hebrew or Latin. One of the first Hebrew translations of the Arabic version of the Almagest was made by Jacob *Anatoli between the years 1231 and 1235 as Ḥibbur ha-Gadol ha-Nikra al-Magesti. Anatoli also translated *Averroes' summary of the Almagest under the title Kiẓẓur al-Magesti, and Kitāb fial-Ḥarakāt al-Samāwiyya ("The Book on the Heavenly Movements") by the ninth-century Arabic astronomer al-Farghānī (Alfraganus) under the title Yesodot ha-Tekhunah. The compendium of Ptolemy's Almagest in Arabic by Ibn Aflaḥ ha-Ishbili (the 12th-century Spanish astronomer), known also as Abu-Muhammad Jābir ibn Aflaḥ, is mentioned by *Maimonides in the Guide of the Perplexed (2:9). Ibn Aflaḥ's book (Kitāb al-Hayʾd, "The Book of Astronomy") is important for its critical appraisal of the Ptolemaic system of the universe, and was translated into Hebrew in two versions: one by Moses ibn *Tibbon (the 13th-century French physician and translator in 1274), and another, apparently, by Jacob b. Machir ibn *Tibbon (Don Profiat), which was abridged by Samuel b. Judah of Marseilles (the 14th-century French physician) in 1335. Moses ibn Tibbon also translated Eisagōgē eis ta Phainomena ("Introduction to Celestial Phenomena") of the first-century b.c.e. Greek philosopher, Geminus, under the title of Ḥokhmat ha-Kokhavim or Hokhmat Tekhunah ha-Kaẓar or Sefer ha-Ḥokhmah ha-Kaddurit, in 1246 at Naples. He also translated Kitâb al-Hay'a ("The Book on Astronomy") by the Arab astronomer al-Biṭrūjī of Seville (d. 1185) under the title Ma'amar bi-Tekhunah in 1259. The latter work had a great influence on Jewish scholars up to the 16th century. Jacob b. Machir translated around 1271 Fi-Hay'at al- ʿÂlam ("On the Astronomy of the Universe" as Sefer ha-Tekhunah) by Abuʿali ibn al-Haytham (11th century), describing the quadrant and astronomy. Samuel b. Judah of Marseilles translated the treatise on the movement of the fixed stars (Ma'amarbi-Tenuʾat ha-Kokhavim ha-Kayyamim), by Abu Ishaq Ibrahim ibn Yaḥya al-Zarqālī (also known as Zarqāla or Zarqallah) of Cordova (second half of the 11th century). Moses b. Elijah the Greek (probably the 15th-century Moses Galeno) translated a study of astronomy by Omar ibn Muhammad under the title, Sefer Mezukkak. The Christian Jacob Christmann translated into Latin the Hebrew translations of the summary of the Almagest by Jacob Anatoli and al-Farghānī's book on astronomy (Frankfurt, 1590). Abraham de *Balmes (d. 1524) translated into Latin Moses ibn Tibbon's Hebrew translation of Geminus' work on astronomy (see above), under the mistaken title Isagogicon Astrologiae Ptolemaei, as well as Jacob b. Machir's Hebrew translation of the above work by Ibn al-Haytham, under the title Liber de Mundo.
The following are among those who published commentaries on the Almagest: Samuel ben Judah of Marseilles (14th century), David ibn *Naḥmias of Toledo (beginning of 14th century), and Elijah *Mizraḥi (d. 1525). Commentaries on the Hebrew translation of al-Farghānī's work were composed by Moses Handali (possibly 13th century), Isaac b. Samuel abu al-Khayr (c. 1340), Maimon of Montpellier (of unknown date; see *Montpellier), and Judah ibn Verga (1457). There exists a shortened version of the Almagest which was possibly written by Ḥayyim *Vital.
At the end of the Middle Ages books in Latin were also translated into Hebrew. The essay by the German astronomer, Johannes de Gamundia (1380–1442), "De ratione componendi et usu novi instrumenti" was translated by David Kalonymus b. Jacob Meir Kalonymus under the title Marot ha-Kokhavim (1466). Theorica Planetarum of Georg Peuerbach (1423–1461) was translated twice: once by Ephraim Mizraḥi, and a second time by Moses b. Baruch *Almosnino (1510–1580). John de Sacrobosco (John of Holywood, the Parisian mathematician and astronomer who died in 1256) wrote the famous Tractatus de Sphaera, which elucidated and incorporated Ptolemy's Almagest and the work of al-Farghānī (see above) and which soon replaced both these books. It was translated into Hebrew around 1399 by *Solomon b. Abraham (Avigdor) of Montpellier as the Mareh ha-Ofannim.
Several Arabic essays were translated into European languages, especially Latin and Spanish. These translations were, in fact, the main channels for the progress of astronomy in medieval Europe. In 1256 Judah b. Moses ha-Kohen of Toledo translated into Spanish the Kitāb al-Kawākib ("Book of the Stars") of 'Abd al-Raḥmān al-Sūfi (tenth century) under the title Libro de las figuras and the astrological treatise Kitāb al-Bārie by Ibn Abu al-Rijāl (11th century) under the title Librocomplido. Commentaries on the Tractatus de Sphaera by Johnde Sacrobosco were published in the 16th and 17th centuries by Moses b. Baruch Almosnino, Mattathias *Delacrut (1550), and Manoah Handil b. Shemariah (Polish author who died in 1612). A commentary on Georg Peuerbach's Theorica Planetarum was written by Moses *Isserles in the early 17th century.
In the Middle Ages Jews compiled most of the astronomical tables. Among these, the heretic Jew Sind ibn Ali (829–33) was a principal contributor to the astronomical tables of Caliph Maimun. *Abraham b. Ḥiyya ha-Nasi compiled (before 1136) tables called "Luḥot ha-Nasi" ("The Tables of the Prince or al-Battānī's Tables"), named after the Arab who died in 929, on whose calculations they were based. Al-Battānī had a great influence on astronomy; Maimonides relied on his tables for computing the sun's path, and his works were also mentioned by *Judah Halevi (12th-century), Abraham *Ibn Ezra (1092–1167), Isaac *Israeli (ninth to tenth century), and several other Hebrew authors. Abraham ibn Ezra compiled astronomical tables on the movements of the seven planets, and translated in 1160 the "Reasons for the al-Khwārizmī Tables" by Ahmad b. Elmenthi. Twelve Jewish astronomers, under the leadership of the Cordovan astronomer, Ibn Arzarkāli (Azarchel), helped to compile the "Toledo Tables" in the 12th century. In 1263 these were translated into Latin by John of Brescia and Jacob b. Machir ibn Tibbon, and later served as a basis in a Spanish version for the famous "Alphonsine Tables." These were prepared in 1272 by a group of astronomers, headed by Isaac *Ibn Sa'id (also Sid). The Latin Tables were translated into Hebrew in 1460 by Moses b. Abraham of Nimes, while a new corrected edition was made by Solomon Davin of Rodez. Commentaries were written by Moses Botarel Farissol in 1465 and Mattathias Delacrut in the 16th century.
Specially significant to the Hebrew astronomers were the "Persian Tables" in Greek, which were compiled late in the 14th century by Georgios Krisokaka. Solomon b. Elijah drew up (in about 1374) a set of astronomical tables with notes, the first section according to the Ptolemaic system and the second "in the manner of the Persians." Before 1525 Elijah Mizraḥi wrote a commentary on the tables "drawn up by the Persian sages."
Astronomical tables were also devised by *Levi b. Gershom (1288–1344), based on sources found in Persia, Egypt, etc. Isaac b. Solomon ibn Elhada (14th to 15th century) prepared tables for periods and seasons based on Ibn al-Raqqān, al-Battāni, and Ibn al-Kammād. Joseph b. Isaac b. Moses *Ibn Waqar, writing in Arabic in 1357, drew up tables for the years 720–840 of the Muslim calendar (i.e., 1342–1462) and in 1396 he translated his book into Hebrew with additions and alterations. Other tables were compiled by Jacob b. Machir (1300), Jacob b. David b. Yom Tov (1361), and Abraham *Zacuto, whose tables and Almanach Perpetuum in Latin and Spanish were used by Columbus on his voyages.
early jewish astronomers
There were comparatively few original works by medieval Jewish astronomers, but of these a number were equal to works of contemporary non-Jewish writers. Of importance was the group of men in the eighth and ninth centuries who took up astronomy professionally. Generally, they practiced as astrologers and their knowledge was derived from Greek and ancient Indian writers. Unfortunately, comparatively few of their writings have been preserved. Some were translated into Latin, and a few works have been found in Hebrew.
Māshaʾallāh, whose Hebrew name was possibly Joab or Joel, lived during the second half of the eighth and the beginning of the ninth century, and served in the courts of the caliphs in Baghdad. His essay, "Sefer be-Kadrut ha-Levanah ve-Ḥibbur ha-Kokhavim u-Tekufat ha-Shanim" has been preserved in Hebrew. The Persian Jewish astronomer Andruzager b. Zadi Faruch, who lived in the ninth century, is often identified with the expert in intercalation Eliezer b. Faruch, to whom the Arab chronologist al-Bīrūnī (early 11th century) attributed the fixing of the Jewish calendar. The "Baraita of Samuel" which dealt with the secrets of intercalation, dates from the ninth century but was attributed to the amora Samuel; it is regarded by some as the first original Hebrew work on astronomy in the Middle Ages.
During the late tenth century Ḥasan ibn Ḥasan wrote three books on intercalation; unfortunately they have not been preserved, but reference to their contents was made by Abraham ibn Ezra and Isaac Israeli. Shabbetai *Donnolo (tenth century) wrote a commentary on the Sefer *Yeẓirah. Although they demonstrate the author's knowledge of the subject, the astronomical terms are confused with concepts belonging to astrology and mysticism. A calendar is given, showing the location of the heavenly bodies in 4706 (summer of 956). This work is important in that it constitutes the main source of *Rashi's astronomy.
The greatest of the Jewish astronomers who wrote in Hebrew at the beginning of the Spanish period was Abraham b. Ḥiyya ha-Nasi, whose works influenced generations of Jewish writers. Those of his works which were translated into Latin had an important influence on the development of European science. Apart from his astronomical calendars and Arabic astrological work which he translated into Latin, Abraham b. Ḥiyya wrote the following important works: Ẓurat ha-Areẓ, an astronomical-geographical text; Sefer ha-Ibbur, which included series of calculations of years, and determinations of new moons and cycles; Ḥeshbon Mahalakhot ha-Kokhavim, a book to which comments were added by Abraham ibn Ezra.
In his hymn, "Keter Malkhut" Solomon ibn *Gabirol describes the structure of the universe according to Aristotle and Ptolemy. This work contains detailed calculations of the length of the cycle of each star and its size in relation to the size of the earth.
Abraham Ibn Ezra, in addition to his works on astrology and his calendars and commentaries, wrote the following texts on theoretical astronomy: Sefer ha-Ibbur which is on the subject of cycles, new moons, seasons, and signs of the Zodiac; Shalosh She'elot, replies to three questions on intercalation posed by David b. Joseph of Narbonne (c. 1139); and Kelei Neḥoshet an explanation of the use of the instruments of the astrolabical type. This last was followed by Kelei Neḥoshet ha-Sheni which analyzes the fundamentals of intercalation and the sources of astronomy. It has been passed on by Maimonides who also gives a detailed description of the laws of the spheres (Yad. Yesodei ha-Torah, 3). He maintains (ibid., 4:10) that it was to this that the talmudists referred in their commentaries on the creation (ch. 1). Maimonides' writings show him to have been a foremost astronomer of his time, and demonstrate a scientific approach in his analysis of apparent contradictory data.
The main Jewish astronomers of the 13th century were Judah b. Solomon ha-Kohen ibn Matkah of Toledo, the author of an encyclopedia, Midrash ha-Ḥokhmah, part of which consists of summaries of the great Greek and Muslim astronomers; *Gershom b. Solomon, whose work Sha'ar ha-Shamayim contains a section on the works of Ptolemy, Aristotle, Avicenna, and Averroes. This book was held in high esteem in the Middle Ages, and Meir *Aldabi (c. 1360) used it extensively in the astronomical section of his Shevilei Emunah.
In the *Zohar – probably a 13th-century Spanish composition – there is a passage which gives as a cause of the day's changing into night the revolution of the earth. Some 250 years before Copernicus the Zohar stated that "the whole earth spins in a circle like a ball; the one part is up when the other part is down; the one part is light when the other is dark, it is day in the one part and night in the other."
Of great importance is Yesod Olam by Isaac b. Joseph *Israeli. This work, written in 1310, includes a study of astronomy and cosmography. The author deals with the system of intercalation and with laws of the sanctification of the month according to Maimonides. He gives a method for calculating the parallax of the moon, the importance of which was appreciated up to the time of Kepler. This was the leading textbook on astronomy written during the Middle Ages, and was held in high esteem for hundreds of years. Commentaries and explanations to it were written by Isaac Alhadib, Elijah Mizraḥi, and others. In the yeshivot of the 19th century it was the main text for the study of the calendar. Isaac ben Solomon Israeli translated a summary of it into Hebrew entitled Kiẓẓur Yesod Olam. Isaac Israeli also wrote Sha'ar ha-Shamayim which dealt with the subject of periods and seasons and Sefer Sha'ar ha-Millu'im on the movement of the planets, their order, and positions.
The greatest of the Jewish astronomers of the Middle Ages was undoubtedly Levi b. Gershom. Curtze, the historian of astronomy, numbers him among the forerunners of Copernicus in that he pioneered new methods of research, from which evolved his own original system of astronomy. Levi b. Gershom was an independent and original scholar, and although he did not produce a work specifically devoted to astronomy, his knowledge of astronomy is clearly brought out in the first section of the fifth book of his Milḥamot Adonai. This section of the work was known to later generations as Sefer ha-Tekhunah. Levi b. Gershom explains in detail: a) his discovery, or improvement, of the cross-staff, a device for measuring angles and spherical distances. The inventor called it "the depth finder," while it became known in Europe as "Jacob's staff " (baculus Jacobi) b) his method of passing a light ray from a star through a small aperture in a darkened chamber on to a board. This is the first recorded use of the camera obscura. By these methods Levi b. Gershom carried out numerous measurements and rectified many erroneous conceptions regarding the position of the stars. Among his achievements was the measurement of the relationship of the diameters of the sun and the moon to the lengths of their apparent orbits, and the relationship between the parts of the surfaces covered during an eclipse, and the size of the total area. As a result of his corrections of the originally accepted distances and data, he was able to arrive at a new conception of the distances separating the bodies of the universe and their position in space, and hence (in ch. 9) at a rejection of the basic assumptions of the astronomy of Ptolemy and al-Biṭrūjī. Chapter 99 of the text contains his "Astronomical Tables" (Luḥot) on which commentaries have been written by Moses Botarel Farissol. The importance of the work may be gauged from the fact that part of the book was translated into Latin during the author's lifetime (in 1342). The entire book was not translated until the 15th century.
Other Jewish inventors of astronomical instruments in the later Middle Ages were Jacob b. Machir, who invented an angle measuring device, a quadrant, which he described in his work Rova Yisrael; Isaac b. Solomon b. Ẓaddik*Al Hadib (also al-Aḥdab) wrote Keli ha-Miẓẓu'a about his invention of a new instrument which was a combination of astrolabe and quadrant; Jacob (Bonet) de *Lattes (15th to 16th centuries) designed a device in the shape of a ring for measuring the height of the sun and the stars. His work on this was written in Latin (De annuli astronomii utilitate) and was reprinted no less than six times within 50 years. Immanuel b. Jacob *Bonfils (the 14th-century physician and astronomer of Tarascon) wrote many works on astronomy including one on the construction of the astrolabe, as well as tables of the determination of Venus from 1300 to 1357, and tables for the declination of the sun, etc.
Abraham Zacuto was an influential astronomer of the 16th century. His main work was originally written in Hebrew, but was very soon translated into Spanish, and the Latin synopsis of it, Almanach Perpetuum ("The Continual Almanac") was translated into Spanish and Arabic. All of Zacuto's works, his improved astrolabe, and his astronomical tables were of great importance, particularly in the voyages of discovery of the Spanish and Portuguese explorers.
Knowledge of Jewish medieval astronomy is limited to a very small part of the extensive writings on the subject. Much material remains undiscovered and most of what is available has yet to be studied carefully. Yet, over 250 Jewish astronomers are known to have lived before 1500.
Jewish Astronomy in the late Renaissance
The Jewish contribution to astronomy after Copernicus was relatively small. Most writers concerned themselves with transcriptions from old writings or with summarizing these. Thus, the writings on astronomy of the 18th century and in the rabbinical literature of the 19th century are basically derived from the Ptolemaic school.
In the 16th century *Judah Loew b. Bezalel had a high reputation as an astronomer. However, apart from his few astrological discussions, nothing can be found in his few writings to support this. Moses Isserles (d. 1573) showed a real knowledge of astronomy, particularly in his books Torat ha-Olah (Prague, 1569) and his commentary on Theorica Planetarum.
David *Gans was well acquainted with the development of astronomical knowledge. He was a colleague of Kepler and Tycho Brahe; for the latter he translated parts of the "Alphonsine Tables" into German. His most important astronomical work was Nehmad ve-Na'im written in 1613 and published in Jessnitz, 1743, which presented the first Hebrew exposition of the Copernican system, but the author rejected it because of his traditional Ptolemaic outlook. Mordecai b. Abraham *Jaffe wrote Levush Eder ha-Yakar in Levush Or Yekarot (Lublin, 1594), which contains a commentary on Maimonides' laws of the sanctification of the month as well as a lesson on astronomy; his Be'urei Yafeh is a commentary on Ẓurat ha-Areẓ by Abraham b. Ḥiyya.
Joseph Solomon *Delmedigo was a pupil of Galileo. In his Elim two chapters are devoted to astronomy: the first, "The Laws of the Heavens" is an exposition of the first two chapters of the Almagest, the second, "The Mightiness of God," is devoted to an explanation of other parts of the Almagest and of writings by Copernicus and al-Battānī. Delmedigo was the first outstanding exponent of the Copernican theory in Hebrew literature within the framework of traditional Judaism. His method was to reply to questions from the viewpoint of the ancients, and from that of the astronomers who followed Copernicus.
Tobias *Cohn, the physician, remained faithful to the ancients, although he was quite familiar with the astronomy of Copernicus. In his Ma'aseh Tuviyyah (Venice, 1707–8) he analyzed the geocentric conception in its classic form, and in the one revised by Tycho Brahe. The heliocentric view is analyzed and rejected, mainly on religious and traditional grounds.
*Jonathan b. Joseph from Ruzhany, another commentator on Ẓurat ha-Areẓ, wrote Yeshu'ah be-Yisrael ("Salvation in Israel," Frankfurt, 1720), an explanation of Maimonides' laws of the sanctification of the month.
Raphael ha-Levi of Hanover (1685–1788) wrote Tekhunatha-Shamayim (Amsterdam, 1756), a study of astronomy as related to Maimonides' law, and "Tables of Intercalation" (pt. 1, Leiden, 1756; pt. 2, Hanover, 1757).
Israel b. Moses ha-Levi of Zamosc in his book, Nezah Yisrael (Frankfurt on the Oder, 1741), classified certain obscure parts of the Talmud which dealt with engineering and astronomy. He also wrote a commentary on Yesod Olam by Isaac Israeli, and a textbook called Arubbot ha-Shamayim. Shevilei de-Raki'a (Prague, 1785) by Elijah b. Hayyim of Hochheim is devoted to an explanation of Maimonides' laws of the sanctification of the month. In it the author distinguishes between the geocentric assumptions of Maimonides, and the theories of the new astronomy. In the 19th century, Israel David b. Mordecai *Jaffe-Margoliot wrote Ḥazon Mo'ed (Pressburg, 1843), dealing with astronomy, the mathematics of intercalation, as well as with the additional day of festivals in the Diaspora.
Jews in Modern Astronomy
The frequently repeated statement that Sir William Herschel, astronomer to King George iii and his sister Caroline, were of Jewish origin has been shown to be not in accordance with the facts. Among those who contributed to the development of astronomy in the 19th century were Wilhelm Beer (1797–1850), specialist in the mapping of the features of the moon; Hermann *Goldschmidt is especially noted for his work from 1852 to 1861 in discovering 14 new asteroids between Mars and Jupiter; Rudolph Wolf (1816–1893), at the turn of the century, organized systematic solar work at Zurich; Adolph Hirsch (1830–1901) conducted mainly geophysical work in Switzerland; Maurice *Loewy invented, at the Paris Observatory, the Coudé telescope; Edmund Weiss (1837–1917), was director of the Vienna Observatory in the mid-19th century; Friedrich Simon Archenhold (1861–1939) was a well-known writer of popular books on astronomy; Adolph Marcuse (1860–1930), participated in several astronomical expeditions; Fritz Cohen and Samuel Oppenheim conducted important work in celestical mechanics; as did Erwin Finlay *Freundlich, first in Berlin and then at St. Andrews in Scotland. During this century, Richard *Prager, at first at the University Observatory, Berlin, and from 1938 at the Harvard Observatory, worked on variable stars through the continuation of the Geschichte und Literatur der Veraenderlichen Sterne. Sir Arthur *Schuster, in England, founded in 1919 the forerunner of the International Astronomical Union, to whose subsequent rapid development was due much of the well-organized effort and success of present-day astronomy. Frank *Schlesinger, in the U.S.A., was the first to devise photographic methods for a large scale derivation of stellar distances ("parallax-determinations"). Karl *Schwarzschild, director of the Astrophysical Observatory in Potsdam, did fundamental work in many fields; for example, the laws of stellar motions, photometry, optics, the astrophysical application of atomic physics, and the theoretical exploration of stellar atmospheres. His son Martin *Schwarzschild, who taught at Princeton, U.S.A., was an expert in stellar evolution, and the design of satellite-borne telescopes. Albert *Einstein was noted also for his researches in astrophysics. Other contemporary American astronomers of Jewish origin were Luigi Jacchia (1911–1996), on solar-terrestrial relationships, and David Layzer (1925– ), who researched in theoretical atomic astrophysics, both at Harvard University. At the University of Texas, Gerard de Vancouleurs (1918–1995) was involved in research into the structure and systems of extragalactic nebulae. Rudolph Minkowski (1895–1976) up to 1934 at Hamburg University, investigated at Pasadena the intricate problems of supernovae. Herbert A. *Friedman, at the U.S. Naval Research Laboratory in Washington, was a leader in the new field of outer-space spectroscopy. At Rochester University, Emil Wolf (1922– ) was concerned with optical research with astrophysical applications. Leo Goldberg (1913–1987), at the Smithsonian Astrophysical Observatory, organized teamwork for the initiation of new solar and stellar space research. At the California Institute of Technology, Jesse L. Greenstein (1909–2002) carried out fundamental astrophysical work, particularly in high-dispersion spectroscopy.
Before going to Israel, George Alter (1890–1972) was at the University of Prague, and at the Sidmouth Observatory in England, where he was mainly concerned with problems of star clusters. Arthur Beer (1900–1980), formerly at Breslau and Hamburg, and, from 1934, at the Universities of London and Cambridge, investigated problems of spectroscopic binaries, new stars, stellar photometry, large-scale spectrophotometric determination of distances of stars in the outer regions of our galaxy, its spiral structure, and problems in the history of astronomy. At the Royal Greenwich Observatory, stellar evolution and the abundance of chemical elements in the stars were investigated by Bernard Pagel (1929– ).
Cosmological and other astronomical work of great originality and ingenuity was developed in Soviet Russia; outstanding among the researchers were Vitoli Lazarevich Ginzburg and Joseph S. Shklovski (d. 1985). Leading French astronomers included: the former general secretary of the International Astronomical Union, Jean-Claude Pecker (Observatoire de Paris), and Evry Schatzman (Institut d'Astrophysique, Paris), both active in studies of stellar evolution.
See also *Physics.
G. Sarton, Introduction to the History of Science, 5 vols. (1927–48), indexes; C. Roth, The Jewish Contribution to Civilisation (19382), 67, 76, 80–81, 189–90; Legacy of Israel (19282), 173–314; G. Forbes, History of Astronomy (1909); M. Steinschneider, in: jqr, 13 (1900/01), 106–10; idem, Jewish Literature from the 8th to the 18th Centuries (1857); W.M. Feldman, Rabbinical Mathematics and Astronomy (1931), includes bibliography; O. Neugebauer, in: huca, 22 (1949), 321–63; J.B.J. Delambre, Histoire de l'astronomie du moyen-âge (1819, repr. 1965); C. Roth, in: jqr, 27 (1936/37), 233–6; A. Marx, in: Essays and Studies… Linda R. Miller (1938), 117–70; S. Gandz, Studies in Hebrew Astronomy and Mathematics (1970).
Astronomy, the oldest of all the sciences, is the scientific study of the universe and all the celestial bodies, gases, dust, and other materials within it. It seeks to describe the structure, movements, and processes of celestial bodies and materials. Included within astronomy are the theories and observations that have been set forth by astronomers over the many thousands of years of its existence. Cosmology, a branch of astronomy, deals with the study of the universe, including theories of how the universe began, such as the big bang theory. Astronomers use physics, mathematics, chemistry, geology, biology, and many other scientific areas when dealing with astronomy. When astronomers deal primarily with physics, for instance, they are often called astrophysicists.
Ancient ruins provide evidence that the most remote human ancestors observed and attempted to understand the workings of the cosmos, or the entire universe. Although not always fully understood, ancient ruins demonstrate that early humans attempted to mark the progression of the seasons as related to the apparent changing positions of the sun, stars, planets, and Moon on the celestial sphere. Archaeologists speculate that such observations made the determination of times for planting and harvesting more reliable in developing agrarian communities and cultures.
The regularity of the heavens (what is commonly referred to as the sky above the Earth’s surface) also profoundly affected the development of indigenous religious beliefs and cultural practices. For example, according to Aristotle (384–322 BC), Earth occupied the center of the cosmos, and the sun and planets orbited Earth in perfectly circular orbits at an unvarying rate of speed. The word astronomy is a Greek term for star arrangement. Although heliocentric (Sun-centered) theories were also advanced among ancient Greek and Roman scientists, the embodiment of the geocentric theory (Earth-centered theory) conformed to prevailing religious beliefs. In the form of the Ptolemaic model, it was subsequently embraced by the growing Christian church, which dominated Western thought until the rise of empirical science and the use of the telescope during the Scientific Revolution of the sixteenth and seventeenth centuries.
In the East, Chinese astronomers carefully charted the night sky, noting the appearance of celestial phenomena such as comets and novae. As early as 240 BC, the records of Chinese astronomers record the passage of a guest star known now as Halley’s Comet, and in AD 1054, the records indicate that one star became bright enough to be seen in daylight. Archaeoastronomers (astronomers who deal primarily with archaeology) argue that this transient brightness was a supernova explosion, the remnants of
which now constitute the Crab Nebula. The appearance of the supernova was also recorded by the Anasazi Indians of the American Southwest.
Observations were not limited to spectacular celestial events. After decades of patient observation, the Mayan peoples of Central America were able to predict accurately the movements of the sun, moon, and stars. This civilization also devised a calendar that predicted accurately the length of a year, to what would now be measured to be within six seconds.
Early in the sixteenth century, Polish astronomer Nicolaus Copernicus (1473–1543) reasserted the heliocentric theory abandoned by the Greeks and Romans. Although sparking a revolution in astronomy, Copernicus’s system was flawed deeply by an insistence on circular orbits. Danish astronomer Tycho Brahe’s (1546–1601) precise observations of the celestial movements allowed German astronomer and mathematician Johannes Kepler (1571–1630) to formulate his laws of planetary motion, which correctly described the elliptical orbits of the planets.
Italian astronomer and physicist Galileo Galilei (1564–1642) was the first scientist to utilize a newly invented telescope to make recorded observations of celestial objects. In a prolific career, Galileo’s discoveries, including phases of Venus and moons orbiting Jupiter, dealt a death blow to geocentric theory.
In the seventeenth century, English physicist and mathematician Sir Isaac Newton’s (1642–1727) development of the laws of motion and gravitation marked the beginning of Newtonian physics and modern astrophysics. In addition to developing calculus, Newton made tremendous advances in the understanding of light and optics critical to the development of astronomy. Newton’s seminal 1687 work, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) dominated the Western intellectual landscape for more than two centuries and provided an impetus for the advancement of celestial dynamics.
During the eighteenth century, important theories of celestial mechanics by French mathematician Joseph-Louis Lagrange (1736–1813), French mathematician Pierre Simon de Laplace, (1749–1827) and Swiss mathematician Leonhard Euler (1707–1783) explained small discrepancies between Newton’s predicted and the observed orbits of the planets. These explanations contributed to the concept of a clockwork-like mechanistic universe that operated according to knowable physical laws.
Just as primitive astronomy influenced early religious concepts during the eighteenth century, advancements in astronomy caused significant changes in Western scientific and theological concepts based upon an unchanging, immutable God who ruled a static universe. During the course of the eighteenth century, there developed a growing scientific disregard for understanding based upon divine revelation and a growing acceptance of an understanding of Nature that was increasingly based upon the development and application of scientific laws. Whether God intervened to operate the mechanisms of the universe through miracles or signs (such as comets) became a topic of lively philosophical and theological debate. Concepts of the divine became identified with the assumed eternity or infinity of the cosmos. Theologians argued that the assumed immutability of a static universe, a concept shaken by the discoveries of Copernicus, Kepler, Galileo, and Newton, offered proof of the existence of God. The clockwork universe was viewed as confirmation of the existence of a God with infinite power who was the prime mover, or creator, of the universe. For many scientists and astronomers, however, the revelations of a mechanistic universe left no place for the influence of the Divine, and they discarded their religious views. These philosophical shifts sent sweeping changes across the political and social landscape.
In contrast to the theological viewpoint, astronomers increasingly sought to explain miracles in terms of natural phenomena. Accordingly, by the nineteenth century, the appearance of comets was viewed no longer as a direct sign from God but rather a natural, explainable, and predictable result of a deterministic universe. Explanations for catastrophic events (e.g., comet impacts, extinctions, etc.) increasingly came to be viewed as the inevitable results of time and statistical probability.
The need for greater accuracy and precision in astronomical measurements, particularly those used in navigation, spurred development of improved telescopes and pendulum driven clocks; this greatly increased the pace of astronomical discovery. In 1781, improved mathematical techniques, combined with technological improvements and the proper application of Newtonian laws, allowed German-born English astronomer William Herschel (1738–1822) to discover the planet Uranus.
Until the twentieth century, astronomy essentially remained concerned with the accurate description of the movements of planets and stars. Developments in electromagnetic theories of light and the formulation of quantum and relativity theories, however, allowed astronomers to probe the inner workings of the celestial objects. German-American physicist Albert Einstein’s (1879–1955) theories of relativity and the emergence of quantum theory influenced Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910-1995). Chandrasekhar first articulated the evolution of stars into supernovas, white dwarfs, and neutron stars and accurately predicted the conditions required for the formation of black holes, which were subsequently found in the later half of the twentieth century. The articulation of the stellar evolutionary cycle allowed rapid advancements in cosmological theories regarding the creation of the universe. In particular, American astronomer Edwin Hubble’s (1889–1953) discovery of red-shifted spectra from stars provided evidence of an expanding universe that, along with increased understanding of stellar evolution, ultimately led to the abandonment of static models of the universe and the formulation of the big bang theory, which was based on cosmological models.
In 1932, American engineer Karl Janskey (1905–1945) discovered the existence of radio waves emanating from beyond the Earth. Janskey’s discovery led to the birth of radio astronomy, which ultimately became one of the most productive means of astronomical observation and spurred continuing studies of the cosmos across all regions of the electromagnetic spectrum.
Profound questions regarding the birth and death of stars led to the stunning realization that, in a real sense, because the heavier atoms of which humans were comprised were derived from nucleosynthesis in dying stars, humankind, too, was a product of stellar evolution. After millenniums of observing the cosmos, by the dawn of the twenty-first century, advances in astronomy allowed humans to gaze into the night sky and realize that they were looking at the light from stars distant in space and time, and that they, also, were made from the very dust of stars.
At its most fundamental, astronomy is based on the electromagnetic radiation emitted by the stars. The ability to gather light is the key to acquiring useful data. The bigger the primary mirror of a telescope, the greater its light-gathering capabilities and the greater the magnification of the instrument. These two attributes allow a large telescope to image fainter, smaller objects than a telescope of lesser size. Thus, astronomers build ever-larger telescopes, such as the 33-ft-diameter (10-m-diameter) Keck telescopes in Hawaii. They also attempt to escape the distorting effects of the atmosphere with orbital observatories like the Hubble Space Telescope and the James Webb Space Telescope, which is expected to be launched in orbit about the Earth on or after 2011 by NASA.
Astronomy is not just about visible light, however. Though the visible spectral region is most familiar to human sight because eyes are optimized for these wavelengths, observation in the visible region shows only a small portion of the activities and processes underway in the universe. When astronomers view the night sky in other regions of the electromagnetic spectrum, it presents an entirely different picture. Hot gases boil when viewed at infrared wavelengths, newly forming galaxies and stars glow with x rays, and mysterious objects generate explosive bursts of gamma rays. Radio wave and ultraviolet observations likewise bring astronomers new insights about stellar objects.
Each spectral region requires different instrumentation and different approaches to data analysis. Radio astronomers, for instance, use 20- and 30-ft-diameter (6- and 9-m) antennas. In addition, they use telescopes like the one in Arecibo, Puerto Rico, in which a 1,000-ft (303-m) diameter natural bowl in the landscape has been lined to act as an enormous radio wave collector. In the Very Large Array in New Mexico, 27 antennas placed as much as one mile apart from one another are linked by computer to make simultaneous observations, effectively synthesizing a telescope with a 22-mi (35-km) aperture—a radio-frequency analog to the Keck telescope. Infrared, x-ray, and gamma-ray telescopes require special materials and designs for both the focusing optics and the detectors and cannot be performed below the Earth’s atmosphere.
Astronomy is based upon the information that scientists can derive by what is observed when gazing at the stars. One of the characteristics of a star that can be determined observationally is its luminosity—the amount of light that the star emits. When combined with other information about a star, such as its size or temperature, luminosity can indicate the intensity of fusion reactions taking place in the stellar core. Luminosity cannot always be determined by direct observation, however, as distance can decrease the apparent luminosity of an object. The Sun, for example, is not excessively luminous as stars go; it only appears brighter than any other stellar object because it is so close to the Earth.
Magnitude is another way of expressing the luminosity of a star. Greek astronomer and mathematician Hipparchus (190–120 BC) developed the magnitude scale for stars, rating their brightness on a scale of 1 to 6. According to the scale, a star of first magnitude is defined as appearing 100 times as bright as a star of sixth magnitude, so the larger the magnitude, the fainter the object. As telescopes have allowed astronomers to peer deeper into the universe, the scale has expanded: the star Sirius, which appears to be the brightest star in the heavens, has an apparent magnitude of –1.27, while the dwarf planet Pluto has a magnitude of 14.
Apparent magnitude, like apparent luminosity, can be deceptive. To avoid invalid comparisons, astronomers have developed the concept of absolute magnitude, which is defined as the apparent magnitude the object would have when viewed at a distance of 32.6 light years (a light year is defined as the distance that light travels in one earth-year). Thus, measuring the distance to various objects is an important task in astronomy and astrophysics.
The color of light emitted by a star indicates its temperature. At the beginning of the twentieth century, astronomers began classifying stars based on color, or spectral classes. The classes are O, B, A, F, G, K, and M. O-type stars are the hottest (63,000°F [34,632°C]) and tend to appear white or blue-white, while M-type stars are the coolest (5,400°F [2,952°C]) and tend to appear red; the Sun, a yellow star, type G, falls in the middle. Another rating—L-type, for dim, cool objects below M-type—has been proposed for addition to the listing.
Astronomers can glean a tremendous amount of information from stellar magnitudes and glasses. Between 1911–13, Danish astronomer Ejnar Hertzsprung (1873–1967) and American astronomer Henry Norris Russell (1877–1957) independently developed what is now known as the Hertzsprung-Russell diagram, which plots the magnitude and color of stars. According to the diagram, most stars fall on a slightly curving diagonal that runs from very bright, hot stars down to very cool, red stars. Most stars follow this so-called main sequence as they gradually burn out. Some stars fall off of the main sequence, for example red giants, which are relatively cool but appear bright because of their enormous size; or white dwarfs, which are bright but so small as to appear faint.
Absolute magnitude —The apparent brightness of a star, measured in units of magnitudes, at a fixed distance of 10 parsecs.
Apparent magnitude —The brightness of a star, measured in units of magnitudes, in the visual part of the electromagnetic spectrum, the region to which our eyes are most sensitive. Emission spectrum—A spectrum consisting of bright lines generated by specific atoms or atomic processes.
Luminosity —The amount of light emitted from a source per unit area.
Spectroscopy —A technique for studying light by breaking it down into its constituent wavelengths.
When we think of astronomy, spectacular, colorful pictures of swirling galaxies, collapsing stars, and giant clouds of interstellar gas come to mind. In reality, however, some of the most useful observational data in astronomy does not involve images at all. Spectroscopic techniques are powerful tools that allow scientists to detect the presence of certain elements or processes in faraway galaxies.
In spectroscopy, incoming light—such as that from a star—is passed through a grating or a prism that splits the light up into its constituent wavelengths, or colors. Normally, a very bright, hot star will emit a continuous spectrum of light that spreads like a rainbow across the electromagnetic spectrum. In the case of lower-density gas masses such as nebulae, however, the light will be emitted only at certain specific wavelengths defined by the elements found in the nebula; hydrogen atoms, for example, generate vivid yellow lines at characteristic wavelengths. The spectra will, thus, consist of a collection of bright lines in an otherwise dark background; this is called an emission spectrum. Similarly, if a cooler atmosphere surrounds a star, the atoms in the atmosphere will absorb certain wavelengths, leaving dark lines in what would otherwise be a continuum. This is known as an absorption spectrum.
Scientists study absorption and emission spectra to discover the elements present in stars, galaxies, gas clouds, or planet-forming nebulae. By monitoring the amount by which spectroscopic lines shift toward red wavelengths or toward blue wavelengths, astronomers can determine whether objects are moving toward or away from the Earth. This technique, based on the Doppler shift, is used not only to help astronomers study the expansion of the universe but also to determine the distance or age of the object under study. By studying the Doppler shift of stellar spectra, astronomers have been able to monitor faint wobbles in the motion of stars that indicate the presence of a companion star or even of extrasolar planets.
Although the sophisticated instruments and analysis techniques of astrophysics assist in the understanding of universe, astronomy is essentially about the observation of light. Using the data produced by a multitude of telescopes around the world and in orbit, astronomers are making new discoveries on a daily basis and, just as often, exposing new puzzles to solve. The basic tools described above help scientists to extract information about stellar objects and thus about the processes at work in the universe.
See also Astrobiology; Astroblemes; Astrolabe; Astrometry; Astronomical unit; Cosmic background radiation; Cosmic ray; Cosmology; Gravity and gravitation; Infrared astronomy; Relativity, general; Relativity, special; Space shuttle; Spacecraft, manned; Spectral classification of stars; Spectral lines; Spectroscope.
Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.
Aveni, Anthony F. Uncommon Sense: Understanding Nature’s Truths Across Time and Culture. Boulder, CO: University Press of Colorado, 2006.
Chaisson, Eric. Astronomy: A Beginner’s Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004. Hawking, Stephen. The Illustrated Brief History of Time.
Sagan, Carl. Cosmos. New York: Random House, 2002.
Home page of <http://www.KidsAstronomy.com> (accessed October 1, 2006).
Nemiroff, Robert, and Jerry Bonnell. “Astronomy Picture of the Day.” National Air and Space Administration and Michigan Technological University <http://antwrp.gsfc.nasa.gov/apod/astropix.html> (accessed October 1, 2006).
K. Lee Lerner
The Babylonian View of the Universe. There are limited and conflicting views of the universe in ancient Meso-potamian cosmology. One envisioned a sixlevel universe with three heavens and three different “earths”: the heaven of the stars, two additional heavens above the sky, the earth, the underground waters of Apsu, and, beneath it, the underworld of the dead. The most common perception of the universe, however, was three-fold. The heavens included everything above the ground. They were where the birds fly, the winds blow, the clouds float, as well as where the moon and the five visible planets drift among the fixed stars and—above them—the Upper Heavens where the gods reside. Below the heavens was the earth, where humankind live. Below the earth lay a body of fresh underground water and, below that, the underworld of the dead. Presumably the earth was considered to be flat, as the only surviving Mesopotamian world map (the so-called Babylonian Map of the World) shows a flat circular disk. It depicts the inhabited world within a large circle entirely surrounded by water, beyond which are triangular uncharted regions, the rest of the world.
The Beginning of Regular Recorded Astronomical Observations. The movements of the sun and of the celestial bodies in the night sky had been keenly observed since earliest times. It was not until the seventeenth century b.c.e. that regularly written astronomical records began to be kept in Mesopotamia. There are earlier references in Mesopotamian sources to astronomical phenomena, such as eclipses; and there are lists of stars and constellations, which were first named by the Sumerians in the third millennium b.c.e. Nevertheless, the observations of the planet Venus recorded at the time of king Ammi-saduqa of Babylon (circa 1646 - circa 1626 b.c.e.) are the earliest known written records of regular observations of the moon and the planets.
Astronomical Observation and Divination. These early observations of the rising and setting times of the planet Venus were cast into an omen framework and used to make predictions about possible future events on earth. This use of observational data to forecast future possibilities was entirely consistent with ancient Mesopotamian thinking. In their worldview, the gods used events in the natural world as divine signs to warn mankind of possible happenings in the future. Astral omens could signal some outcome for the state; that is, the king, the country, or the city. Other kinds of omens might predict imminent vicissitudes for ordinary individuals. The signs sent by the gods could be observed by anyone, and their ominous meanings could affect anybody, but only the trained specialist could interpret the effects of a circumstance revealed by means of a sign. Through divination the signs were interpreted, and the will of the gods was made known.
Record Keeping and Signs. In the second millennium B.C.E., there was little difference between purely descriptive nonmathematical “astronomy” and the “science” of celestial omens. Data were accumulated for divinatory purposes. Divination was a highly respected scholarly pursuit for the intellectual elite of Assyria and Babylonia.
Celestial Divination. Celestial divination provided the astronomical background for the later growth of mathematical astronomy in Mesopotamia. Celestial omens were based on a variety of observed astral or meteorological events. Some of these omens concerned weather-related events, such as thunder, lightning, rain, wind, hail, rainbows, or cloud formations. Others included such optical events as halos, flashes of light, or the color and brightness of constellations. Still others included specifically lunar and planetary phenomena, such as first and last visibilities of heavenly bodies, conjunctions of planets and stars (the time when two heavenly bodies are at the same celestial longitude), and eclipses. In a typical astral omen, the celestial event is described in the protasis (the subordinate “if” clause of a conditional sentence), while the apodosis (the main “then” clause of that sentence) predicts some corresponding earthly event for the king or state.
Textual Evidence. The first suggestion of the practice of celestial divination, such as the observation of an eclipse followed by the death of a king, goes back to the late third millennium b.c.e. Solid textual evidence for the first celestial omens comes later, in the Old Babylonian period (circa 1894 - circa 1595 b.c.e.). The bulk of celestial-omen material is even later, mostly from the Neo-Assyrian period (circa 934 - 610 B.C.E.), when celestial divination was the prime means of forecasting possible events for the king. Extispicy, the time-honored technique of predicting the future by examining the entrails of sacrificial animals, was not abandoned; sometimes it was used with celestial divination when there was uncertainty about the interpretation of an ominous astral event. Extispicy always had the advantage that it could be performed whenever it was needed, and it was considered well tested and proven. Still, celestial divination became the most prevalent form of divination toward the end of the Assyrian Empire.
Celestial Omen Series. Celestial omens began to be collected into series as early as the beginning of the second millennium b.c.e., and this practice continued into the first millennium B.C.E. The earliest known are the Old Babylonian lunar and solar omens, such as the eclipse omens of circa 1700 b.c.e. These astral omens were forerunners of those later organized into the major series of celestial-omen tablets, known today, as in antiquity, as Enuma Anu Enlih—“When (the gods) Anu (and) Enlil.” These series are the primary sources of Mesopotamian celestial omens.
Enuma Anu Enlil. The title of this series comes from the incipit, the opening words, of its mythological introduction, which credits the gods Anu, Enlil, and Ea with the creation of the order of heaven and earth in the universe.
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Enuma Anu Enlil was probably first compiled around 1000 B.C.E. The tablets continued to be copied, and new omens were added, even after the seventh century B.C.E. Most of the surviving tablets are copies made during the reign of Assyrian king Ashurbanipal (668 - circa 627 b.c.e.) in his library at Nineveh. The series comprises observations of the moon and sun, especially eclipses; planets and stars; and weather and earthquakes. These natural occurrences were linked to forecasts about the king, affairs of state, and the fate of the kingdom in general. Typically, these omens would be stated in the form “If x occurs in the sky, then y will occur on earth.” The series consisted of at least seventy tablets, originally containing some 6,500–7,000 omens. In modern times they were organized into four sections: lunar omens, solar omens, weather omens, and omens about stars and planets.
The Venus Tablet. Tablet 63 of Enuma Anu Enlil, the so-called Venus Tablet of Ammi-saduqa, with its fifty-nine omens, is the best-known tablet in the series. It contains an observational record of the appearances and disappearances of Venus during the reign of the Old Babylonian king Ammi-saduqa (circa 1646 - circa 1626 B.C.E.), linking these phenomena with possible consequences on earth:
(If) in month XI, 15th day, Venus disappeared in the west; it stayed away in the sky for three days, and in month XI, 18th day, Venus became visible in the east, (then) springs will open, Adad will bring his rain, Ea his floods, (and) a king will send messages of reconciliation to a(nother) king. (If) In month VIII, 11th day, Venus disappeared in the east; it stayed away in the sky two months, 7 days, and in month X, nineteenth day, Venus became visible in the west: (then) the harvest of the land will prosper. (Reiner and Pingree, 1975)
The sequence of observations in these tablets has tempted many a scholar to attempt a chronology for the Old Babylonian period. The obstacles are many, including the choice of data to use and the arbitrary intercalation, or insertion, of extra months into the calendar at this time. Conflicting interpretations of observations in Tablet 63 have resulted in different views of Old Babylonian dating. There are three common proposals, and others are possible. The Low Chronology puts the beginning of Ammi-saduqa’s reign at 1582 b.c.e., the Middle Chronology places the event at 1646 B.C.E., and the High (or Long) Chronology points to 1702 b.c.e., Although the Middle Chronology, which dates the end of Hammurabi’s reign at 1750 b.c.e., is the most commonly used as a matter of convention, including throughout this volume, scholars continue to disagree about which of these three chronologies, if any, to use.
Scholars’ Letters and Reports to Assyrian Kings. The omens of Enuma Anu Enlil were well known to, and regularly consulted by, the experts in celestial divination who served the Neo-Assyrian kings of the late eighth and seventh centuries b.c.e. Their correspondence—now called “letters and reports”—shows that these scholars, living in cities all over Assyria and Babylonia, made regular observations of astral phenomena, interpreted their meanings according to the omen compendia, and sent observations, predictions, and advice to the king at court. For example, the Babylonian scribe Munnabitu wrote to one of the later Neo-Assyrian kings during the first half of the seventh century b.c.e.:
If on the 16th day the moon and sun are seen together: one king will send messages of hostility to another; the king will be shut up in his palace for the length of a month; the step of the enemy will be set towards his land; the enemy will march around in his land victoriously. … The king must not be negligent about these observations of the moon; let the king perform either a namburbi or some (other) ritual which is pertinent to it. (Hunger)
The kings depended on such communications to undertake state and religious activities at propitious times, to avoid bad portents by the performance of the appropriate rituals, and to keep themselves, their families, and their kingdom safe and well.
MUL.APTN (The Plow Constellation). Another astral compendium connected with celestial omens is the two-tablet Assyrian series MUL.APlN, likely composed around the beginning of the first millennium b.c.e.MUL.APIN is more astronomical in character than Enuma Anu Enlil. It sets out rules for the intercalation of months in an ideal and schematic 12-month, 360-day calendar and provides a prediction scheme for the risings and settings of planets. Its six catalogues of stars provide a reference system for the identification of Mesopotamian constellations. One catalogue describes the division of the fixed stars and planets into three paths on the eastern horizon, over which they and the moon rise: the northernmost path of Enlil, “Lord Wind,” the divine ruler of Earth and its inhabitants; the central path of Anu, god of the sky; and the southernmost path of Ea, god of fresh underground waters. MUL.APIN also has shadow lists for equinox and solstice days, and for those days it lists weights of water to be used in a water clock to measure time intervals during the day and night.
Observational Astronomy in the First Millennium B.C.E.. After some seven to eight centuries of descriptive astronomy, scholars turned their attentions to what would today be classified as true astronomy. For the first time, there is evidence of regular and continuous record keeping of astronomical observations in chronological sequence. Clear proof of such record keeping is provided by the “astronomical diaries,” the earliest datable to 652 b.c.e. There are compelling arguments, however, that diaries were likely being written a full century earlier, in the reign of the Babylonian king Nabonassar, who ruled from 747 to 734 B.C.E. First, there are eclipse reports and tables of eclipses and eclipse possibilities from this time. Second, the astronomer Claudius Ptolemy, working in Alexandria circa 130–175 c.e., wrote that he had access to continuous astronomical records from the time of Nabonassar on, but nothing from before that time. In book 3 of his Almagest, Ptolemy wrote: “The beginning of the reign of Nabonassar is the era beginning from which the ancient observations are preserved down to our own time.” Ptolemy certainly used these Mesopotamian observations in his own work. The last datable astronomical diary comes from 61 b.c.e., demonstrating that Babylonian astronomers continued to compose these texts for at least six, and most likely seven, centuries. If diaries were still being written up to the time of the last datable cuneiform text in the mid first century c.e., the tradition of diary writing was maintained eight centuries, not only under native Mesopotamian rulers but also under the succeeding Achaemenid Persians, Hellenized Macedonians, Seleucids, and Arsacid Parthians.
Astronomical Diaries. “Astronomical diaries” is the modern name given to the collection of day-by-day accounts of celestial and meteorological phenomena once housed in a vast but still undiscovered astronomical archive in Babylon. The basic format of a diary was fixed. A half-year diary covering either the first or last half of a Babylonian year had six sections, seven in an intercalary year, each
“This text has been suppressed due to author restrictions”
spanning one lunar month. Each monthly unit was filled with almost daily observations, made almost exclusively in Babylon. The major emphasis was on the behavior of the moon, by far the swiftest of all the “planets,” and the lunar month was the basic unit of the Mesopotamian calendar and the construction of each diary. The diary began with what was considered to be the beginning of the new month, the first visibility of the new moon at sunset. This observation was followed by a statement about the length of the preceding month; that is, whether the first sighting of the new moon was seen on the thirtieth evening (meaning the previous month had twenty-nine days) or on the thirty-first (meaning the previous month had thirty days). Next the observers recorded the monthly progress of the moon among the stars and the planets, the first and last appearances of planets, stationary points, and oppositions; and time intervals of various phenomena, which helped the astronomers predict the date on which the next month would begin. (When viewed from the earth, the planets appear to speed up, slow down, and change direction of travel—a phenomenon called “retrograde motion”—as a result of the differences between the earth’s and the planets’ velocities around the sun. The point in a planet’s orbit when its apparent motion changes to retrograde motion is called the first stationary point, and the point where it resumes “forward” motion is termed its second stationary point. A planet is said to be in “opposition” when it is 180 degrees from the sun.) Observers also wrote down detailed accounts of local weather conditions by night and day, because these had an impact on visibility. Eclipses were dated and described; equinoxes and solstices were recorded; and dates of rising and settings of the bright star Sirius were noted. An observational passage for November 271 B.C.E. includes the following information:
Month VIII. … Night of the 1st, clouds crossed the sky. The 1st, in the afternoon, clouds crossed the sky. Night of the 2nd, the moon was 11/2 cubits behind Jupiter, the moon being one cubit high to the north. The 2nd, Merfcury’s first appearance in the east in Libra?…]. Night of the 3rd, very overcast, lightning, thunder, rain … gusty wind. The 3rd, clouds crossed the sky, it thundered, rain shower. Night of the 4th, overcast, rain, but the sandal was not removed. … (Sachs and Hunger)
(The reference to the sandal means that the rain was not heavy enough to cause sticky mud to remove one’s sandals.) Meteors and comets were mentioned, including what is now known to have been Halley’s comet in February 234 b.c.e., September/October 164 B.C.E., and August 87 b.c.e. At the end of the daily observations, there was a final statement about the last appearance of the moon, a measurement of the interval between moonrise and sunrise, and then a recapitulation of planetary positions at the end of the month. Then there was a list of the market values of the same six commodities (barley, dates, mustard, cress, sesame, and wool) and measurements of the changes in the water levels of the Euphrates. Finally, there might be some anecdotal historical information. The diaries occupy a unique position in the study of ancient history. In sheer bulk, continuity, detail, and kind of information, they are unmatched. Most importantly, because of the astronomical content of the diaries, particularly the continuous attention to the changing position of the moon, it is possible to date these texts confidently to the day, when they can be dated at all. Thus, any evidence extracted from them—whether astronomical, meteorological, economic, or historical—can be dated with certainty.
Goal-Year Texts and Almanacs. The diaries were used not only to preserve an astronomical record but in the last centuries of the first millennium B.C.E. (in the Seleucid and Parthian periods) to provide material for the construction of tables of eclipses and other chronologically arranged lunar and planetary phenomena. The diaries were also the source of two other kinds of astronomical texts, “goal-year texts” and “almanacs.” The goal-year texts, composed from at least 236 b.c.e. on, predicted the behavior of the moon and planets for some given year, the goal year. They presented data derived from the diaries that antedated the goal year by some astronomically significant period, such as a period of eight years for Venus to complete its travel from a given starting point on the ecliptic back again to that same point. (The ecliptic is the great circle that is the apparent orbit of the sun among the stars.) Scribes could pick an appropriate year in the past, find the diary data for that year, and list the phenomena expected to occur in the following goal year. These texts, being based on the periodic character of planetary and lunar behavior, show that period relations of recurring phenomena of the moon and five planets (Jupiter, Venus, Mercury, Saturn, and Mars) were well known to Babylonian astronomers of this time. Later, these became a fundamental notion in Babylonian theoretical astronomy. In fact, Otto Neugebauer has called them “the very backbone of Babylonian mathematical astronomy.” In mathematical terms, period relations state that s intervals or events of one kind equal t intervals or events of another. For example, 235 lunar months equal 19 solar years (the so-called Metonic Cycle). For a planet, the period relations state that x number of phenomena of one kind equal y number of revolutions around the ecliptic. For example, 391 like phenomena of Jupiter occur in 36 revolutions around the ecliptic. The sun travels one complete revolution of the 360 degrees of the ecliptic in a year (the definition of a year), and for each cycle by Jupiter from one appearance of a phenomenon to the next of the same kind, the sun travels once around the ecliptic plus the increase in longitude Jupiter makes for that cycle. This period then can be translated into years by stating that 391 such phenomena take 391 times around the ecliptic plus 36 more revolutions. The “period relations of Jupiter” would then be 391 + 36 = 421 years. Derived from goal-year texts, almanacs predicted month-by-month lunar and planetary phenomena: beginning of the month planetary positions; dates of entry into a zodiacal sign; dates of solstices and equinoxes; dates of Sirius risings and settings; and eclipses. Normal Star almanacs gave dates when planets moved into certain positions among the stars near the ecliptic that were used as reference points to give the position of the moon and planets.
Mathematical Astronomy. Around the mid-first millennium B.C.E., after more than a millennium and a half of systematic observation of celestial phenomena and the collection of celestial omens, a new kind of astronomy developed in Babylonia. Scholars began to create original methods of calculation to find solutions for what had become the main focus of scholarly endeavor, the prediction of astronomical phenomena. For the moon, the Babylonians were interested in the dates of the new and full moon, the lunar visibilities near these phases, times and magnitudes for eclipses, and the lengths of the months. For the planets, they wished to predict appearances and disappearances, stations and oppositions. The new strategies were mathematical and computational. The basic elements of the new methodology were the sexagesimal (base 60) number system and place-value notation; period relations; the 19-year luni-solar cycle (19 solar years = 235 lunar, or synodic, months); the “Saros cycle” of eclipse possibilities (in 223 synodic months, approximately 18+ years, there are 38 eclipse possibilities with the average interval of 5+ months between eclipses); the planetary periods, the number of years it took a planet to complete a cycle from one appearance of a particular phenomenon to the next of the same kind; and a zodiacal reference system.
The Zodiacal Reference System. The zodiac is defined as an imaginary belt of the celestial sphere extending about eight or nine degrees on either side of the ecliptic (the apparent orbit of the sun among the stars); within the zodiacal belt occur the apparent paths of the moon and planets. It is likely that sometime in the mid fifth century b.c.e. Babylonian astronomers invented a reference system of zodiacal signs. The astronomers divided the ecliptic into twelve equal thirty-degree segments, or “signs” (each named for a different ecliptical constellation), through which the moon and planets travel. Each sign could be further subdivided into twelve microzodiacal signs to enable observers to further refine a planet’s position. This zodiacal system became the basic reference for Mesopotamian astronomers and, later, for all Babylonian mathematical astronomy. Before this time, the so-called Normal Stars, a group of more than thirty-one stars near the ecliptic and on the zodiacal belt, served as reference points for the movement of the moon and the planets. During the Seleucid period (311–129 b.c.e.), the Babylonian signs of the zodiac, both individually and in astrologically significant combinations, became popular as designs for personal finger-ring intaglios.
Ephemerides. Beginning in the fourth century b.c.e., theoretical astronomical texts began to be composed in scholarly centers at Babylon and at Uruk. There are some four hundred to five hundred of these tablets, many surviving only in small fragments. These texts fall into two categories. Some present dates and a particular function necessary to compute ephemerides, successive phenomena of the moon, or the planets, which, for planets, might include the dates and times of first and last visibility, opposition, and stationary points. The rest are lunar or planetary procedure texts, which give directions for calculating the ephemerides. Nowhere are the theories that underlie the instructions stated. They have been reconstructed by modern scholars using the ephemerides and procedure texts. Two broad categories of Babylonian astronomy emerged: lunar and planetary theory. Intrinsic in this work are “System A” and “System B.” These theoretical texts underscore the valuable contribution of the Babylonians to the science of astronomy.
System A and System B. Two kinds of mathematical models were used to describe and predict the periodic movements of the moon, the sun, and the planets, none of which travels at a constant velocity. These arithmetical models are distinguished by modern scholars as “System A” and “System B.” They differ in the way in which they treat the velocity of celestial bodies as they travel around the ecliptic. In System A, the ecliptic is divided into zones inside which phenomena, or events, of celestial bodies progress in steps of different velocities. In System A, for example, the sun is assumed to travel through the ecliptic with two distinct velocities throughout the year, a constant value of 30 degrees per month in its travel from Virgo 13 to Pisces 27 and a constant value of about 28 degrees per month (28;7,30 in sexagesimal numbers) from Pisces 27 back to Virgo 13. The ecliptic is thus divided into two parts, a fast arc and a slow arc, or two steps. System A, therefore, can be described as a step-function scheme. In contrast, System B assumes that velocity can be characterized as a linear zigzag function in which a variable is reduced to an arithmetic progression alternately increasing and decreasing by a constant amount in successive intervals of time between some fixed maximum and minimum values. System A was designed to calculate variable quantities from one initial value, such as a sequence of longitudes for a corresponding sequence of dates from one starting point. System A was useful, for example, for finding the changing positions in longitude as a celestial body progressed from some particular phase to the next phase of the same kind. In the case of a planet, the model could be used to compute the longitudes as the planet traveled from one synodic phenomenon to the next same phenomenon, for example, from one first stationary point to another. System B was useful for describing deviations from mean values of solar, lunar, and planetary movements, such as the length of the month. System A, with its use of the step function, is the earlier of the two models and a fresh innovation designed to fit the requirements of the new computational astronomy. System B was constructed soon after, relying on the linear zigzag function, which had already been devised as early as the Old Babylonian period to describe variations of daylight; later it was used in Enuma Anu Enlil for the prediction of two periodic functions: the time within a month that the moon was visible at night and the variation of the increment of this time over the course of the year. Recycled into System B, the linear zigzag function served in an innovative way to describe periodic phenomena. Both models are ingenious in the way they are able to separate and describe, one by one, any single component of more complex astronomical phenomena and then combine them again to make predictions. The creation of such mathematical models for numerical predictions of astronomical phenomena is one of the finest Babylonian accomplishments.
Transmission of Babylonian Mathematical Astronomy. The surviving astronomical tablets come almost exclusively from three sites: Nineveh, Babylon, and Uruk. The library of Ashurbanipal at Nineveh was destroyed in 612 b.c.e. Texts from Uruk cease about 150 b.c.e., about
the time southernmost Babylonia was seized by the Parthians from Iran. However, the astronomers at the astronomical center of Babylon, which eventually fell to the Parthians as well, continued to write tablets into the first century c.e. By this time, Babylonian astronomical knowledge had spread and was used throughout the Hellenistic world. Certainly, much of Babylonian astronomy underlies the work of the astronomer Ptolemy in the Almagest, written circa 150 c.e. Ptolemy used sexagesimal fractions, the degree as the basic unit of angular measure, and the zodiacal reference system—all inventions of Babylonian astronomers. He also used Babylonian observations of celestial phenomena, including eclipses that went back to those recorded in the reign of Nabonassar; Babylonian parameters, such as the value for the mean synodic month, the length of the year, and period relations for the moon and planets; and Mesopotamian constellation names. His knowledge of Babylonian astronomy likely came from the works of the astronomer Hipparchus, who worked in Rhodes, circa 150–125 b.c.e., and who not only used Babylonian observations and parameters but also had access to their mathematical techniques for prediction. The mathematical astronomy of the Babylonians has recently been found on papyrus fragments from Roman Egypt. The astronomy of the Greek and Roman world may well have been inspired by the Babylonian conception that it was possible to build astronomical models for prediction purposes.
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ASTRONOMY. Colonial Americans lacked instruments and libraries. They had difficulty communicating with each other and often relied on English correspondents for news of other colonialists. During the seventeenth century, European astronomy was focused on extending Isaac Newton's mathematical description of the solar system, and a few Americans contributed their observations to the Royal Society of London. American observations served European theories.
When Venus passed in front of the sun in 1761 and 1769, transits revealed the distance of the earth from the sun. John Winthrop, professor of mathematics and natural philosophy at Harvard, organized an expedition to Newfoundland to observe the first transit. The Massachusetts Assembly assigned a ship to transport Winthrop's group and Harvard permitted him to take college instruments, provided they were insured against loss or damage. The observations were sent to Europe for analysis.
Winthrop lectured his students that determination of the distance of the earth from the sun would result in a deeper insight into God's wonderful works. Enlightenment faith in the discernable regularity of the universe also encouraged the study of astronomy in early American colleges. In its appeal to the Pennsylvania Assembly for funds to observe the 1769 transit, the American Philosophical Society, founded in Philadelphia in 1743, cited a more utilitarian goal, "the Promotion of Astronomy and Navigation, and consequently of Trade and Commerce." In a period of increasing cultural nationalism, the society also wanted to win recognition for American achievements.
Many of America's astronomers were surveyors. The self-taught American astronomer David Rittenhouse made his living as a clockmaker, but he was also a surveyor. In 1767, he constructed in Philadelphia his famous orrery, or mechanical planetarium, which represented with great precision the motions of the planets around the sun. The Pennsylvania assembly paid for it. The onset of the Revolutionary War suspended hope to build an observatory.
With political independence came a desire for cultural independence. However, little public patronage was forthcoming for astronomy in the early national period. In 1825, President John Quincy Adams pointed out that Europe had 130 "lighthouses of the skies" but the United States none. Yet his request for funds for a national observatory was denied.
The American Academy of Arts and Sciences, founded in Boston in 1780 by John Adams, published astronomical observations by Nathaniel Bowditch, a self-educated Salem seaman. His The New American Practical Navigator (1800) became the most widely used nautical guide, and in 1811, he observed a solar eclipse to improve the determination of the longitude of Cambridge. The European scientific community applauded Bowditch's translation and commentary on Pierre Laplace's Mécanique céleste. The American Academy offered to pay for publication, but Bowditch waited until he could afford to publish it himself.
Through much of the nineteenth century, the United States was a nation in development. While some of their European brethren made observations and contributed to the advance of knowledge, American astronomers often struggled with more mundane problems, including writing textbooks, acquiring books and journals for libraries, and building, equipping, and financing observatories. Elias Loomis, a professor at Western Reserve College in Ohio, at the University of the City of New York, and at Yale University, published An Introduction to Practical Astronomy in 1855 and a A Treatise on Astronomy in 1876, both of which went through numerous editions. At both New York and Yale, Loomis arranged to receive publications from European observatories on an exchange basis. His will left funds to pay observers and publish their results.
College observatories consisted of a small building and telescope intended for the education of undergraduate students, but they could not pay researchers or provide funds for publication. The University of North Carolina built an observatory in 1831, which lasted several years. Observatories were constructed at Yale (1830s), at Williams College (1838), at Western Reserve College (1838), at the Philadelphia High School (1838), at West Point (1839), and at Georgetown (1843). In 1839, Harvard lured William Cranch Bond from his private observatory to supply his own instruments and work for no salary. The great comet of 1843 aroused public interest, which manifested itself in public support to construct and endow the Harvard College Observatory. Harvard ordered from the German firm Merz and Mahler a twin to the Russian Pulkova Observatory's fifteen-inch (lens diameter) telescope, then the largest in the world.
There were also public observatories: the Cincinnati Observatory, whose cornerstone John Quincy Adams laid in 1843, and the Dudley Observatory in Albany, built between 1852–1856. The tribulations of the Cincinnati Observatory illustrate the obstacles to practicing astronomy in mid-nineteenth-century America. Ormsby MacKnight Mitchel, a West Point graduate, moved to Cincinnati and became professor of mathematics, civil engineering, mechanics, and machinery at Cincinnati College. His public lectures on astronomy led to the founding of the Cincinnati Astronomical Society and the municipal Cincinnati Observatory, funded by public subscription. After five hours of teaching, Mitchel would supervise construction of the observatory in the afternoons. Cincinnati purchased an 11.25-inch telescope from Merz and Mahler, but Mitchel spent much of his time displaying the heavens to subscribers, from 4:00 to 10:00 p.m. daily. He tried publishing a journal to raise money for auxiliary instruments and for his salary, the observatory having no endowment for operating expenses, and did make money from a book on popular astronomy and from surveying a railroad route. His observations of singular phenomena, a kind of natural history of the heavens, fell short of a new professional emphasis on measurement and theory, requiring considerable mathematical competence. Economic forces discouraged sustained, structured research.
A few would-be professional astronomers received training and employment with the Coast Survey, established in 1807 in response to commercial interests of seaboard states. In legislation for the Coast Survey in 1832, Congress explicitly declared that it did not authorize construction or maintenance of a permanent astronomical observatory. A decade later, the Naval Observatory was created surreptitiously, as part of the Depot of Charts and Instruments. Not until 1866, however, would the observatory begin a program of fundamental research in astronomy. Meanwhile, the Nautical Almanac, located in Cambridge, Massachusetts, was established under the Naval Observatory budget in 1849. It reported directly to the secretary of the navy, provided training and employment for a few astronomers, and improved navigation and raised America's scientific standing with an annual astronomical almanac more accurate and theoretically advanced than the British Nautical Almanac. Simon New-comb, one of America's best-known scientists at the end of the century, got his start at the Nautical Almanac, and also worked at the Naval Observatory. He analyzed the motions of the moon and planets.
There were also a few private observatories in America. Lewis Rutherfurd, a wealthy New Yorker and trustee of Columbia College, had a nine-inch diameter telescope, and also a small transit instrument belonging to Columbia College at his observatory at Second Avenue and Eleventh Street. The Coast Survey used this observatory in 1848 to determine the longitude of New York. Rutherfurd was a pioneer in astronomical photography. Not until late in the century, though, would individual American fortunes fund the establishment and sustenance of large observatories with systematic programs of scientific investigation carried on by full-time, paid employees.
The second half of the nineteenth century saw advances in telescope production, especially by the Boston firm of Alvan Clark & Sons. Their metal tubes were stiffer yet lighter than wooden telescopes. Larger pieces of optical glass were now available, and the Clarks figured the lens for the world's largest refracting telescope on five occasions: an 18.5-inch lens for the University of Mississippi in 1860, a 26-inch lens for the Naval Observatory in 1873 (with which Asaph Hall discovered Mars's moons in 1877), a 30-inch lens for the Pulkova Observatory in 1883, a 36-inch lens for the Lick Observatory of the University of California in 1887, and a 40-inch lens for the Yerkes Observatory of the University of Chicago in 1897. James Lick, a California land speculator during the gold rush, and Charles Yerkes, a Chicago street car magnate, put up the funds for their eponymous observatories, under university auspices, and Boston investor Percival Lowell directed his own observatory. All three observatories were far removed from cities, and Lick's and Lowell's were on mountain peaks. With the largest telescopes in the best locations, American observatories now surpassed all others.
Growing interest in astrophysics and in distant stars and nebulae encouraged the development of new observatories with large steerable reflecting (light focused by a curved mirror) telescopes suitable for photography and auxiliary instruments for the analysis of starlight. George Ellery Hale founded the Astrophysical Journal in 1895, the American Astronomical and Astrophysical Society in 1899, the Mount Wilson Observatory in 1904, and the International Astronomical Union in 1918. Hale was an early prototype of the high-pressure, heavy-hardware, big-spending, team-organized scientific entrepreneur. In 1902, Andrew Carnegie, rich from innovations in the American steel industry, created the Carnegie Institution of Washington to encourage investigation, research, and discovery in biology, astronomy, and the earth sciences. Its ten million dollars were more than the total of endowed funds for research in all American universities combined. Hale left the Yerkes Observatory to build, with Carnegie money, the Mount Wilson Observatory on a mountain above Los Angeles. There George Willis Ritchey, who accompanied Hale from Yerkes, made the photographic reflecting telescope the basic instrument of astronomical research, constructing a 60-inch telescope in 1908 and a 100-inch telescope in 1919. They were the largest telescopes in the world and revolutionized the study of astronomy. Harlow Shapley found that the system of stars is a hundred times larger than previous estimates and that the sun is far from the center. Edwin Hubble showed that spiral nebulae are independent island universes beyond our galaxy and that the universe is expanding. Cosmology, previously limited to philosophical speculations, joined mainstream astronomy.
The Mount Wilson Observatory depended on its relationship with physicists at the nearby California Institute of Technology for its dominance of astrophysics during the first half of the twentieth century. A scientific education was fast becoming necessary for professional astronomers, as astrophysics came to predominate, and the concerns of professionals and amateurs diverged. As late as the 1870s and 1880s, the self-educated American astronomer Edward Emerson Barnard, an observaholic with indefatigable energy and ocular acuteness, could earn positions at the Lick and Yerkes observatories with visual observations of planetary details and discoveries of comets and moons. Already, however, he was an exception and an anachronism. Soon an advanced academic degree and considerable theoretical understanding were required of professional astronomers in America.
Supposedly, only men could withstand the rigors of observing the heavens all night in unheated telescope domes. Women were first employed to examine photo-graphs of stellar spectra and to catalog the spectra. Edward Pickering, director of the Harvard College Observatory in 1881 and an advocate of advanced study for women, was so exasperated with his male assistant's inefficiency that he declared even his cook could do a better job of copying and computing. Pickering hired her and she did do a better job, as did some twenty more females over the next several decades, recruited for their steadiness, adaptability, acuteness of vision, and willingness to work for low wages. In 1925, Cecilia Payne, a graduate student, determined the relative abundances of eighteen chemical elements found in stellar atmospheres. Her Ph.D. thesis has been lauded as the most brilliant written in astronomy. Her degree, however, was from Radcliffe College, before Harvard granted degrees to women, and in subsequent employment at Harvard she was initially budgeted as "equipment."
Radio astronomy began in America in 1933. Karl Jansky, a radio engineer with the Bell Telephone Company, detected electrical emissions from the center of our galaxy while studying sources of radio noise. Optical astronomers were not interested, nor were Jansky's practical-minded supervisors. Grote Reber, an ardent radio amateur obsessed with distance communication, was interested, and built for a few thousand dollars a 31.4-foot-diameter pointable radio antenna in his backyard in Wheaton, Illinois. In 1940, he reported the intensity of radio sources at different positions in the sky. Fundamental knowledge underlying radio astronomy techniques increased during World War II, especially with research on radar.
Advances in nuclear physics during the war made possible quantitative calculations of the formation of elements in a supposed primeval fireball. The Russian-American physicist George Gamow sought to explain the cosmic abundance of elements as the result of thermo-nuclear reactions in an early hot phase of an expanding universe, consisting of high-energy radiation. In 1963, unaware of Gamow's work, Arno Penzias and Robert Wilson at the Bell Telephone Laboratories detected radiation of cosmic origin. Meanwhile, Robert Dicke at Princeton University had independently thought of the cosmic background radiation and set a colleague to work calculating its strength. When Dicke learned in 1965 of Penzias and Wilson's measurement, he correctly interpreted it as Gamow's predicted radiation. A Nobel Prize went to Penzias and Wilson. Their discovery won general acceptance of the big bang theory and refuted the rival steady state theory.
World War II changed the relationship between science and the state. Radar, missiles, and the atomic bomb established state-sponsored and state-directed research and development. Furthermore, groups of scientists brought together in wartime proved effective. After the war, engineers and physicists with their instruments, techniques, training, and ways of operating moved into astronomy. Then came Sputnik in 1957, the world's first satellite. This Soviet triumph challenged American supremacy in military might and world opinion.
After Sputnik, the National Science Foundation supplied many millions of dollars for construction of the Kitt Peak National Observatory on a mountain near Tucson, Arizona. It is the largest collection of big telescopes in the Northern Hemisphere. Seventeen universities came together in AURA, the Association of Universities for Re-search in Astronomy, to manage the observatory.
Another response to Sputnik was the creation of the National Aeronautics and Space Administration (NASA). Among its accomplishments are automated observatories launched into space, including the Hubble Space Telescope. Its primary mirror is eight feet in diameter. Including recording instruments and guidance system, the telescope weighs twelve tons. It has been called the eighth wonder of the world, and critics say it should be, given its cost of 1.5 billion dollars! The telescope is as much a political and managerial achievement as a technological one. Approval for a large space telescope was won in a political struggle lasting from 1974 to 1977, but not until 1990 were a plethora of problems finally overcome and the telescope launched into space, only to discover that an error had occurred in the shaping of the primary mirror. One newspaper reported "Pix Nixed as Hubble Sees Double." The addition of a corrective mirror solved the problem.
NASA also funds X-ray astronomy. Captured German rockets provided the first proof of X-rays from the sun. Astronomers did not expect to find X-ray sources and were skeptical that brief and expensive rocket-borne experiments were worthwhile. NASA, however, had more money than there were imaginative scientists to spend it, and the military, even more. One imaginative and eager scientist was the Italian-born Riccardo Giacconi, who in 1960, funded by the Air Force Cambridge Research Laboratories, discovered a cosmic X-ray source, and in 1963, detected a second. NASA adjudicates questions of scientific priority and supplies money for space observatories; industry helps build them; universities or consortiums of universities design and operate them and analyze the data. NASA then funded a rocket survey program and a small satellite for X-ray astronomy and in 1978 the Einstein X-ray telescope. Unlike the relatively quiescent universe seen by earth-bound astronomers, the universe revealed to engineers and physicists observing from satellites is violently energetic.
Major changes have occurred in both the size and scope of American astronomy over the centuries, but never more rapidly nor more dramatically than at the beginning of the Space Age. There were some five hundred American astronomers in 1962 and three times that many a decade later. Only four worked on X-rays in 1962 compared to over forty times that many in 1972. Over eighty percent of them were migrants from experimental physics, with expertise in designing and building instruments to detect high-energy particles.
Astronomers now realize that important cosmological features can be explained as consequences of new theories of particle physics, and particle physics increasingly drives cosmology. Conversely, particle physicists, having exhausted the limits of particle accelerators and public funding for yet larger instruments, turn to cosmology for information regarding the behavior of matter under extreme conditions, such as those prevailing in the early universe.
The spectacular rise of American astronomy roughly parallels the remarkable evolution of the nation, itself, from British colonies to world super power. Once limited to visual observations and determining positions, astronomy now includes cosmology, the study of the structure and evolution of the universe, and analysis of the physical and chemical composition of the universe and its components. Once peripheral, now American astronomers, men and women, formally educated in a variety of fields, working in large teams, on systematic long-term projects, and enjoying government patronage, lead world advances in instrumentation, observation, and theory.
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Astronomy, from the Greek astron, star, plus nomos, law—thus the laws or regular patterns of the stars—is now defined as the science of objects beyond the Earth's atmosphere, including their physical and chemical properties. This science of what is beyond the Earth paradoxically served as the model for the early modern effort to create a science of terrestrial phenomena. Because of their apparently more simple and necessary order, astral phenomena were the first to be subject to explanations in the form of "laws," the methods of which were then extended in modern physics to explain the dynamics of falling bodies at or near the Earth. Yet just as modern physics emerged to give human beings greater powers over material affairs than ever before, and thus pose a challenge to ethics, so subsequent developments in astronomy deprived humans of an order that could be perceived as a transcendent and normative guide for human conduct. Immanuel Kant (1724–1804) could still wonder at the correspondence between the "starry heavens above and the moral law within" (Critique of Practical Reason, p. 288), but the achievements of modern astronomy have left the moral law within to fend for itself.
Astronomy has been called the world's second oldest profession. Notations found on artifacts scattered over Africa, Asia, and Europe dating from 30,000 b.c.e. appear to be rudimentary calendars based on the phases of the moon (Hartmann and Impey 1994). The transition from hunter-gatherers to life in stable villages, occurring around 10,000 b.c.e. with the rise of agriculture, required a refined estimation of the timing of seasonal changes. The sky, although no doubt deeply mysterious to these ancient cultures, was also reassuringly deterministic. By 4000 b.c.e., for instance, Egyptian astronomers knew that the first appearance of the brightest star in the dawn sky, Sirius, marked the beginning of the Nile's annual flooding. Many, probably most, cultures timed their agricultural activities based on similar annual celestial events.
The stars of course were also used for navigation. The Minoans of the island of Crete employed the stars to navigate the Mediterranean and to forge relationships with the Greeks as long ago as 2600 b.c.e. In developing this technology, they grouped the stars into pictures that gave rise to some of the constellations that we still know today (Hartmann and Impey 1994). The navigational prowess of the Polynesians is legend. The courage and faith these seafarers had in the heavens' ability to guide their way is astonishing. Crossing vast expanses of the Pacific, Polynesians discovered that if they sailed north until the Southern Cross dropped to a hand's length above the horizon, they would be at the latitude of Hawaii. To return, they would point their outriggers south until two stars, Sirius and Pollux, set together.
The megalithic monument Stonehenge on the Salisbury Plain in Great Britain had a utilitarian as well as spiritual design. On the longest day of summer, at solstice, the sun rose over a huge, notched boulder, the "Heel Stone," as seen from the center of concentric rings of massive boulders. Some weighed thirty to fifty tons (Hawkins and White 1965). The accompanying midsummer ritual 4000 years ago would have been an annual part of the cultural weaving of astronomy, beliefs, and values for the participants. Enormously demanding achievements such as the construction of Stonehenge and of the Egyptian pyramids are testament to the power the heavens exerted on the societies that built them.
Possibly the most extraordinary early example of institutional astronomy was that of the Mayans. The priest-astronomers that observed the heavens and performed the calculations to produce their calendars were publicly supported for at least 200 years around 400 c.e. The Mayan calendar did not only chart the seasons for agriculture. It also predicted eclipses, experienced by the Mayans as traumatic and darkly mysterious. Mayan astronomers computed the complex motions of Venus, believing it to be one god in the evening, and another when it reappeared in the morning. Venus's quasi-periodic disappearance and reemergence on the other side of the world was seen as a journey and transformation in the underworld (Aveni and Hotaling 1994). It appears that in all early cultures, astronomy and religion were deeply interconnected. Astronomy, by giving an accurate description of the motions of heavenly bodies, was at the same time a very powerful tool for sustaining civilization and exploring the world.
However it goes about it, religion seeks to provide guidance for living in harmony with the Earth, with other people, and with the universe. But peace, it could be argued, is only possible for human beings if they have in some way accepted what their lives mean. Religion addressed the human question of meaning, by defining our relationship with the cosmos. So astronomical questions, such as what brought forth the universe, how old it is, and what our place in it is, were religious questions. It has been suggested that the starkly hierarchical medieval (Aristotelian) cosmology, with the universe consisting of ten concentric spheres around the Earth (the outermost being heaven), was reflected in the rigidly hierarchical society that oppressed the vast majority of people (Abrams and Primack 2001).
The astronomical observations of Galileo Galilei (1564–1642), using the new technology of the telescope, began the fracture of science and religion that is today a deep chasm. As is well known, Galileo kept his head because he recanted his conclusions that the sun was at the center of the solar system and that the celestial bodies were not flawless. With improving technologies and the bold modern project begun by René Descartes (1596–1650), Francis Bacon (1561–1626), and John Locke (1632–1704), however, science and religion diverged under the auspices of an uneasy truce. As the quest for truth in the universe became a scientific endeavor, it was no longer part of the institution that spoke directly to meaning in human lives, to guidance for living in harmony, and for rules that guide human behavior.
Modern Astronomy and the Rise of Scientific Cosmology
Modern astronomy can be described in terms of its institutional structures, its intellectual debates, and its scientific discoveries.
NATIONAL, PRIVATE, AND UNIVERSITY OBSERVATORIES. Astronomy may have grown from a fundamental desire to understand the universe, but the use of heavenly motions as a powerful technology for navigation grew with it. Systematic observations of the heavens for centuries allowed us to chart the limits of our world, and to navigate confidently within it.
By the end of the nineteenth century, large national observatories existed in the United Kingdom, France, the United States, and Russia. Although originally designed to survey the heavens for applications in geodetics and navigation, these institutions also began to branch out and address more fundamental questions (Struve and Zebergs 1962). Especially as instrumentation improved, astronomers were increasingly making observations in attempts to understand the structure, history, and origin of the universe. Larger and larger telescopes would enable astronomers to see further into the universe and with ever greater sharpness. The excitement of this quest was felt keenly by a number of American philanthropists, and the late nineteenth century saw the rise of large, privately funded observatories such as the Lick (1888), the Lowell (1894), and the Yerkes (1897). Following these, construction of the last of the giant, privately funded observatories was completed with the McDonald Observatory in 1939 and the Palomar Observatory in 1947. The flagship of Palomar is the 200"-diameter Hale telescope, which reigned supreme as the largest and most capable telescope in the world until the launch of the Hubble Space Telescope into Earth orbit in 1990.
Hubble was born of the dreams of astronomer Lyman Spitzer (1914–1997), who, in the heady days of the postwar technology boom, first advocated a telescope in space to explore the universe with unprecedented clarity. Above the veil of obscuring atmosphere and luminous clamor of the Earth, a moderate telescope in space would see the universe 100 times clearer than the behemoths on Earth. This meant that it could see 100 times further away and 100 times further back in time. This it has done, and the images of the universe that it has returned have astonished us and enriched our lives.
Light is the only form of electromagnetic energy that is directly perceived by human beings. Electromagnetic waves are produced by a vast array of physical phenomena in the universe, including stars, planets, galaxies, supernovae remnants, black holes, and almost everything in between. Many of these emissions have wavelengths that are much longer than those of light; these are radio waves. Because they are absorbed by dust and gas less readily than is light, radio waves traveling through space allow a glimpse of parts of the Milky Way that cannot be seen by optical telescopes. In addition, radio waves are produced by different processes than those that create light, giving scientists insights into the physical processes and compositions of many objects in space.
Primitive radio receivers were first pointed at the sky in the early 1930s. It became clear soon thereafter that radio waves can be detected from all parts of the sky, but most especially from the center of the Milky Way. The rapid advances in electronics due to the technological efforts in waging World War II paved the way for vast improvements in radio telescope sensitivity. Serious construction of large astronomical radio telescopes began in 1947. Some are steerable, such as the 250-foot wire-mesh dish at Jodrell Bank in Great Britain. The largest is Arecibo, the immovable 1,000-foot dish carved into a limestone sinkhole in Puerto Rico. Today, enormous arrays of radio dishes are icons of modern astronomy, probing the universe's mysteries and listening for signs of alien minds.
THE ISLAND UNIVERSE DEBATE. On a clear night away from city lights, a ghostly swath cuts through the sky. It is thickest in the constellations of Sagittarius and Scorpio, and thins as its path is traced northeast through Cassiopeia or southwest through the summer constellations of Cygnus and Aquila. One of the great conceptual leaps of humanity was the realization that this apparition was our view of a great island universe, a galaxy, from the inside. The peculiar smudgy swirls seen in early telescopes, such as Galileo's, were vast communities of stars, comparable to ours but unimaginably far away. The close ones, such as Andromeda, can be seen to be in the shape of a pinwheel with a bright central bulge. As we look to Sagittarius, we look into the core of our galaxy from the inside of the disk. On the other side of the sky where the Milky Way is more diffuse, we can see dark lanes of dust obscuring stars, and the outline of spiral arms. Our sun is one dot in the multitudes that blend together with such promiscuity that they form the milk of the Milky Way.
By the end of the nineteenth century, astronomers knew that the Milky Way was a vast field of stars in which the sun and solar system were embedded. Systematic star counts led to estimates of the size and shape of our galaxy, but also to the erroneous conclusion that the sun was at the center of it. In spite of the Copernican revolution, subtle assumptions on the centrality and primacy of humans in the universe remained, skewing scientific interpretations of the observational data.
Our view of the Milky Way galaxy from within was sharpened considerably by the observations of Harlow Shapley (1885–1972). Shapley noticed that globular clusters—beautiful, tightly packed spherical aggregates of stars—tended to form a vast spherical halo around the nucleus of the Milky Way. His observations successfully set the stage for the twentieth-century view: that the sun exists in an enormous, flattened disk of stars, about two-thirds of the way from the center to edge. This final dethroning of the role of humans in the cosmos played out during the 1910s and 1920s and was one of the great classic scientific debates of the century. The new picture did little at first to illuminate what the universe was, or its extent. Was our disk, 100,000 light years wide and 10,000 light years thick, with a central bulge and 100 billion stars, the universe? What was outside of it, and how did it come to be? These questions could only be answered with improvements in telescope and photographic technology, which followed rapidly.
Kant proposed, in the eighteenth century, that the Milky Way we are inside of was a disk-shaped spiral, similar to the far-away spiral nebulae seen in telescopes at the time. He called these spirals "island universes." Kant's famous intuition turned out to be largely correct, although the scientific path to this conclusion did not end until the middle 1920s. During that decade, the shape of our galaxy's spiral arms came into focus, and the correspondence to the shapes of the far-off spiral nebulae became scientifically accepted. Until then, it was generally thought that the Milky Way was all that there was, and the large variety of spiral nebulae were smaller aggregates of stars within or just outside of it. As telescopic and photographic technology progressed in the twentieth century, and ever more detailed images of the deep heavens were acquired, this view began to change.
It was Edwin Powell Hubble (1889–1953) who eventually solved the mystery of the celestial spirals. It had long been known that a special class of variable stars, known as Cepheid variables, exhibited a well-determined relationship between periodicity and intrinsic brightness. Distance determinations to celestial objects were bootstrapped to ever more distant objects by noting the parallax shifts of nearby stars (including Cepheids) due to the earth's orbit around the sun. This technique was used to calibrate Cepheid variables at far more distant locales. Using the 100" telescope at Mt. Wilson observatory above Pasadena, then the largest instrument in existence, Hubble was able to resolve individual Cepheid variables in the Andromeda galaxy. Extrapolating from the period-luminosity relation for these variables in our own galaxy, in 1923 Hubble conclusively showed that the Andromeda galaxy was far, far away, about ten times further than the diameter of our own galaxy. So spiral galaxies are indeed island universes, vast collections of stars very much like our Milky Way, many with 100 billion stars or more. The press for larger, more powerful instruments in the early part of the twentieth century was on, driven almost entirely by a thirst for understanding the depth and breadth of all existence. This thirst was very much felt by society in general, and was part of the great scientific excitement of the time, which included the development of quantum mechanics and the deeper understanding of space and time worked out by Albert Einstein (1879–1955).
We now know that the Andromeda galaxy is only one of more than 100 billion such whirlpools of stars, making the observable universe an inconceivably large place, containing 100 billion times 100 billion stars, and perhaps almost as many solar systems. On a cloudless night in autumn, the Andromeda galaxy is clearly visible to the unaided eye. It is the farthest thing we humans can perceive directly. Light reaching us today left the galaxy 2.2 million years ago, traveling 10,000,000,000,000,000,000 miles before leaving its impression on our retinas and minds.
In his famous book The Realm of the Nebula, Hubble classified the vast diversity of extragalactic forms into a more-or-less coherent taxonomy (1926). The realization that spiral nebulae and their brethren, giant elliptical galaxies, were island universes, coequal with our own vast Milky Way, paved the way for one of the most extraordinary scientific discoveries of all time and gave birth to modern cosmology. In 1929, Hubble announced his discovery that the recessional velocities of galaxies were proportional to how far away they were. The furthest galaxies were receding the fastest, as measured by the Doppler shifts of their emitted light. The constant of proportionality became known as the Hubble constant. The implications of this relationship are profound. The simplest way to explain it is that at some time in the very distant past, all the galaxies were packed together. If we reverse the movie of the universe, all the galaxies speed in toward each other until—what? Georges-Henri Lemaitre (1894–1966) hypothesized that the movie takes us back to the primeval egg, a cosmology that poetically phrased the juxtaposition of myth and science. But how far one can extend the movie and continue to rely on the laws of physics as we know them is at the heart of modern cosmology. At the beginning of time and space, the galaxies or their precursors were propelled somehow from the egg. In this picture, the reciprocal of the Hubble constant is the age of the universe, and its extent is approximately the distance that light travels in this time. This theory became known as the Big Bang. Science has thus looked directly at the question: What is the origin of everything? We cannot go back: The countless and varied myths, societies' identification with the infinite, have been supplanted by the power of scientific truth.
THE MORALITY OF SUPERNOVAE. One of the great natural wonders of the universe is the supernova. In schoolchildren, descriptions of the great power of these exploding stars excite a keen intellectual wonder in the natural world. Stars are a great balance between gravity trying to squeeze them small, and nuclear-generated heat trying to pull them apart. The story of the supernova is awesome and kinetic, its wonders easily readable in the faces of children who listen to it. A single, supergiant star approaches the end of its life. As its final generation of fuel is exhausted, the giant radiation engine that supports the star shuts down. Massive collapse ensues, on a scale that is well beyond human comprehension. The implosion rebounds ferociously, spewing the alchemy of the old star into the cosmos. The transmuted elements are made nowhere else but here, the hellish belly of the most powerful beast of the universe. And these elements disperse through the cosmos—and become us.
Supernovae are so rare that one occurs in our galaxy, with 100 billion stars, only about once a century. For about a month, though, the maelstrom from that single, dying star is brighter than all of its 100 billion siblings combined. Overall, in the 100 billion galaxies that we can see from our vantage point, that means we have seen and measured and analyzed many hundreds of supernovae.
It isn't hard to see how a driving scientific curiosity could be drawn to trying to understand this thing. Indeed, supercomputer models of unimaginable explosions are quite refined, and scientific models of how stars explode have been highly successful. What is curious is that they are aided by a rather keen interest in an entirely different field: the nature and yield of human-made nuclear explosions. As declassification of the fundamental nuclear science of the 1940s and 1950s proceeded during the last decades of the twentieth century, there was a highly successful synergy between the study of the most fantastic, wondrous, violent explosions in our universe and the efficiency and effectiveness of nuclear weapons.
For 200,000 years, human beings have had an intense, powerful relationship with the skies above them. We all evolved within societies for which the sky was a pervasive source of magic, awe, religion, and art. For every human being, for 99.9 percent of the history of human-kind, there was a personal relationship with the sky. For 10,000 generations, the sky had personal meaning to people, figuring in much of what they did and how they behaved, how they moralized, and how they loved. We were born with humanity's relationship to the sky in our genes. The scientific study of astronomy doesn't change this, although it has changed the feelings we have about our place in the universe. As humanity explores and understands the natural world, the ever-growing power it wields over nature demands clarity and wisdom. Shortly before his death in 1695, the eminent Danish astronomer Christiaan Huygens (1625–1695) wrote in Kosmotheoros, for his time and ours:
This shows us how vast those Orbs must be, and how inconsiderable the Earth, the Theater upon which all our mighty Designs, all our Navigations, and all our Wars are transacted, is when compared to them. A very fit Consideration, and matter of Reflection, for those Kings and Princes who sacrifice the Lives of so many People, only to flatter their Ambition in being Masters of some pitiful corner of this small Spot.
MARK A. BULLOCK
SEE ALSO Cosmology.
Abrams, N. E., and J.R. Primack. (2001). "Cosmology and 21st-Century Culture." Science 293: 1769–1770. One of the few works that investigates the impact cosmology has had on culture.
Aveni, A.F., and L.D. Hotaling. (1994). "Monumental inscriptions and the observational basis of Maya planetary astronomy." Archaeoastronomy 19: S21–S54. The definitive popular work on Mayan astronomy by a leading expert in the field of archeoastronomy.
Hartmann, William K., and Chris Impey. (1994). Astronomy: The Cosmic Journey, 5th edition. Belmont, CA: Wadsworth. A standard undergraduate astronomy text book.
Hawkins, Gerald S., with John B. White. (1965). Stonehenge Decoded. Garden City, NY: Doubleday. A fascinating popular book on the mysteries of Stonehenge.
Huygens, Christiaan. (1698). Kosmotheoros. London: Timothy Childe. A philosophical treatise addressed to Huygen's brother Constantijn on the construction of the universe and the habitability of planets.
Struve, Otto, and Velda Zebergs. (1962). Astronomy of the 20th Century. New York: Macmillan. A comprehensive text on the history of modern astronomy up to the early 1960s.
Astronomy, the oldest of all the sciences, seeks to describe the structure, movements and processes of celestial bodies.
History and impact of astronomy
Ancient ruins provide evidence that the most remote ancestors observed and attempted to understand the workings of the Cosmos. Although not always fully understood, these ancient ruins demonstrate that early man attempted to mark the progression of the seasons as related to the changing of the apparent changing positions of the Sun , stars, planets and Moon on the celestial sphere . Archaeologists speculate that such observation made more reliable the determination of times for planting and harvest in developing agrarian communities and cultures.
The regularity of the heavens also profoundly affected the development of indigenous religious beliefs and cultural practices. For example, according to Aristotle (384–322 b.c.), Earth occupied the center of the Cosmos, and the Sun and planets orbited Earth in perfectly circular orbits at an unvarying rate of speed. The word astronomy is a Greek term for star arrangement. Although heliocentric (Sun-centered) theories were also advanced among ancient Greek and Roman scientists, the embodiment of the geocentric theory conformed to prevailing religious beliefs and, in the form of the Ptolemaic model subsequently embraced by the growing Christian church, dominated Western thought until the rise of empirical science and the use of the telescope during the Scientific Revolution of the sixteenth and seventeenth centuries.
In the East, Chinese astronomers, carefully charted the night sky, noting the appearance of "guest stars" (comets , novae, etc.). As early as 240 b.c., the records of Chinese astronomers record the passage of a "guest star" known now as Comet Halley, and in a.d. 1054, the records indicate that one star became bright enough to be seen in daylight. Archaeoastronmers argue that this transient brightness was a supernova explosion, the remnants of which now constitute the Crab Nebula. The appearance of the supernova was also recorded by the Anasazi Indians of the American Southwest.
Observations were not limited to spectacular celestial events. After decades of patient observation, the Mayan peoples of Central America were able to accurately predict the movements of the Sun, Moon, and stars. This civilization also devised a calendar that accurately predicted the length of a year, to what would now be measured to be within six seconds.
Early in the sixteenth century, Polish astronomer Nicolaus Copernicus (1473–1543) reasserted the heliocentric theory abandoned by the Greeks and Romans. Although sparking a revolution in astronomy, Copernicus's system was deeply flawed by an insistence on circular orbits. Danish astronomer Tycho Brahe's (1546–1601) precise observations of the celestial movements allowed German astronomer and mathematician Johannes Kepler (1571–1630) to formulate his laws of planetary motion that correctly described the elliptical orbits of the planets.
Italian astronomer and physicist Galileo Galilei (1564–1642) was the first scientist to utilize a newly invented telescope to make recorded observations of celestial objects. In a prolific career, Galileo's discoveries, including phases of Venus and moons orbiting Jupiter dealt a death blow to geocentric theory.
In the seventeenth century, English physicist and mathematician Sir Isaac Newton's (1642–1727) development of the laws of motion and gravitation marked the beginning of Newtonian physics and modern astrophysics . In addition to developing calculus , Newton made tremendous advances the understanding of light and optics critical to the development of astronomy. Newton's seminal 1687 work, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) dominated the Western intellectual landscape for more than two centuries and proved the impetus for the advancement of celestial dynamics.
Theories surrounding celestial mechanics during the eighteenth century were profoundly shaped by important contributions by French mathematician Joseph-Louis Lagrange (1736–1813), French mathematician Pierre Simon de Laplace, (1749–1827) and Swiss mathematician Leonhard Euler (1707–1783) that explained small discrepancies between Newton's predicted and the observed orbits of the planets. These explanations contributed to the concept of a clockwork-like mechanistic universe that operated according to knowable physical laws.
Just as primitive astronomy influenced early religious concepts, during the eighteenth century, advancements in astronomy caused significant changes in Western scientific and theological concepts based upon an unchanging, immutable God who ruled a static universe. During the course of the eighteenth century, there developed a growing scientific disregard for understanding based upon divine revelation and a growing acceptance of an understanding of Nature based upon the development and application of scientific laws. Whether God intervened to operate the mechanisms of the universe through miracles or signs (such as comets) became a topic of lively philosophical and theological debate. Concepts of the divine became increasing identified with the assumed eternity or infinity of the Cosmos. Theologians argued that the assumed immutability of a static universe, a concept shaken by the discoveries of Copernicus, Kepler, Galileo and Newton, offered proof of the existence of God. The clockwork universe viewed as confirmation of the existence of a God of infinite power who was the "prime mover" or creator of the universe. For many scientists and astronomers, however, the revelations of a mechanistic universe left no place for the influence of the Divine, and they discarded their religious views. These philosophical shifts sent sweeping changes across the political and social landscape.
In contrast to the theological viewpoint, astronomers increasingly sought to explain "miracles" in terms of natural phenomena. Accordingly, by the nineteenth century, the appearance of comets was no longer viewed as direct signs from God but rather a natural, explainable and predictable result of a deterministic universe. Explanations for catastrophic events (e.g., comet impacts, extinctions, etc.) increasingly came to be viewed as the inevitable results of time and statistical probability.
The need for greater accuracy and precision in astronomical measurements, particularly those used in navigation, spurred development of improved telescopes and pendulum driven clocks that greatly increased the pace of astronomical discovery. In 1781, improved mathematical techniques combined with technological improvements along with the proper application of Newtonian laws, allowed English astronomer William Herschel to discover the planet Uranus .
Until the twentieth century, astronomy essentially remained concerned with the accurate description of the movements of planets and stars. Developments in electromagnetic theories of light and the formulation of quantum and relativity theories, however, allowed astronomers to probe the inner workings of the celestial objects. Influenced by German-American physicist Albert Einstein's (1879–1955) theories of relativity and the emergence of quantum theory, Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910–1995) first articulating the evolution of stars into supernova, white dwarfs, neutron stars and accurately predicting the conditions required for the formation of black holes subsequently found in the later half of the twentieth century. The articulation of the stellar evolutionary cycle allowed rapid advancements in cosmological theories regarding the creation of the universe. In particular, American
can astronomer Edwin Hubble's (1889–1953) discovery of red shifted spectra from stars provided evidence of an expanding universe that, along with increased understanding of stellar evolution , ultimately led to the abandonment of static models of the universe and the formulation of big bang based cosmological models.
In 1932, American engineer Karl Janskey (1905–1945) discovered existence of radio waves of emanating from beyond the Earth. Janskey's discovery led to the birth of radio astronomy that ultimately became one of the most productive means of astronomical observation and spurred continuing studies of the Cosmos across all regions of the electromagnetic spectrum .
Profound questions regarding the birth and death of stars led to the stunning realization that, in a real sense, because the heavier atoms of which he was comprised were derived from neucleosynthesis in dying stars, man too was a product of stellar evolution. After millenniums of observing the Cosmos, by the dawn of the twenty-first century, advances in astronomy allowed humans to gaze into the night sky and realize that they were looking at the light from stars distant in space and time, and that they, also, were made from the very dust of stars.
The science of astronomy
At its most fundamental, astronomy is based on the electromagnetic radiation emitted by the stars. The ability to gather light is the key to acquiring useful data. The bigger the primary mirror of a telescope, the greater its light-gathering capabilities and the greater the magnification of the instrument. These two attributes allow a large telescope to image fainter, smaller objects than a telescope of lesser size. Thus, astronomers build everlarger telescopes, such as the 33-ft-diameter (10-m) Keck telescopes in Hawaii, or escape the distorting effects of the atmosphere with orbital observatories like the Hubble Space Telescope .
Astronomy is not just about visible light, however. Though the visible spectral region is most familiar to us because our eyes are optimized for these wavelengths, observation in the visible region shows only a small portion of the activities and processes underway in the universe. When astronomers view the night sky in other regions of the electromagnetic spectrum , it presents an entirely different picture. Hot gases seethe and boil when viewed at infrared wavelengths, newly forming galaxies and stars glow with x rays , and mysterious objects generate explosive bursts of gamma rays. Radio wave and ultraviolet observations likewise bring astronomers new insights about stellar objects.
Each spectral region requires different instrumentation, and different approaches to data analysis. Radio astronomy, for example, is performed by 20- and 30-ft-diameter (6- and 9-m) antennas, or even telescopes like the one in Arecibo, Puerto Rico, in which a 1,000-ft (303-m) diameter natural bowl in the landscape has been lined to act as an enormous radio wave collector. In the Very Large Array in New Mexico, 27 antennas placed as much as a mile apart from one another are linked by computer to make simultaneous observations, effectively synthesizing a telescope with a 22-mi (35-km) aperture—a radio-frequency analog to the Keck telescope. Infrared, x-ray, and gamma-ray telescopes require special materials and designs for both the focusing optics and the detectors, and cannot be performed below the Earth's atmosphere.
Quantifying light—luminosity and spectral classes
Astronomy is based upon the information we can derive by what we observe when we gaze at the stars. One of the characteristics of a star that can be determined observationally is its luminosity—the amount of light that the star emits. When combined with other information about a star such as its size or temperature , luminosity can indicate the intensity of fusion reactions taking place in the stellar core. Luminosity cannot always be determined by direct observation, however, as distance can decrease the apparent luminosity of an object. The Sun, for example, is not excessively luminous as stars go; it only appears brighter than any other stellar object because it is so close to us.
Magnitude is another way of expressing the luminosity of a star. The Greek astronomer Hipparchus developed the magnitude scale for stars, rating their brightness on a scale of 1 to 6. According to the scale, a star of first magnitude is defined as appearing 100 times as bright as a star of sixth magnitude, so the larger the magnitude, the fainter the object. As telescopes have allowed astronomers to peer deeper into the universe, the scale has expanded: Sirius, which appears to be the brightest star in the heavens, has an apparent magnitude of −1.27, while Pluto has a magnitude of 14.
Apparent magnitude, like apparent luminosity, can be deceptive. To avoid invalid comparisons, astronomers have developed the concept of absolute magnitude, which is defined as the apparent magnitude the object would have when viewed at a distance of 32.6 light years. Thus, measuring the distance to various objects is an important task in astronomy and astrophysics.
The color of light emitted by a star indicates its temperature. At the beginning of the century, astronomers began classifying stars based on color, or spectral classes. The classes are O, B, A, F, G, K, and M. O-type stars are the hottest (63,000°F [34,632°C]) and tend to appear white or blue-white, while M-type stars are the coolest (5,400°F [2,952°C]) and tend to appear red; our yellow sun, type G, falls in the middle. Another rating—L-type, for dim, cool objects below M-type—has recently been proposed for addition to the listing.
Astronomers can glean a tremendous amount of information from stellar magnitudes and glasses. Between 1911–13, Danish astronomer Ejnar Hertzsprung (1873–1967) and American astronomer Henry Norris Russell (1877–1957) independently developed what is now known as the Hertzsprung-Russell diagram that plots the magnitude and color of stars. According to the diagram, most stars fall on a slightly curving diagonal that runs from very bright, hot stars down to very cool, red stars. Most stars follow this so-called main sequence as they gradually burn out. Some stars fall off of the main sequence, for example red giants, which are relatively cool but appear bright because of their enormous size; or white dwarfs, which are bright but so small as to appear faint.
When we think of astronomy, spectacular, colorful pictures of swirling galaxies, collapsing stars, and giant clouds of interstellar gas come to mind. In reality, however, some of the most useful observational data in astronomy does not involve images at all. Spectroscopic techniques are powerful tools that allow scientists detect the presence of certain elements or processes in faraway galaxies.
In spectroscopy , incoming light—such as that from a star—is passed through a grating or a prism that splits the light up into its constituent wavelengths, or colors. Normally, a very bright, hot star will emit a continuous spectrum of light that spreads like a rainbow across the electromagnetic spectrum. In the case of lower density gas masses such as nebulae, however, the light will be emitted only at certain specific wavelengths defined by the elements found in the nebula—hydrogen atoms, for example—generate vivid yellow lines at characteristic wavelengths. The spectra will thus consist of a collection of bright lines in an otherwise dark background; this is called an emission spectrum. Similarly, if a star is surrounded by a cooler atmosphere, the atoms in the atmosphere will absorb certain wavelengths, leaving dark lines in what would otherwise be a continuum. This is known as an absorption spectrum.
Scientists study absorption and emission spectra to discover the elements present in stars, galaxies, gas clouds, or planet-forming nebulae. By monitoring the amount by which spectroscopic lines shift toward red wavelengths or toward blue wavelengths, astronomers can determine whether objects are moving toward or away from the Earth. This technique, based on the Doppler shift, is not only used to help astronomers study the expansion of the universe, but to determine the distance or age of the object under study. By studying the Doppler shift of stellar spectra, astronomers have been able to monitor faint wobbles in the motion of stars that indicate the presence of a companion star or even of extrasolar planets .
Although the sophisticated instruments and analysis techniques of astrophysics assist in the understanding of universe, astronomy is essentially about the observation of light. Using the data produced by a multitude of telescopes around the world and in orbit , astronomers are making new discoveries on a daily basis, and just as often exposing new puzzles to solve. The basic tools described above help scientists to extract information about stellar objects, and thus about the processes at work in the Universe.
See also Astrobiology; Astroblemes; Astrolabe; Astrometry; Astronomical unit; Cosmic background radiation; Cosmic ray; Cosmology; Gravity and gravitation; Infrared astronomy; Relativity, general; Relativity, special; Space shuttle; Spacecraft, manned; Spectral classification of stars; Spectral lines; Spectroscope.
Croswell, Ken. Magnificent Universe. New York: Simon & Schuster, 1999.
Crosswell, Ken. See the Stars: Your First Guide to the NightSky. Boyds Mills PA: Boyds Mills Press, 2000.
Hawking, Stephen. The Illustrated Brief History of Time, Updated and Expanded. New York: Bantam, 2001.
Rees, Martin J. Our Cosmic Habitat. Princeton, NJ: Princeton University Press, 2001.
Sagan, Carl. Cosmos New York: Random House, 2002.
Nemiroff, Robert, and Jerry Bonnell. National Air and Space Administration and Michigan Technological University.
"Astronony Picture of the Day" [cited February 5, 2003]. <http://antwrp.gsfc.nasa.gov/apod/astropix.html>.
K. Lee Lerner
KEY TERMS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
- Absolute magnitude
—The apparent brightness of a star, measured in units of magnitudes, at a fixed distance of 10 parsecs.
- Absorption spectrum
- Apparent magnitude
—The brightness of a star, measured in units of magnitudes, in the visual part of the electromagnetic spectrum, the region to which our eyes are most sensitive.
- Emission spectrum
—A spectrum consisting of bright lines generated by specific atoms or atomic processes.
—The amount of light emitted from a source per unit area.
—A technique for studying light by breaking it down into its constituent wavelengths.
astronomy, branch of science that studies the motions and natures of celestial bodies, such as planets, stars, and galaxies; more generally, the study of matter and energy in the universe at large.
Astronomy is the oldest of the physical sciences. In many early civilizations the regularity of celestial motions was recognized, and attempts were made to keep records and predict future events. The first practical function of astronomy was to provide a basis for the calendar, the units of month and year being determined by astronomical observations. Later, astronomy served in navigation and timekeeping. The Chinese had a working calendar as early as the 13th cent. BC About 350 BC, Shih Shen prepared the earliest known star catalog, containing 800 entries. Ancient Chinese astronomy is best known today for its observations of comets and supernovas. The Babylonians, Assyrians, and Egyptians were also active in astronomy. The earliest astronomers were priests, and no attempt was made to separate astronomy from astrology. In fact, an early motivation for the detailed study of planetary positions was the preparation of horoscopes.
The highest development of astronomy in the ancient world came with the Greeks in the period from 600 BC to AD 400. The methods employed by the Greek astronomers were quite distinct from those of earlier civilizations, such as the Babylonian. The Babylonian approach was numerological and best suited for studying the complex lunar motions that were of overwhelming interest to the Mesopotamian peoples. The Greek approach, on the contrary, was geometric and schematic, best suited for complete cosmological models. Thales, an Ionian philosopher of the 6th cent. BC, is credited with introducing geometrical ideas into astronomy. Pythagoras, about a hundred years later, imagined the universe as a series of concentric spheres in which each of the seven "wanderers" (the sun, the moon, and the five known planets) were embedded. Euxodus developed the idea of rotating spheres by introducing extra spheres for each of the planets to account for the observed complexities of their motions. This was the beginning of the Greek aim of providing a theory that would account for all observed phenomena. Aristotle (384–322 BC) summarized much of the Greek work before him and remained an absolute authority until late in the Middle Ages. Although his belief that the earth does not move retarded astronomical progress, he gave the correct explanation of lunar eclipses and a sound argument for the spherical shape of the earth.
The Alexandrian School and the Ptolemaic System
The apex of Greek astronomy was reached in the Hellenistic period by the Alexandrian school. Aristarchus (c.310–c.230 BC) determined the sizes and distances of the moon and sun relative to the earth and advocated a heliocentric (sun-centered) cosmology. Although there were errors in his assumptions, his approach was truly scientific; his work was the first serious attempt to make a scale model of the universe. The first accurate measurement of the actual (as opposed to relative) size of the earth was made by Eratosthenes (284–192 BC). His method was based on the angular difference in the sun's position at the high noon of the summer solstice in two cities whose distance apart was known.
The greatest astronomer of antiquity was Hipparchus (190–120 BC). He developed trigonometry and used it to determine astronomical distances from the observed angular positions of celestial bodies. He recognized that astronomy requires accurate and systematic observations extended over long time periods. He therefore made great use of old observations, comparing them to his own. Many of his observations, particularly of the planets, were intended for future astronomers. He devised a geocentric system of cycles and epicycles (a compounding of circular motions) to account for the movements of the sun and moon.
Ptolemy (AD 85–165) applied the scheme of epicycles to the planets as well. The resulting Ptolemaic system was a geometrical representation of the solar system that predicted the motions of the planets with considerable accuracy. Among his other achievements was an accurate measurement of the distance to the moon by a parallax technique. His 13-volume treatise, the Almagest, summarized much of ancient astronomical knowledge and, in many translations, was the definitive authority for the next 14 centuries.
Development of Modern Astronomy
The Copernican Revolution
After the fall of Rome, European astronomy was largely dormant, but significant work was carried out by the Muslims and the Hindus. It was by way of Arabic translations that Greek astronomy reached medieval Europe. One of the great landmarks of the revival of learning in Europe was the publication (1543) by Nicolaus Copernicus (1473–1543) of his De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres). According to the Copernican system, the earth rotates on its axis and, with all the other planets, revolves around the sun. The assertion that the earth is not the center of the universe was to have profound philosophical and religious consequences. Copernicus's principal claim for his new system was that it made calculations easier. He retained the uniform circular motion of the Ptolemaic system, but by placing the sun at the center, he was able to reduce the number of epicycles. Copernicus also determined the sidereal periods (time for one revolution around the sun) of the planets and their distance from the sun relative to the sun-earth distance (see astronomical unit).
Brahe and Kepler
The great astronomer Tycho Brahe (1546–1601) was principally an observer; a conservative in matters of theory, he rejected the notion that the earth moves. Under the patronage of King Frederick II, Tycho established Uraniborg, a superb observatory on the Danish island of Hveen. Over a period of 20 years (1576–97), he and his assistants compiled the most accurate and complete astronomical observations to that time. At his death his records passed to Johannes Kepler (1571–1630), who had been his last assistant. Kepler spent nearly a decade trying to fit Tycho's observations, particularly of Mars, into an improved system of heliocentric circular motion. At last, he conceived the idea that the orbit of Mars was an ellipse with the sun at one focus. This led him to the three laws of planetary motion that bear his name (see Kepler's laws).
Galileo Galilei (1564–1642) made fundamental discoveries in both astronomy and physics; he is perhaps best described as the founder of modern science. Galileo was the first to make astronomical use of the telescope. His discoveries of the four largest moons of Jupiter and the phases of Venus were persuasive evidence for the Copernican cosmology. His discoveries of craters on the moon and blemishes on the sun (sunspots) discredited the ancient belief in the perfection of the heavens. These findings were announced in The Sidereal Messenger, a small book published in 1610. Galileo's Dialogue on the Two Chief Systems of the World (1632) was an eloquent argument for the Copernican system over the Ptolemaic. However, Galileo was called before the Inquisition and forced to renounce publicly all doctrines considered contrary to Scripture.
Isaac Newton (1642–1727), possibly the greatest scientific genius of all time, succeeded in uniting the sciences of astronomy and physics. His laws of motion and theory of universal gravitation provided a physical, dynamic basis for the merely descriptive laws of Kepler. Until well into the 19th cent., all progress in astronomy was essentially an extension of Newton's work. Edmond Halley's prediction that the comet of 1682 would return in 1758 was refined by A. C. Clairault, who included the perturbing effects of Jupiter and Saturn on the orbit to calculate the nearly exact date of the return of the comet. In 1781, William Herschel accidentally discovered a new planet, eventually named Uranus. Discrepancies between the observed and theoretical orbits of Uranus indicated the existence of a still more distant planet that was affecting Uranus's motion. J. C. Adams and U. J. J. Leverrier independently calculated the position where the new planet, Neptune, was actually discovered (1846). Similar calculations for a large "Planet X" led in 1930 to the discovery of Pluto, now classed as a dwarf planet.
By the early 19th cent., the science of celestial mechanics had reached a highly developed state at the hands of Leonhard Euler, J. L. Lagrange, P. S. Laplace, and others. Powerful new mathematical techniques allowed solution of most of the remaining problems in classical gravitational theory as applied to the solar system. In 1801, Giuseppe Piazzi discovered Ceres, the first of many asteroids. When Ceres was lost to view, C. F. Gauss applied the advanced gravitational techniques to compute the position where the asteroid was subsequently rediscovered. In 1838, F. W. Bessel made the first measurement of the distance to a star; using the method of parallax with the earth's orbit as a baseline, he determined the distance of the star 61 Cygni to be 60 trillion mi (about 10 light-years), a figure later shown to be 40% too large.
Modern Techniques, Discoveries, and Theories
Astronomy was revolutionized in the second half of the 19th cent. by the introduction of techniques based on photography and spectroscopy. Interest shifted from determining the positions and distances of stars to studying their physical composition (see stellar structure and stellar evolution). The dark lines in the solar spectrum that had been observed by W. H. Wollaston and Joseph von Fraunhofer were interpreted in an elementary fashion by G. R. Kirchhoff on the basis of classical physics, although a complete explanation came only with the quantum theory. Between 1911 and 1913, Ejnar Hertzsprung and H. N. Russell studied the relation between the colors and luminosities of typical stars (see Hertzsprung-Russell diagram). With the construction of ever more powerful telescopes (see observatory), the boundaries of the known universe constantly increased. E. P. Hubble's study of the distant galaxies led him to conclude that the universe is expanding (see Hubble's law). Using Cepheid variables as distance indicators, Harlow Shapley determined the size and shape of our galaxy, the Milky Way. During World War II Walter Baade defined two "populations" of stars, and suggested that an examination of these different types might trace the spiral shape of our own galaxy (see stellar populations). In 1951 a Yerkes Observatory group led by William W. Morgan detected evidence of two spiral arms in the Milky Way galaxy.
Various rival theories of the origin and overall structure of the universe, e.g., the big bang and steady state theories, have been formulated (see cosmology). Albert Einstein's theory of relativity plays a central role in all modern cosmological theories. In 1963, the moon passed in front of the radio source 3C-273, allowing Cyril Hazard to calculate the exact position of the source. With this information, Maarten Schmidt photographed the object's spectrum using the 200-in. (5-m) reflector on Palomar Mt., then the world's largest telescope. He interpreted the result as coming from an object, now known as a quasar, at an extreme distance and receding from us at a substantial fraction of the speed of light. In 1967 Antony Hewish and Jocelyn Bell Burnell discovered a radio source a few hundred light years away featuring regular pulses at intervals of about 1 second with an accuracy of repetition of one-millionth of a second. This was the first discovered pulsar, a rapidly spinning neutron star emitting lighthouse-type beams of energy, the end result of the death of a star in a supernova explosion.
The discovery by Karl Jansky in 1931 that radio signals were emitted by celestial bodies initiated the science of radio astronomy. Most recently, the frontiers of astronomy have been expanded by space exploration. Perturbations and interference from the earth's atmosphere make space-based observations necessary for infrared, ultraviolet, gamma-ray, and X-ray astronomy. The Surveyor and Apollo spacecraft of the late 1960s and early 1970s helped launch the new field of astrogeology. A series of interplanetary probes, such as Mariner 2 (1962) and 5 (1967) to Venus, Mariner 4 (1965) and 6 (1969) to Mars, and Voyager 1 (1979) and 2 (1979), provided a wealth of data about Jupiter, Saturn, Uranus, and Neptune; more recently, the Magellan probe to Venus (1990) and the Galileo probe to Jupiter (1995) have continued this line of research (see satellite, artificial; space probe). The Hubble Space Telescope, launched in 1990, has made possible visual observations of a quality far exceeding those of earthbound instruments.
See A. Berry, Short History of Astronomy (1961); J. L. Dreyer, History of Astronomy from Thales to Kepler (2d ed. 1953); A. Koyré, The Astronomical Revolution (1973); P. Maffei, Beyond the Moon (1978); P. Moore, ed. The International Encyclopedia of Astronomy (1987); S. Maran, ed., The Astronomy and Astrophysics Encyclopedia (1991); C. Peterson and J. C. Brandt, Astronomy with the Hubble Space Telescope (1995).
ASTRONOMY There are astronomical references of chronological significance in the Vedas. Due to precession of Earth, the seasons shift at a rate of about a month every two thousand years. Some Vedic notices mark the beginning of the year and that of the vernal equinox in Orion; this was the case around 4500 b.c. There are other astronomical references from the subsequent millennia, which indicates the memory of a long period over which astronomy developed into a science. Fire altars, with astronomical basis, have been found in the third millennium cities of India. The texts that describe their designs are conservatively dated to the first millennium b.c., but their contents appear to be much older.
Vedic ritual was based on the times for the full and the new moons, the solstices and the equinoxes. There were two years: the ritual year started with the winter solstice (mahāvrata), and the civil one started with the spring equinox (vishuva). The passage of the rising of the sun in its northward course from the winter solstice to the summer solstice (vishuvant) was called gavām ayana, or the sun's walk. The solar year was divided into two ayanas: in the uttarāyana, the sun travels north; in the dakshināyana, it travels south.
The movement of the moon was marked by its nightly conjunction with one of the 27 or 28 nakshatras (stars or star clusters). The Rig Veda 1.164 also speaks of another tradition of dividing the zodiac into twelve equal parts. It appears that these divisions were called the Ādityas.
The incommensurability between the lunar and the solar reckonings led to the search for ever-increasing cycles to synchronize the motions of the sun and the moon. This is how the yuga (world cycle) astronomical model was born. In the lunar month, there were separate traditions of counting the beginning of the month by the full-moon day and the new-moon day.
During the earliest times in India, there existed a centennial calendar with a cycle of 2,700 years. Called the Saptarshi calendar, it is still in use in several parts of India. Its current beginning is taken to be 3076 b.c. Notices by the Greek historians Pliny and Arrian suggest that, during the Mauryan times, the calendar used in India began in 6676 b.c. It is very likely that this calendar was the Old Saptarshi calendar with a beginning at 6676 b.c. Other major Indian eras are that have wide currency are Kaliyuga (3102 b.c.), Vikrama (58 b.c.), and Shaka (a.d. 78).
The shifting of seasons through the year and the shifting of the North Pole allow us to date several other statements in the Vedic books. Thus the Shatapatha Brāhmaṇa statement that the Krittikās never swerve from the east corresponds to 2950 b.c.
The Maitrāyaniya Brāhmaṇa Upanishad refers to the winter solstice being at the midpoint of the Shravishthā (Delphini) segment and the summer solstice at the beginning of Māgha. This indicates 1660 b.c. The Vedānga Jyotisha, the text that describes some of the astronomical knowledge of the times of altar ritual, has an internal date of circa 1350 b.c.
The year was known to be somewhat more than 365 days and a bit less than 366 days. In one tradition, an extra eleven days were added to the lunar year of 354 days. According to one text, five more days are required over the nominal year of 360 days to complete the seasons.
The central idea behind the Vedic system is the notion of bandhu (connections) between the astronomical, the terrestrial, and the physiological. The connections were represented in sacred ritual and sacred books. This knowledge was also coded in the organization of the Rig Veda, which was taken to be a symbolic altar of hymns. The examination of the Rig Veda is of unique significance since this ancient book has been preserved with incredible accuracy.
Vedic ritual was generally performed at an altar. The altar design was based on astronomical numbers related to the reconciliation of the lunar and solar years. The fire altars symbolized the universe, and there were three types of altars representing the earth, the space, and the sky. The altar for the earth was drawn as circular, whereas the sky (or heaven) altar was drawn as square. The geometric problems of circulature of a square and that of squaring a circle are a result of equating the earth and the sky altars. These problems are among the earliest considered in ancient geometry.
The fire altars were surrounded by 360 enclosing stones; of these, 21 were around the earth altar, 78 around the space altar, and 261 around the sky altar. Thus the earth, the space, and the sky are symbolically assigned the numbers 21, 78, and 261.
The main altar was built in five layers. The basic square shape was modified to several forms, such as that of a falcon and a turtle. These altars were built in five layers, of a thousand bricks of specified shapes. The construction of these altars required the solution to geometric and algebraic problems. The main altar was an area that was taken to be equivalent to the nominal year of 360 days.
The altar ritual dealt with the difference between the two years: lunar, which is a fraction more than 354 days (360 tithis); and solar, which is in excess of 365 days (between 371 and 372 tithis). A well-known altar ritual says that altars should be constructed in a sequence of 95, with progressively increasing areas. The residual excess in 95 years adds up to 89 tithis; it appears that this was distributed in some manner over the 95-year period. The 95-year cycle corresponds to the tropical year being equal to 365.24675 days.
The Vedic astronomical system as given in the Vedānga Jyotisha is a luni-solar system. It considers a five-year yuga, employing two intercalary lunar months, with the condition that at the beginning of each yuga both the sun and the moon would be at the Shravishthā nakshatra, and it will be the winter solstice. For these conditions to be met, several corrections had to be made at the end of the yuga. For example, an additional day was needed to make sure that the new yuga would start with the new-moon day.
Nature of the Planetary System
The Āryabhatīmya of Āryabhata (b. a.d. 476) is a milestone of astronomy for two reasons. In it Earth is taken to spin on its axis, and the orbits of the planets are considered with respect to the sun. This idea of a spinning Earth causing night and day was a major advance in astronomy. Since the inner planets were already seen close to the sun, it made it easy to refer their orbital motions with respect to the sun. In contrast to this, in the Greek view the planets and stars were on concentric crystalline spheres centered on Earth. Each planet, the sun, and the moon were on their own sphere; the stars were placed on the largest sphere surrounding all of the rest.
A pure heliocentrism is to be found in the following statement in the Vishnu Purāṇa: "The sun is stationed for all time, in the middle of the day. The rising and the setting of the sun being perpetually opposite to each other, people speak of the rising of the sun where they see it; and, where the sun disappears, there, to them, is his setting. Of the sun, which is always in one and the same place, there is neither setting nor rising."
By examining early Vedic sources, the stages of the development of the earliest astronomy become apparent. After the Rig Vedic stage comes the period of the Brāhmaṇas, in which we place the Vedānga Jyotisha astronomy. The third stage is early Siddhāntic and early Purāṇic astronomy.
The concepts of the shīghrocca and mandocca cycles are peculiar to Indian astronomy. They indicate that the motion of the planets was taken to be fundamentally around the sun, which, in turn, was taken to go around Earth. The mandocca, in the case of the sun and the moon, is the apogee where the angular motion is the slowest; in the case of the other planets, it is the aphelion point of the orbit. For the superior planets, the shīghrocca coincides with the mean place of the sun, and in the case of an inferior planet, it is an imaginary point moving around Earth with the same angular velocity as the angular velocity of the planet around the sun; its direction from Earth is always parallel to the line joining the sun and the inferior planet.
The mandocca point serves to slow down the motion from the apogee to the perigee and speed up the motion from the perigee to the apogee. It is a representation of the nonuniform motion of the body, and so it can be seen as a direct development of the idea of the non-uniform motion of the sun and the moon. The shīghrocca maps the motion of the planet around the sun to the corresponding set of points around Earth. The sun, with its winds that hold the solar system together, is, in turn, taken to go around Earth. The antecedents of this system can be seen in the earlier texts.
In these standard texts of Indian astronomy, which became popular about two thousand years ago, the calculations are not done with respect to the nakshatras but rather with respect to the twelve signs of the zodiac. There is speculation that this change arose out of the interaction with the Greeks, but the twelve-division zodiac was a part of the early Indian astronomical tradition.
The mean longitudes were computed from the number of days elapsed from the beginning of long periods called the kalpa and the yuga, with the current yuga (Kaliyuga) having commenced on 17/18 February 3102 b.c. Planetary motions were computed using epicyclic and eccentric circles. Eclipses were computed more accurately by applying corrections due to parallax. Computations were based on arithmetic, geometric and algebraic techniques; plane and spherical trigonometry was also used.
The problems dealt with in the siddhāntas include: the determination of the longitudes of the planets and also of the ascending and descending nodes of the moon; corrections of these computations with the passage of time; lunar and solar eclipses; problems relating to the shadow; the phases of the moon; helical rising and setting of the planets; occultation of stars and planets; and astronomical instruments.
The astronomical texts may be divided into three types: siddhāntas, karanas, and koshthakas. While the siddhāntas are comprehensive and commence the calculations from the kalpa or a yuga, the karanas are practical manuals to facilitate calculations from a specific epoch with zero corrections at that point. The koshthakas or saranis are astronomical tables for the casting of horoscopes by astrologers. There are also texts that focus only on instruments.
The prominent astronomers after Āryabhata include his later rival Brahmagupta (seventh century), Bhāskara II (b. 1114), and the many mathematician-astronomers of the Kerala school that flourished during the years of the Karnataka (Vijayanagara) empire. The two most prominent names of this school are Mādhava (c. 1340–1425) and Nīlakantha (c. 1444–1545). Their contributions include power series for trigonometric functions, demonstration that π is irrational, and contributions to calculus. Nīlakantha presented an improved version of the Āryabhata's scheme in which the five planets orbit the sun and in turn they all orbit Earth.
Size of the Universe
The Brāhmaṇas consider noncircular motion of the sun and, by implication, of the moon, and the sun is taken to be about 500 Earth diameters away from Earth. Much later, Āryabhata considers the orbit of the sky as 4.32 million times greater than the orbit of the sun. Clearly, this was inspired by cosmological ideas. The Purāṇas consider the size of the universe to be 500 million yojanas (or over 4.5 billion miles). They also speak of other universes beyond ours. The conception of such a large size, and the noncentrality of Earth for the universe sets this tradition apart from Western astronomy.
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Rao, S. Balachandra. Indian Astronomy: An Introduction. Hyderabad: Universities Press, 2000.
Astronomy, the oldest of all the sciences, seeks to describe the structure, movements, and processes of celestial bodies.
Some ancient ruins provide evidence that the most remote ancestors observed and attempted to understand the workings of the Cosmos. Although not always fully understood, these ancient ruins demonstrate that early man attempted to mark the progression of the seasons as related to the changing positions of the Sun , stars, planets, and Moon on the celestial sphere. Archaeologists speculate that such observation made more reliable the determination of times for planting and harvest in developing agrarian communities and cultures.
The regularity of the heavens also profoundly affected the development of indigenous religious beliefs and cultural practices. For example, according to Aristotle (384-322 b.c.), Earth occupied the center of the Cosmos, and the Sun and planets orbited Earth in perfectly circular orbits at an unvarying rate of speed. The word astronomy is a Greek term for star arrangement. Although heliocentric (Sun centered) theories were also advanced among ancient Greek and Roman scientists, the embodiment of the geocentric theory conformed to prevailing religious beliefs and, in the form of the Ptolemaic model subsequently embraced by the growing Christian church, dominated Western thought until the rise of empirical science and the use of the telescope during the Scientific Revolution of the sixteenth and seventeenth centuries.
In the East, Chinese astronomers carefully charted the night sky, noting the appearance of "guest stars" (comets , novae, etc.). As early as 240 b.c., the records of Chinese astronomers record the passage of a "guest star" known now as Comet Halley, and in a.d. 1054, the records indicate that one star became bright enough to be seen in daylight. Archaeoastronmers argue that this transient brightness was a supernova explosion, the remnants of which now constitute the Crab Nebula. The appearance of the supernova was also recorded by the Anasazi Indians of the American Southwest.
Observations were not limited to spectacular celestial events. After decades of patient observation, the Mayan peoples of Central America were able to accurately predict the movements of the Sun, Moon, and stars. This civilization also devised a calendar that accurately predicted the length of a year, to what would now be measured to be within six seconds.
Early in the sixteenth century, Polish astronomer Nicolas Copernicus (1473–1543) reasserted the heliocentric theory abandoned by the Greeks and Romans. Although sparking a revolution in astronomy, Copernicus' system was deeply flawed by an insistence on circular orbits. Danish astronomer Tycho Brahe's (1546–1601) precise observations of the celestial movements allowed German astronomer and mathematician Johannes Kepler (1571–1630) to formulate his laws of planetary motion that correctly described the elliptical orbits of the planets.
Italian astronomer and physicist Galileo Galilei (1564–1642) was the first scientist to utilize a newly invented telescope to make recorded observations of celestial objects. In a prolific career, Galileo's discoveries, including phases of Venus and moons orbiting Jupiter, dealt a death blow to geocentric theory.
In the seventeenth century, English physicist and mathematician Sir Isaac Newton's (1642–1727) development of the laws of motion and gravitation marked the beginning of Newtonian physics and modern astrophysics. In addition to developing calculus, Newton made tremendous advances in the understanding of light and optics critical to the development of astronomy. Newton's seminal 1687 work, Philosophiae Naturalis Principia Mathematica (Mathematical principles of natural philosophy) dominated the Western intellectual landscape for more than two centuries and proved the impetus for the advancement of celestial dynamics.
Theories surrounding celestial mechanics during the eighteenth century were profoundly shaped by important contributions by French mathematician Joseph-Louis Lagrange (1736–1813), French mathematician Pierre Simon de Laplace (1749–1827), and Swiss mathematician Leonhard Euler (1707–1783) that explained small discrepancies between Newton's predicted and the observed orbits of the planets. These explanations contributed to the concept of a clockwork-like mechanistic universe that operated according to knowable physical laws.
Just as primitive astronomy influenced early religious concepts, during the eighteenth century, advancements in astronomy caused significant changes in Western scientific and theological concepts based upon an unchanging, immutable God who ruled a static universe. During the course of the eighteenth century, there developed a growing scientific disregard for understanding based upon divine revelation and a growing acceptance of an understanding of Nature based upon the development and application of scientific laws. Whether God intervened to operate the mechanisms of the universe through miracles or signs (such as comets) became a topic of lively philosophical and theological debate. Concepts of the divine became increasingly identified with the assumed eternity or infinity of the Cosmos. Theologians argued that the assumed immutability of a static universe, a concept shaken by the discoveries of Copernicus, Kepler, Galileo, and Newton, offered proof of the existence of God. The clockwork universe viewed as confirmation of the existence of God of infinite power who was the "prime mover" or creator of the universe. For many scientists and astronomers, however, the revelations of a mechanistic universe left no place for the influence of the Divine, and they discarded their religious views. These philosophical shifts sent sweeping changes across the political and social landscape.
In contrast to the theological viewpoint, astronomers increasingly sought to explain "miracles" in terms of natural phenomena. Accordingly, by the nineteenth century, the appearances of comets were no longer viewed as direct signs from God but rather a natural, explainable and predictable result of a deterministic universe. Explanations for catastrophic events (e.g., comet impacts, extinctions, etc.) increasingly came to be viewed as the inevitable results of time and statistical probability.
The need for greater accuracy and precision in astronomical measurements, particularly those used in navigation, spurred development of improved telescopes and pendulum driven clocks that greatly increased the pace of astronomical discovery. In 1781, improved mathematical techniques, combined with technological improvements, along with the proper application of Newtonian laws, allowed English astronomer William Herschel to discover the planet Uranus.
Until the twentieth century, astronomy essentially remained concerned with the accurate description of the movements of planets and stars. Developments in electromagnetic theories of light and the formulation of quantum and relativity theories, however, allowed astronomers to probe the inner workings of the celestial objects. Influenced by German-American physicist Albert Einstein's (1879–1955) theories of relativity and the emergence of quantum theory , Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910-1995) first articulated the evolution of stars into supernova, white dwarfs, neutron stars and accurately predicted the conditions required for the formation of black holes subsequently found in the later half of the twentieth century. The articulation of the stellar evolutionary cycle allowed rapid advancements in cosmological theories regarding the creation of the universe. In particular, American astronomer Edwin Hubble's (1889–1953) discovery of red shifted spectra from stars provided evidence of an expanding universe that, along with increased understanding of stellar evolution , ultimately led to the abandonment of static models of the universe and the formulation of big bang based cosmological models.
In 1932, American engineer Karl Jansky (1905–1945) discovered existence of radio waves of emanating from beyond the earth. Janskey's discovery led to the birth of radio astronomy that ultimately became one of the most productive means of astronomical observation and spurred continuing studies of the Cosmos across all regions of the electromagnetic spectrum .
Profound questions regarding the birth and death of stars led to the stunning realization that, in a real sense, because the heavier atoms of which he was comprised were derived from nucleosynthesis in dying stars, man too was a product of stellar evolution. After millennia of observing the Cosmos, by the dawn of the twenty-first century, advances in astronomy allowed humans to gaze into the night sky and realize that they were looking at the light from stars distant in space and time, and that they, also, were made from the very dust of stars.