ASTRONOMY

ASTRONOMY. The movement of the stars and planets has fascinated humans for thousands of years. For the vast majority of ancient astronomers, the stars seemed to be equally distant from Earth in what was an Earth-centered (or "geocentric") cosmos. The ancient Greeks observed that the stars revolved westward around the north celestial pole every twenty-three hours and fifty-six minutes, thus constituting a kind of objective clock. Most envisaged these revolving stars to be located on a sphere, composing an incorruptible, celestial orb that could easily be contrasted with the world of change, generation, and corruption on Earth (to which irregular meteorological phenomena such as comets were also understood to belong). Against the backdrop of this outer sphere, the Sun was seen to move along a path termed the ecliptic, while the planets moved within eight degrees of this path. These usually moved eastward, though occasionally they moved in the opposite direction, thus exhibiting retrograde motion. Many Greek astronomers suggested that the Sun could be placed on the equator of an inner sphere that revolved once a year, hence constituting a second sphere in addition to the stellar orb. Following the theories of the fourth-century-b.c.e. scholar Eudoxus, astronomers and natural philosophers became increasingly committed to the idea that the motions of each planet could be accounted for by means of their own specific, homocentric spheres.

DEFENDING AN EARTH-CENTERED UNIVERSE

The retrograde motion of the planets offended against the apparent simplicity of heavenly motions, as well as the dictum that their orbits were circular. From the third century b.c.e., astronomers began to conceive of planets rotating in small circles (epicycles) around a point that moved on a carrier orbit (or deferent). This accounted for the apparent motions of the planets, including their retrograde motion, although others followed Aristotle in positing the existence of homocentric spheres. After Ptolemy's composition of the Almagest in the second century c.e., the movements of the planets could be accurately represented by means of techniques involving the use of epicycles, deferents, eccentrics (whereby planetary motion is conceived as circular with respect to a point displaced from Earth), and equants (a device that posits a constant angular rate of rotation with respect to a point displaced from Earth). These approaches, and arguments over the reality of the celestial orbs, continued to be refined in the following centuries by first Muslim and then Christian astronomers. The geocentric cosmos was readily appropriated by Christians, many of whom believed that God existed outside the stellar sphere and (as evidenced by Dante's fourteenth-century poem Divine Comedy ) that angels turned the epicycles and spheres in the intervening spaces.

Astronomy had a primarily practical function, and by the thirteenth century it played a central role in determining the dates of religious festivals. Ptolemy's mathematically sophisticated Almagest, with various commentaries, formed the basis for advanced astronomy in university curricula from this period onward. In the late fifteenth century, the arrival of the printing press coincided with the transmission to western Europe of a number of important Greek mathematical and astronomical manuscripts. These became a new focal point for doing astronomy, as ancient texts could be seen and assessed in their original form (as opposed to being translated into Latin from Syriac or Arabic). Georg von Peuerbach (1423–1461) and his pupil Regiomontanus produced important works that formed the basis of astronomical training for the next century.

Astronomy was equally significant for providing data from which astrological predictions could be made (judicial astrology). The effects that heavenly actions had on earthly events, and the manner in which they were carried out, constituted a problem for most human societies. Astrology was a major part of both daily life and intellectual culture, and was deeply implicated in politics, medicine, and agriculture. However, its use was increasingly questioned by elites in the late sixteenth century; both Catholics and Protestants argued that the notion that the relations between stars and planets influenced or governed human behavior detracted from the primary Christian belief in free will. Nevertheless, this argument was often expressed as a condemnation of bad astrology, the implication being that astrology was a credible art or science that could be reformed. However, although it was still taken seriously by Johannes Kepler at the beginning of the seventeenth century, it was almost universally despised by social elites a hundred years later.

In Aristotle's De Caelo, heavenly bodies were treated as physical objects and thus as a branch of natural philosophy, or physics. Astronomy thus enjoyed a somewhat bifurcated position within the university system, taught partly in tandem with medicine and astrology, and partly as a higher-level, more technically difficult mathematical enterprise. This mirrored the scholarly distinction between the treatment of the physical reality of the supposedly "crystalline" spheres and such ideas as the epicycles, the actual existence of which was debatable. When Nicolaus Copernicus (1473–1543) published his heliocentric (Sun-centered) De Revolutionibus Orbium Coelestium (On the revolutions of the heavenly orbs) of 1543, a number of astronomers believed that his system could be treated as another convenient set of devices for explaining the celestial phenomena without any commitment to the reality of the general cosmology and physics that was prominent in the first part of the book. Indeed, the Lutheran theologian Andreas Osiander (1498–1552) reassured readers in an anonymous preface to the work that Copernicus had merely devised "hypotheses" that facilitated the calculation of celestial positions.

THE COPERNICAN REVOLUTION

However, Copernicus had indeed upset the standard disciplinary division in universities, and he implied that a mathematical astronomer was entitled to speak about the physical nature of things—the traditional preserve of the more prestigious role of the natural philosopher. Not only did he offer the first modern heliocentric system (although he appealed to the authority of ancient heliocentric systems such as that of Pythagoras), but he asserted that the demonstrations in his work could only be understood by mathematically competent astronomers. He condemned the uncertainty of the calendar, and also of the inability of astronomers to determine the precise motions of the heavenly bodies. He lambasted the inconsistent use of homocentric circles and epicycles, and argued that the systems produced by recent astronomers lacked aesthetic credibility. Instead, with all the different techniques and philosophies in play, they had produced a sort of astronomical "monster."

Although he retained epicycles, Copernicus dispensed with the equant and showed that retrograde motion could be explained by means of a heliocentric system. Indeed, contra Osiander, he could hardly have been more assertive about the truth of his Sun-centered cosmos. He proffered tentative physical explanations of why objects on a revolving earth would not be ejected into the heavens, and argued that the stellar sphere had to be an enormous distance further away than that accepted by traditional astronomers. This meant that his inability to detect stellar parallax (the feature by which the apparent position of a star would vary depending on the time of year, since an observer on Earth would view it from a different position) would not be an argument against his system. Ominously, he also suggested that biblical passages that appeared to support geocentricity were the result of the author "accommodating" his discourse to the capacities of ordinary people. Momentous as it now seems, Copernicus's system had only a handful of adherents in the sixteenth century, although Erasmus Reinhold's Prutenic Tables of 1551, which were based on methods pioneered by Copernicus, were used to produce the Gregorian calendar (decreed by Pope Gregory XIII) in 1582.

In the last three decades of the sixteenth century, the Danish astronomer Tycho Brahe (1546–1601) was single-handedly responsible for two major initiatives in astronomy. First, he devised a "geoheliocentric" system that, for the half-century before Copenicus's system was broadly accepted, provided the major alternative to a geocentric cosmos. In this, the Sun again revolved around the Earth, with the five planets (not including the Moon) revolving around the Sun; since the Martian and solar orbits intersected, this system could not accommodate the reality of the spheres. Tycho rejected a heliocentric system, partly because he could not abide the massive distances required by Copernicus's system, partly because he could not find stellar parallax, and partly because he was strongly opposed to heliocentrism on scriptural grounds.

Most important, Tycho made a series of naked-eye observations, conducted from 1574 as part of a monumental observation program at his observatory (Uraniborg) on the island of Hven. He designed and made new instruments that were as much as ten times more accurate than those of his predecessors. He also conceived of a means of using each of his devices to cross-check results obtained from other instruments in his collection. His instruments were less cumbersome than previous examples and he introduced new techniques for more precisely dividing them into minutes and seconds of a degree. He turned himself and his workplace—which had its own printing press—into the hub of an extensive network of correspondents, and he used this and personal contacts to train a number of the best astronomers of the next generation. Two of his measurements, linked to exceptional celestial events in the 1570s, demonstrated the precision of his observations. First, he determined the extreme distance from Earth of the terrifying supernova of 1572, and second, he showed for the first time that the comet of 1577 existed beyond the lunar sphere and hence was technically part of the heavens. Independent of any implications of De Revolutionibus, both seemed to suggest that the celestial sphere could no longer be considered immutable.

As Tycho's work came to a close, Johannes Kepler (1571–1630) burst on to the stage with his heliocentric Mysterium Cosmographicum (The secret of the universe) of 1596. Kepler was trained at Tübingen by the pro-Copernican Michael Mästlin (1550–1631), and throughout his adult life remained committed to a heliocentric system. In the Mysterium he famously attempted to show that the distances between the planets could be represented by nested regular solids, although this ultimately failed to fit the astronomical data produced by Tycho and his colleagues. At the end of 1600 Kepler traveled to Prague to work with Tycho, who had only recently been appointed imperial mathematician to Rudolf II. When Tycho died within the year, Kepler gained access to his data concerning the orbit of Mars. Tycho had observed the Martian orbit with astonishing accuracy, finding a discrepancy between the observed orbit and an ideal circular trajectory of eight feet that could not be explained by instrument error, and Kepler sought strenuously over the next few years to provide a harmonious and geometrically satisfying orbit to fit these observations. After many different shapes had been tried, he decided that Mars and the other planets traveled in ellipses, one of whose foci was the Sun, and that each planet swept out equal areas in equal times (considering the area to be drawn out by a line linking the planet to the Sun).

Kepler published his first two laws of planetary motion in his Astronomia Nova (New astronomy) of 1609, although they are merely a handful of many mathematical relations that he posed for planetary orbits. In this work he also appealed to the magnetic philosophy devised by the English physicist William Gilbert in his De Magnete (On the magnet) of 1600, in order to explain the physical causes of heavenly motion. He suggested that the planets, which all traveled in the same direction and (virtually) in the same plane, were controlled by a motive force emanating from the Sun. His third and final law, first enunciated in his Harmonice Mundi (Harmony of the world) of 1619, is more complicated and states that the square of the mean orbital period of a planet is proportional to the cube of its mean distance from the Sun. Kepler combined a Platonic-Pythagorean concern for the reality of harmonies and ratios with a magnetic physics, but from a disciplinary point of view he was explicitly asserting the right of an astronomer such as himself to produce a "celestial physics" that gave the true (non-Aristotelian) causes of heavenly motion. In a work on astrology of 1601, he also attempted to uncover the physical causes underlying the influence that planetary conjunctions had on earthly activities.

THE SCIENTIFIC REVOLUTION

In 1609, the same year that Kepler published his first two laws, Galileo Galilei (1564–1642) used a combination of lenses to look at the heavens, having heard of a similar invention that had been introduced the year before. Within weeks, he had deduced that the Moon had mountains and valleys (and was thus not perfectly smooth), had noted that the Milky Way was actually composed of numerous stars, and proposed that Jupiter possessed four satellites that revolved around it as the Moon revolved around Earth. For his extraordinary discoveries, which appeared in Sidereus Nuncius (Starry messenger) of 1610, he was rewarded with the position of court philosopher to his onetime pupil, Cosimo de' Medici (Cosimo II), grand duke of Tuscany. Galileo was now a court intellectual to rival Johannes Kepler, who had succeeded Tycho Brahe as imperial mathematician to Rudolf II. Given the courtly locations of both Kepler and Galileo, twentieth- and twenty-first-century historians have pointed to the prevalence of nonuniversity locations as the most important settings for innovative astronomy. Kepler's friendly stance toward Galileo (he wrote a book praising Galileo's discoveries within months of the work appearing) ensured that there was minimal animosity between them, although this may have been helped by Galileo's apparent complete ignorance of Kepler's own discoveries.

In the next two and a half decades, adherence to the Copernican system met with determined opposition and, notoriously, René Descartes felt obliged to suppress his not sufficiently anti-Copernican Le monde (The world) in 1633 soon after Galileo's pro-Copernican Dialogue had been condemned by the Roman Inquisition in July. However, the vast majority of astronomers and natural philosophers were avowed Copernicans by the middle of the seventeenth century, and even those, such as the Jesuits, who were slow to accept the Copernican worldview, made substantial contributions to astronomy. Indeed, Galileo's successes can be overstated as contributions to the demise of the notion of a perfect celestial sphere, for previous events and observations had begun to shake confidence in heavenly incorrigibility. Of all his discoveries, only his sightings of the phases of Venus provided overt support for Copernicus's system, although Tycho's system could also account for them. Nevertheless, Galileo's visually striking discoveries—and their implications—were easily understood by a new audience of scholars and gentlemen, and they had a dramatic impact on writers and poets such as John Donne (1572–1631).

It is important to note, as Allan Chapman has properly argued, that the great theoretical advances in cosmology achieved by Kepler, Isaac Newton, and others were facilitated and indeed made possible by improvements in angular astronomical measurement and not by superior visual acuity. Tycho's advances in instrument design and observational accuracy were made possible by the cadre of excellent craftsmen he had at his disposal, and similarly, John Flamsteed, the first British astronomer royal, whose observations were to prove crucial for Newton's enunciation of universal gravitation, had innovative and highly skilled instrument makers working with him. Only with patient astronomy of this sort could precise measurements be made of celestial magnitudes such as distance and size.

In the 1660s telescopic sights were added to quadrants and a zenith sector in the attempt by the French to measure the length of a degree of meridian in France. In Restoration England a number of episodes occurred that acted as a spur to creating an alliance between accurate measurement and theoretical innovation. The Royal Society of London was founded in 1660, followed by the Royal Greenwich Observatory—founded to aid navigation and the determination of longitude by improving astronomy—in 1675. The Observatory was badly stocked with instruments at first, but gradually, with some private support and with the help of occasionally brilliant suggestions from Robert Hooke (1635–1703) for automating observations, Flamsteed was able to build up a stock of the best instruments then available. By the 1690s he had a degree clock that allowed star positions to be measured with extraordinary accuracy, and the ten-to-twelveseconds error of his mural arc (that is, a large quadrant set on a wall) was a sixfold improvement on the accuracy of Tycho's instruments. Flamsteed used telescopic sights on his instruments but also a filar micrometer, that is, a system of thin wires, minutely movable by means of a carefully graduated screw, that could be placed inside a telescope to finesse its accuracy.

Isaac Newton (1642–1727) developed an early interest in astronomy and became famous initially because of his development of a reflecting telescope in the late 1660s. Flamsteed's data was crucial for Newton in the winter of 1684–1685, when the latter was trying to determine what mutual influence Jupiter and Saturn might have on each other, and again in late 1685, when Newton wanted three items of data (accurate to a minute) on the path of the Great Comet of 1680–1681. This data would constitute crucial evidence for the cosmological system that Newton published in his momentous Principia Mathematica (The mathematical principles of natural philosophy) of 1687, for he could now analyze observed deviations from perfect elliptical orbits by means of his concept of universal gravitation. Perhaps of equal significance were the observations Flamsteed put his way in 1694–1695 when Newton had another go at the Moon. This ultimately unprofitable endeavor was part of an effort to solve the (insoluble) three-body problem of the mutual interactions of Sun, Moon, and Earth, all of which Newton later described as the most difficult science he ever did. The pair fell out irreconcilably soon after this, and Newton behaved abominably toward the astronomer royal, practically stealing Flamsteed's laboriously crafted star catalog by claiming it as the property of the state. Not the least of Newton's actions was to downgrade and even efface (in his Principia ) the contributions made by Flamsteed, who had generously provided the observations that allowed Newton to corroborate and then rework his supreme theory. Whatever his personal dealings with others, Newton's theory provided the basic theory of the heavens that we now take to be true, and his achievements included the recognition that some comets travel in periodic elliptical orbits.

By the early eighteenth century, the London instrument-making trade was widely held to produce the highest quality instruments; Pierre-Louis Moreau de Maupertuis (1698–1759), for example, took a zenith sector and clock constructed by the outstanding London instrument maker George Graham (1673–1751) to Lapland in 1736–1737. It was this expedition that went furthest in determining the shape of Earth, confirming Newton's calculation that it was an oblate spheroid (flattened at the poles). In England, Graham and others made instruments for the astronomers royal who followed Flamsteed, namely Edmond Halley (in 1720) and James Bradley (in 1742). Bradley, who discovered stellar aberration in 1727 and who confirmed Newton's analysis of the extent of the nutation of Earth's axis in the 1740s, combined access to the best instruments of the day with an obsession for accuracy. By the middle of the eighteenth century, measurements were confined to the meridian, and, among other activities, experiments were being undertaken to better ascertain longitude and latitude—an activity seen by the British, the French, and many other naval powers as essential for improving navigation. With massively expensive instrumentation that only large institutions could afford, astronomy had changed beyond all recognition from the medieval period. Religious and other value systems no longer placed barriers on believing in and publishing particular accounts of the cosmos, and all serious intellectuals were heliocentrists.

See also Aristotelianism ; Astrology ; Brahe, Tycho ; Calendar ; Copernicus, Nicolaus ; Cosmology ; Descartes, René ; Galileo Galilei ; Kepler, Johannes ; Newton, Isaac ; Scientific Instruments ; Scientific Revolution .

BIBLIOGRAPHY

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.

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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