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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.


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 (14231461) 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 (14731543) 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 (14981552) reassured readers in an anonymous preface to the work that Copernicus had merely devised "hypotheses" that facilitated the calculation of celestial positions.


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 thingsthe 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 (15461601) 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 workplacewhich had its own printing pressinto 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 (15711630) 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 (15501631), 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.


In 1609, the same year that Kepler published his first two laws, Galileo Galilei (15641642) 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 discoveriesand their implicationswere easily understood by a new audience of scholars and gentlemen, and they had a dramatic impact on writers and poets such as John Donne (15721631).

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 Observatoryfounded to aid navigation and the determination of longitude by improving astronomyin 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 (16351703) 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 (16421727) 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 16841685, 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 16801681. 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 16941695 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 (16981759), for example, took a zenith sector and clock constructed by the outstanding London instrument maker George Graham (16731751) to Lapland in 17361737. 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 latitudean 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 .


Primary Sources

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.

Secondary Sources

Chapman, Allan. Dividing the Circle: The Development of Critical Angular Measurement in Astronomy, 15001850. 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.

Rob Iliffe

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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.


Christianson, Gale E. Edwin Hubble: Mariner of the Nebulae. New York: Farrar, Stauss, Giroux, 1995.

Edmundson, Frank K. AURA and its US National Observatories. New York: Cambridge University Press, 1997.

Hetherington, Norriss S. Hubble's Cosmology: A Guided Study of Selected Texts. Tucson, Ariz.: Pachart Publishing, 1996.

Hindle, Brook. David Rittenhouse. New Jersey: Princeton University Press, 1964.

Hoyt, William Graves. Lowell and Mars. Tuscon: University of Arizona, 1976.

Jones, Bessie Judith Zaban, and Lyle Gifford Boyd. The Harvard College Observatory: The First Four Directorships, 1839–1919. Cambridge, Mass. Belknap Press, 1971.

Lankford, John. American Astronomy: Community, Careers, and Power, 1859–1940. Chicago: University of Chicago Press, 1997.

Levy, David H. Clyde Tombaugh: Discoverer of Planet Pluto. Tucson: University of Arizona Press, 1991.

Osterbrock, Donald E. Eye on the Sky: Lick Observatory's First Century. Berkeley: University of California Press, 1988.

———. Yerkes Observatory 1892–1950: The Birth, Near Death, and Resurrection of a Scientific Research Institution. Chicago: University of Chicago, 1997.

———. Pauper and Prince: Ritchey, Hale, & Big American Telescopes. Tucson: University of Arizona Press, 1993.

Sheehan, William. The Immortal Fire Within: The Life and Work of Edward Emerson Barnard. New York: Cambridge University Press, 1995.

Smith, Robert W. The Space Telescope: A Study of NASA, Science, Technology, and Politics. New York: Cambridge, 1989.

Tucker, Wallace, and Karen Tucker. The Cosmic Inquirers: Modern Telescopes and Their Makers. Cambridge: Harvard University Press, 1986.

Warner, Deborah Jean. Alvan Clark & Sons: Artists in Optics. Richmond, Va.: Willmann-Bell, 1995.


See alsoCoast and Geodetic Survey ; Hubble Space Telescope ; Observatories, Astronomical ; Physics ; Surveying .

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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.

Ancient Astronomy

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.

Greek Innovations

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's Telescope

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.

Astrophysical Discoveries

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).

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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 (14731543) 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 (15461601) precise observations of the celestial movements allowed German astronomer and mathematician Johannes Kepler (15711630) to formulate his laws of planetary motion that correctly described the elliptical orbits of the planets.

Italian astronomer and physicist Galileo Galilei (15641642) 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 (16421727) 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 (17361813), French mathematician Pierre Simon de Laplace (17491827), and Swiss mathematician Leonhard Euler (17071783) 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 (18791955) 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 (18891953) 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 (19051945) 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.

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25. Astronomy

See also 100. COSMOLOGY ; 259. MARS ; 271. METEORITES ; 280. MOON ; 318. PLANETS ; 387. SUN .

the branch of astronomy that studies meteors.
the branch of astronomy that studies aerolites, or stony meteors.
the ratio between the light reflected from a surface and the total light falling upon that surface, as the albedo of the moon.
the point in the orbit of a heavenly body where it is farthest from the sun. Cf. perihelion.
in an orbit around a moon, the point furthest from the moon. Cf. perilune.
the astronomical studies of the planet Mars. areologist, n. areologic, areological, adj.
Rare. a constellation or small group of unrelated stars. asterismal, adj.
the art of navigating in space. Cf. astronavigation. astrogator, n.
astrogeny, astrogony
the theory of the evolution of heavenly bodies.
a geological specialty that studies celestial bodies.
the branch of astronomy that studies the fixed stars.
a scientific analysis and mapping of the stars and planets. astrographic, adj.
the worship of the heavenly bodies. Also called Sabaism . astrolater, n.
1. a form of divination involving studying the stars.
2. Rare. astrology. Also called sideromancy. astromancer, n. astromantic, adj.
the branch of astronomy that studies the dimensions of heavenly bodies, especially the measurements made to determine the positions and orbits of various stars. astrometric, astrometrical, adj.
the science of space travel, concerned with both the construction and the operation of vehicles that travel through interplanetary or interstellar space. Also called cosmonautics . astronautic, astronautical, adj. astronaut, n.
a type of navigation involving observations of the apparent positions of heavenly bodies. Also called celestial navigation, celo-navigation . astronavigator, n.
the science that studies the stars and other features of the material universe beyond the earths atmosphere. astronomer, n. astronomical, adj.
a person strongly attracted to knowledge about the stars. astrophilic, adj.
a form of photography used to record astronomical phenomena.
the branch of astronomy concerned with the origin, and the chemical and physical nature of heavenly bodies. astrophysicist, n.
celestial navigation
astronavigation. Also called celo-navigation .
the study of stars through a telescope in which the star appears as a ring of light, in order to observe the stars scintillation. chromatoscope, n.
the fundamental theoretical basis of modern astronomy, first demonstrated in the early 16th century by Copernicus, who showed that the earth and the other planets orbit around the sun. Cf. Ptolemaism .
Obsolete, an instrument, like an astrolabe, used for astronomical observations.
astronautics. cosmonaut. n. cosmonautical, adj.
an instrument used in astronomy to show the apparent movement of the sun.
an instrument originally designed for measuring the suns diameter, now used for measuring the angular distance between stars.
the practice of measuring the angular distance between stars by means of a heliometer. heliometric, heliometrical, adj.
the period between the old moon and the new when the moon is invisible each month. interlunar, adj.
an imaginary great circle in the sphere of the heavens, passing through the poles and the zenith and nadir of any point and intersecting the equator at right angles. See also 178. GEOGRAPHY. meridian, meridional, adj.
the entire system of galaxies, including the Milky Way. metagalactic, adj.
the periodic oscillation that can be observed in the precession of the earths axis and the precession of the equinoxes. See also 133. EARTH. nutational, adj.
the inclination of the earths equator or the angle between the plane of the earths orbit and the plane of the equator (23°27). Also called obliquity of the ecliptic . See also 133. EARTH. obliquitous, adj.
the process of one heavenly body disappearing behind another as viewed by an observer.
a false moon, in reality a bright spot or a luminous ring surrounding the moon.
the point in the orbit of a heavenly body where it is nearest the sun. Also spelled perihelium. Cf. aphelion.
in orbit around a moon, the point nearest the moon. Cf. apolune.
Rare. a work or treatise on astronomy or celestial bodies.
1. a representation of the planetary system, particularly one in which the movements of the planets are simulated by projectors.
2. a room or building housing such an apparatus.
the branch of astronomy that studies the planets. planetologist, n. planetologic, planetological, adj.
a map showing half or more of the sphere of the heavens, indicating which part is visible at what hour from a given location. planispheric, planispherical, adj.
the complicated demonstration of Ptolemy, 2nd-century geographer and astronomer, that the earth is the fixed center of the universe around which the sun and the other planets revolve; now discredited. Cf. Copemicanism .
a supporter of the Ptolemaic explanation of planetary motions.
the branch of astronomy that studies radio frequencies emitted by the sun, planets, and other celestial bodies.
Sabianism, Sabaeanism, Sabeanism
the religion of the Sabians, a group sometimes associated with worship of the sun, moon, and stars. See also 349. RELIGION .
the combination or configuration of the aspects of the planets and other heavenly bodies.
the scientific analysis and mapping of the moons physical features. selenographer, selenographist, n. selenographic, selenographical, adj.
the branch of astronomy that studies the moon. selenologist, n. selenologic, selenological, adj.
a form of divination involving observations of the stars. Also called astromancy . sideromancer, n. sideromantic, adj.
an abnormal fear of the stars.
Obsolete, astronomy.
the branch of astronomy that deals with the description of the heavens by constructing maps and charts, especially of the fixed stars. Also called uranology . uranographer, uranographist, n. uranographic, uranographical, adj.
1. a written description of the heavens and celestial bodies.
2. another term for astronomy.
uranometry, uranometria
1. a treatise recording the positions and magnitudes of heavenly bodies.
2. the science of measuring the real or apparent distances of heavenly bodies from Earth. uranometrical, adj.

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Astronomy is the scientific study of the universe as a whole and of objects that exist naturally in space, such as the Moon, planets, stars, and galaxies. The Sun is a rather typical star moving around the center of the Milky Way at a distance of 20,000 light years. This Milky Way, our galaxy, is made of hundreds of millions of stars with vast empty regions and diffuse gas in between all held together by gravity. There are at least another hundred billion galaxies in the observable universe.

Every day and night astronomers collect data on these objects in all bands of the electromagnetic spectrum of light. From radio waves through X-rays, small and large telescopes on the ground and in space record immense amounts of data. Analyzing these data is one of the major challenges of modern astronomy. Sophisticated time-dependent, three-dimensional models of astronomical objects and the universe as a whole are now routinely constructed to help astronomers interpret their observations.

In 1823 the president of the Royal Astronomical Society presented the society's gold medal to British mathematician Charles Babbage (17911871) for the invention of the first digital calculating machine. The president remarked: "In no department of science or of the arts does this discovery promise to be so eminently useful as in that of astronomy The practical astronomer is interrupted in his pursuit, is diverted from his task of observation, by the irksome labour of computation;" Before the first electronic computers became available to astronomers, many observatories employed humans referred to as computers for this "irksome labour of computation."

In the 1950s theoretical astronomers began tapping the potential of programmable computing systems. Their first programs derived the solution of the restricted three-body problem (the gravitational interaction of three massive bodies) and also the solution to the equations of stellar structure. Model stellar atmospheres were also constructed from atomic data to be compared with observations of stellar spectra. The first practical applications were computations of precise apparent star positions and analysis routines transforming photoelectric observational data into meaningful scientific quantities.

By the 1960s computers had become an integral part of research in astronomy. In particular, progress in the theoretical understanding of stellar structure and evolution was driven largely by results of numerical investigations.

Astronomical research stimulates new advances in computing. A prime example is SETI@home, a project that uses the computing power of idle home and office computers. SETI, which stands for Search for ExtraTerrestial Intelligence, involves the analysis of the many gigabytes of data collected daily at radio telescopes . A freely distributed screensaver connects to a central data server and collects a small part of these data. When the home computer is otherwise idle, its processing power is used to look for extraterrestial signals. Upon completion of the analysis, the results of the computation are relayed back to the central server. Although no alien life has been found to date, millions of SETI@home users are contributing to this scientific endeavor.

The rapid growth in computing technologies also enables new areas of astronomical research. The data obtained at telescopes are now all stored in large databases that can be accessed by astronomers around the globe. These archived data repositories act as virtual observatories. Scientist can retrieve (observe) data on the same astronomical objects in many different wave-bands in a very short time without even having access to a telescope. Any relevant scientific studies are cross-referenced with the observed objects and are readily accessible on the Internet worldwide.

Supercomputers play an important role in astronomy today. Perhaps one of the most important uses of supercomputers is the physical modeling of the origin and evolution of astrophysical objects. Computational astrophyics has become a mature science in the past decades, driven not only by the ever-increasing power of computers but especially by new algorithmic breakthroughs. Novel techniques allow scientists to capture the vast space and time scales that astronomical objects span. It has become possible to study in detail such difficult problems as the collision of black holes; the formation of planets, stars, and galaxies; and the large-scale structure of the universe.

In detailed three-dimensional models, computational astrophysicists follow the time evolution of magnetic fields, radiation, gas dynamics, chemistry, gravitational interactions, cosmic ray interactions, and many more physical processes. Physicists can experiment in laboratories to test their theories. Only numerical experiments allow the astronomers and astrophysicists to test their theories in the quest to understand the nature of astronomical objects.

Rapidly evolving computing technologies and their uses in astronomical research will continue to lead to profound new insights into the nature, origin, and evolution of the cosmos and the complex structures within.

see also Navigation; Physics; Weather Forecasting.

Tom Abel


Duffett-Smith, Peter. Astronomy with Your Personal Computer. New York: Cambridge University Press, 1990.

Foust, Jeff, and Ron LaFon. Astronomer's Computer Companion. San Francisco: No Starch Press, 2000.

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An Old Science. For almost two thousand years the greatest European thinkers agreed with the Greek scientist Aristotles conception of the universe. Aristotle believed that the Earth was at the center of the universe, surrounded by the Moon, the Sun, the planets, and the stars. Each heavenly body orbited the Earth in a perfect circular motion. There was only one Moon in the universe. The Suns orbit around Earth was between the spheres of Venus and Mars. The realm of fixed stars was at the outer edge of a small, finite, limited universe. Copernicus, Johannes Kepler, Galileo, and Sir Isaac Newton had different views. During the same decades that Columbus discovered America, Jacques Cartier discovered the Saint Lawrence River, and John Smith founded Jamestown, Europeans were restructuring the universe. They believed that the Sun, not the Earth, was the center. Moreover, the Sun was the center not of the universe but of one solar system in one galaxy in a possibly infinite universe. The Earth was one of several planets orbiting the Sun in an elliptical path. Other planets besides the Earth had moons. The universe operated according to precise forces, such as gravity, which made the universe logical and predictable. The observations of astronomers helped humans understand the fundamental laws of the universe.

Colonials. Beginning in the 1650s the ideas of the new science began to cross the Atlantic to America. The first colonial astronomer of note was John Winthrop Jr., son of the founder of Massachusetts Bay. Winthrop owned two telescopes, which he used to observe comets and planets. In 1672, he donated a three-and-one-half-foot telescope to Harvard College. In 1680 Thomas Brattle used it to make precise observations of Newtons Comet. By this time Harvard had established itself as the leader of scientific observation in the colonies. In ensuing decades Isaac Greenwood, Charles Morton, and other professors taught the theories of Copernicus and Newton. Graduates such as Increase Mather, Samuel Sew-all, and Thomas Robie observed the aurora borealis (northern lights), comets, eclipses, and planetary motions. Robie, a Harvard teacher and physician, calculated the distances of the planets to the Sun, observed the solar eclipse of November 1722 and the transit of Mercury in 1723, and tried to explain the aurora borealis. James Logan of Philadelphia and Cadwallader Colden of New York also made significant astronomic observations. Although few in number, these colonial Americans turned their attention from the practical affairs of everyday life to observe the motions of the heavens.

Other Planets. Since antiquity humans have speculated on the possibility of life on other planets within and beyond the solar system. Scientists fueled these speculations by showing that the Earth was not unique but like the other planets in that it orbited the Sun. And if the Earth orbited a star (the Sun), why should there not be in the universe other stars with other planets capable of supporting life? There was no better example of the impact of the new science on Americans than the growing concerns about extraterrestrial life. Cotton Mather so believed in the efficiency of the universe and the Creator that he assumed some of the stars in the universe must support life. Benjamin Franklin speculated on extraterrestrial life in the course of his lengthy discussion on astronomy in Poor Richards Almanack for 1753 and 1754. After discussing the tremendous heat of Mercury, Franklin observed that it does not follow, that Mercury is therefore uninhabitable; since it can be no Difficulty for the Divine Power and Wisdom to accomodate the Inhabitants to the place they are to inhabit; as the Cold we see Frogs and Fishes bear very well, would soon deprive any of our Species of Life. Franklin put himself in the place of possible inhabitants of Mars and wondered what they would see. The Earth and Moon will appear to them, thro Telescopes if they have any such Instruments, like two Moons, a larger and a smaller, sometimes horned, sometimes Half or three Quarters illuminated, but never full. Franklin was not willing to deny the possibility of life on the Moon, Jupiter, Saturn, and even comets. If there are any Inhabitants in the Comets, they must live a Life wholly inconceivable to us. Franklin closed his discourse in astronomy speculating on the vast numbers of stars, the planets orbiting them, and the variety of life that must exist on those planets in the universe.


George Daniels, Science in American Society: A Social History (New York: Knopf, 1971);

Benjamin Franklin, The Complete Poor Richard Almanacks, 2 volumes (Barre, Mass.: Imprint Society, 1970);

Raymond Stearns, Science in the British Colonies of North America (Urbana: University of Illinois Press, 1970).

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Astronomy of the Middle Ages was grounded in the work of Ptolemy, a scientist of ancient Alexandria, whose work The Almagest set out the Ptolemaic system of an earth-centered universe. The Ptolemaic view was accepted by philosophers and sanctioned by the church, and his work was the foundation of studies and commentaries by the medieval scholars Georg von Peuerbach, Johann Müller (known as Regiomontanus), and Georg Joachim Rheticus. To verify their observations astronomers drew on the Alfonsine Tables that were set down in Toledo, Spain, in 1252, and that were based on the Ptolemaic system. The earth was the center of the universe around which the sun, planets, and stars revolved; the heavens were permanent and unchanging; a perfect harmony and balance existed in which, according to astrologers, celestial phenomena had their effect on events and people on the terrestrial globe.

As the skill of observers improved, however, the Ptolemaic system came under question. The Polish astronomer Nicolaus Copernicus, while using the Alfonsine Tables in 1504 in observing a conjunction of Mars and Saturn, found them inaccurate. Trusting in his own calculations, Copernicus began questioning the Ptolemaic system and concluded that a heliocentric (sun-centered) structure accounted more accurately for the motions of the planets. This theory was revolutionary and, in Copernicus's view, dangerous, as it questioned the accepted wisdom supported by the church for centuries. He did not allow his theory to circulate in print until the end of his life, although heliocentrism became a common topic of debate among clergy and scientists in the early sixteenth century. The Alfonsine Tables would be replaced by the Prutenic Tables of Erasmus Reinhold, which were based on the Copernican heliocentric universe.

In 1572, the Danish astronomer Tycho Brahe discovered a new star and, in 1577, a comet. Through these observations Brahe showed that the heavens were ever-changing, producing new objects and phenomena that were not accounted for in traditional astronomy. The invention of the telescope in the early seventeenth century allowed astronomers more accurate observations that led to improvements in the Copernican system. Using a telescope and colored lenses, in 1611 Christoph Scheiner observed sunspotsdark spots that appear at times on the sunproving that the sun was a mutable body, and not a perfect sphere of fire or light. In studying the motion of Mars, Johannes Kepler concluded that the circular orbits of traditional astronomy were a mirage. Instead, according to Kepler, the planets moved in elliptical orbits, with the sun lying at one focus of the ellipse. According to Kepler's harmonic law, the orbital period of a planet varied with the distance of the planet from the sun. Astronomers would later use this law to calculate the dimensions of the solar system.

The complex equations theorized by Kepler and others replaced the fixed and unchanging doctrines of the past. Sir Isaac Newton introduced the law of universal gravitation, which explained the relationship between orbit and velocity. The Rudolphine Tables created by Tycho Brahe and Kepler in 1627 replaced the Prutenic Tables. Astronomy became a science, carried out through accurate observation. Renaissance astronomers dared to question traditional wisdom, even at the risk of losing their reputations and their lives. The Italian scientist Galileo Galilei, using a telescope, discovered the moons of Jupiter and the mountains and craters of the moon. For his theories on the nature of the solar system; and his discovery of worlds previously unknown, he was threatened by the Inquisition of the Catholic Churcha tribunal that punished heresyand his works were censored. Nevertheless, the work of Galileo and others placed the traditional view of the heavens through a transformation, explaining the universe was a skill passing out of the hands of philosophers and the church and to a class of scientific specialists who rejected medieval traditions of astrology and religious doctrine altogether.

See Also: astrology; Copernicus, Nicolaus; Galilei, Galileo; Kepler, Johannes

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astronomy Branch of science studied since ancient times and concerned with the universe and its components in terms of the relative motions of celestial bodies, their positions on the celestial sphere, physical and chemical structure, evolution and the phenomena occurring on them. It includes celestial mechanics, astrophysics, cosmology, and astrometry. Waves in all regions of the electromagnetic spectrum can now be studied either with ground-based instruments or by observations and measurements made from satellites, space probes and rockets.


Astronomy was first used practically to develop a calendar, the units of which were determined by observing the heavens. The Chinese had a calendar in the 14th century bc. The Greeks developed the science between 600 bc and ad 200. Thales introduced geometrical ideas and Pythagoras saw the universe as a series of concentric spheres. Aristotle believed the Earth to be stationary but he explained lunar eclipses correctly. Aristarchus put forward a heliocentric theory. Hipparchus used trigonometry to determine astronomical distances. The system devised by Ptolemy was a geometrical representation of the Solar System that predicted the motions of the planets with great accuracy. From then on astronomy remained dormant until the scientific revolution of the 16th and 17th centuries, when Copernicus stated his theory that the Earth rotates on its axis and, with all the other planets, revolves round the Sun. This had a profound effect upon contemporary religion and philosophy. Kepler and his laws of planetary motion refined the theory of heliocentric motion, and his contemporary, Galileo, made use of the telescope and discovered the moons of Jupiter. Isaac Newton combined the sciences of astronomy and physics. His laws of motion and universal theory of gravitation provided a physical basis for Kepler's laws and enabled the later prediction of Halley's comet and the discovery of the planets Uranus, Neptune, and Pluto. by the early 19th century the science of celestial mechanics (the study of the motions of bodies in space as they move under the influence of their mutual gravitation) had become highly advanced and new mathematical techniques permitted the solution of the remaining problems of classical gravitation theory as applied to the Solar System. In the second half of the 19th century astronomy was revolutionized by the introduction of techniques based on photography and spectroscopy. These encouraged investigation into the physical composition of stars. Ejnar Hertzsprung and H. N. Russell studied the relationship between the colour of a star and its luminosity. by this time larger telescopes were being constructed, which extended the limits of the universe known to man. Harlow Shapley determined the shape and size of our galaxy and E. P. Hubble's study of distant galaxies led to his theory of an expanding universe. The Big Bang and steady-state theory of the origins of the universe were formulated. In recent years, space exploration and observation in different parts of the electromagnetic spectrum have contributed to the discovery and postulation of such phenomena as the quasar, pulsar, and black hole. There are various branches of modern astronomy. Optical astronomy studies sources of light in space. Light rays can penetrate the atmosphere but, because of disturbances, many observations are now made from above the atmosphere. Gamma-ray, infrared, ultraviolet, and X-ray astronomy study the emission of radiation (at all wavelengths) from astronomical objects. Higher wavelengths can be studied from the ground, while lower wavelengths require the use of satellites and balloons. Other branches within astronomy include radar astronomy and radio astronomy.

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Astronomy is the scientific study of the objects visible in the night sky by means of telescopes and associated instruments that analyze the radiation received from these objects. Using such instruments, astronomers determine their positions, apparent motions, distances, sizes, and total radiation emitted. From their spectra (the decomposition of light received from them into wavelengths) astronomers determine their chemical composition and radial motion. Astronomy distinguishes planets from stars, and identifies the way stars are spatially associated in star clusters, galaxies, and clusters of galaxies. Astronomy has ancient roots arising from peoples' attempts to relate the annual change of seasons to positions of stars in the sky. Astronomy is to be distinguished from astrology, which purports to relate the events in human lives to positions of the planets at the time of one's birth.

See also Cosmology, Physical Aspects

george f. r. ellis

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44. Astronomy

  1. Aristarchus of Samos (fl. c. 270 B.C.) Greek astronomer; first to maintain that Earth rotates and revolves around Sun. [Gk. Hist.: EB, I: 514]
  2. Copernicus, Nicholas (14531543) Polish astronomer; author of the Copernican theory that planets orbit the sun. [Polish Hist.: NCE, 652]
  3. Galileo (15641642) Italian mathematician, astronomer, and physicist. [Ital. Hist.: EB, IV: 388]
  4. Halley, Edmond (16561742) British mathematician and astronomer; calculated orbit of comet named after him. [Br. Hist.: EB, IV: 860]
  5. Hipparchus (fl. 146127 B.C.) astronomer who calculated the year and discovered the precession of the equinoxes. [Turkish Hist.: EB, V: 55]
  6. Ptolemy (85165) eminent Greek astronomer. [Gk. Hist.: Hall, 255]
  7. Urania muse of astronomy. [Gk. Myth.: Jobes, 374]

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astronomy XIII. — (O)F. astronomie — L. astronomia — Gr. astronomíā, f. astronómos, f. astronomeîn observe the stars, f. ástron STAR; see -NOMY.
So astronomer XIV. ME. astronom(i)er, f. astronomy; earlier -ien — OF. astronomical XVI. f. F. -ique or L. -icus — Gr. -ikós.

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as·tron·o·my / əˈstränəmē/ • n. the branch of science that deals with celestial objects, space, and the physical universe as a whole.

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astronomyfumy, gloomy, plumy, rheumy, roomie, roomy, spumy •excuse-me • mushroomy • perfumy •Brummie, chummy, crumby, crummy, dummy, gummy, lumme, mummy, plummy, rummy, scrummy, scummy, slummy, tummy, yummy •academy • sodomy • blasphemy •infamy •bigamy, polygamy, trigamy •endogamy, exogamy, heterogamy, homogamy, misogamy, monogamy •hypergamy • alchemy • Ptolemy •anomie • antinomy •agronomy, astronomy, autonomy, bonhomie, Deuteronomy, economy, gastronomy, heteronomy, metonymy, physiognomy, taxonomy •thingummy • Laramie • sesame •blossomy •anatomy, atomy •hysterectomy, mastectomy, tonsillectomy, vasectomy •epitome •dichotomy, lobotomy, tracheotomy, trichotomy •colostomy • bosomy •squirmy, thermae, wormy •taxidermy

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