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Planetary Theories Before Copernicus
THE RECORDED HISTORY OF ASTRONOMY UNTIL THE SEVENTEENTH century is occupied not so much with discoveries of previously unknown phenomena as with a succession of attempts to systematize and to interpret certain facts of which the earliest recognition lies altogether beyond the horizon of history. The ancient and medieval schools of astronomy were concerned with celestial processes which had been conspicuous to unaided human perception from of old. For ages the stars had been observed to form permanent groups, or constellations, that wheeled in hourly and seasonal sequences across the vault of the night sky. The moon had been watched, waxing and waning on its monthly circuit through the central belt of the constellations, and the sun, varying its daily course, and rising and setting with different stars, according to the season of the year. Less conspicuous than sun or moon, but nevertheless distinguished from the stars since prehistoric times, were the planets, circulating slowly and erratically through the constellations. Add to these such occasional spectacles as eclipses and the visitations of comets, and we have before us all the principal phenomena with which astronomers were concerned prior to the invention of the telescope some three hundred and fifty years ago.
To the age-long quest for system and significance in these celestial phenomena a fresh turn was given in the sixteenth century by the astronomer Nicolaus Copernicus, whose contribution to solving the classic problem of the cosmos forms the central theme of this book. Copernicus inherited from antiquity a fully developed philosophy of nature which, on the one hand, conditioned his own novel approach to the problem, and, on the other, served as the basis for the long-continued opposition to his views. To his predecessors also he owed both the geometrical technique which he employed to represent the planetary motions and many of the recorded observations which he utilized to define the constants of his geometrical theory. Unless we know something of the history of astronomy in the ages before Copernicus, we shall scarcely follow his arguments and calculations or judge his achievements aright. Accordingly, we shall devote this opening chapter to a brief preparatory study of such developments in ancient and medieval cosmology as are relevant to what follows.
§ 1. ANTIQUITY
The mighty stream of modern astronomy can be traced back to the confluence of two tributaries derived from the contrasting civilizations of Babylonia and Greece. Even at the level of barbarism, practical life must have demanded of man some knowledge of the basic celestial phenomena, for direction-finding and for regulating the agricultural and ritual cycle of the year. However, conditions favorable to the germination of science are first found in the historic civilizations which arose on the alluvial lands of the great rivers of antiquity, notably in Mesopotamia.
The Babylonian priests at first observed the heavens in order to regulate their lunar calendar and to draw omens from all striking celestial or atmospheric phenomena. The stepped towers, or ziggurats, of the temples were their observatories; and through the stormy centuries of Babylonian history the temple schools were able to develop elaborate systems of star lore, since invaders generally spared the national shrines of the old gods of the land. During the past century a growing light has been shed upon the astronomical achievements of the Babylonians by the decipherment of the clay tablets upon which their observations, calculations, and ephemerides were recorded. In the earliest stage, observations were, for the most part, indiscriminate and lacked numerical precision, the phenomena being treated merely as portents of impending crises. The next stage, well established by the eighth century B.C., was characterized by dated records, numerical specifications, and estimates of the periods of recurrent celestial phenomena. The discovery of periodicity marked the beginning of scientific astronomy; it led, in the three centuries preceding the Christian era, to the highest level of Babylonian astronomical attainment, represented by the construction of ephemerides that served to predict celestial phenomena years in advance.
The Babylonians attached especial significance to the movements of the seven "planets," namely, the sun, the moon, and the bodies to which (following the Romans) we give the names of Mercury, Venus, Mars, Jupiter, and Saturn, and which we class as planets in the modern sense of the term. They associated or identified their chief gods with these bodies. The planetary phenomena to which they paid particular attention were (a) the heliacal risings and settings of a planet, when it was observed to rise before sunrise for the first time or to set after sunset for the last time (occasions which marked the limits of its period of extinction in the sun's rays); (b) the stationary points where a planet's course among the constellations was arrested and reversed; (c) oppositions, when the planet was in the opposite quarter of the sky to the sun; (d) conjunctions, when the planets appeared to pass close to one another or to bright stars; and (e) eclipses of the sun or moon.
The phenomenon of stationary points calls for some further explanation, because of the complications which it necessitated in planetary theories until the time of Copernicus. The sun and moon, in their periodic circuits round the heavens, travel continuously through the constellations from west by south to east; but the apparent motions of the five planets are more complicated. For example, if the planet Mars is observed in the southern sky night after night for some weeks, it will, in general, be found to be slowly moving from west to east in relation to the background of stars. (Such motion is, of course, to be distinguished from the apparent diurnal revolution about the earth, which the planet has in common with the stars.) At fairly regular intervals (about 780 days for Mars), however, the planet's eastward motion is arrested (its apparent path showing a stationary point) and reversed, and Mars moves from east to west through an arc of retrogression of about 15[degrees] before resuming its normal eastward motion. The same is true of the planets Jupiter and Saturn, although their arcs of retrogression are less considerable; the planets Mercury and Venus have the additional peculiarity that the angular distance of each from the sun never exceeds a moderate limiting value (about 25[degrees] for Mercury and 45[degrees] for Venus).
From at least the beginning of the fourth century B.C. the Babylonians were in the habit of defining the apparent positions of the planets in the sky by reference to a series of bright stars distributed fairly regularly round that belt of the heavens in which all the planets move, and which (following the Greeks) we term the zodiac. The place of a planet was defined by the specification of its angular separation from the standard star nearest to it. A scale of angular measurement was afforded by the division of the zodiac into twelve equal parts (the signs of the zodiac), with further subdivisions, and the system was gradually developed so that such measurements might be made both along the zodiac and at right angles to it. This system led eventually to the conception of the ecliptic—the great circle of the celestial sphere traversed by the sun in its annual circuit through the constellations. Thus, by the second century B.C., the celestial longitudes and latitudes of the planets and of other celestial bodies were being defined by reference to the circle of the ecliptic, much as we define the geographical longitudes and latitudes of places on the earth by reference to the terrestrial equator.
By diligent observations, continued over several centuries, the Babylonians were able to arrive at remarkably accurate estimates of the principal time periods associated with the heavenly bodies—the year, the several kinds of months, the sidereal period of each planet (in which, on the average, the planet performs a complete circuit of the heavens relative to the background of stars), and its synodic period (the time for a complete circuit relative to the sun). By the second century B.C., the Babylonians had arrived at an estimate of the sidereal year: 365 days, 6 hours, 13 minutes, 43.4 seconds, only about four and a half minutes in excess of the modern estimate for that age (F. X. Kugler: Sternkunde und Sterndienst in Babel, II, 8). And about the fourth century B.C. they had already discovered that lunar eclipses form sequences which recur periodically at intervals of about eighteen years. In their intricate ephemerides of the sun, moon, and planets, the Babylonians made allowance for the principal periodic non-uniformities observable in the apparent motions of these bodies through the constellations.
It is difficult to give in a few words any idea of the refinement and complexity of the Babylonian methods. Tables for showing the dates of successive new moons (second century B.C.) included columns of corrections for the yearly inequality in the sun's motion and for the monthly inequality in the moon's motion; the amounts of the corrections for successive months oscillated in a regular manner between maximum and minimum values. As regards the remaining five planets, the Babylonians took account not only of the element of inequality in the motion of each which depends upon its position in relation to the sun (and which we now know to be due to the earth's motion) but also of the further element, complicating the first, which depends upon the planet's position in the zodiac (and which results from the planet's elliptic motion). They represented this latter inequality by assigning to the planet, as it moved round the zodiac, a succession of rates of angular motion, which varied between maximum and minimum values and recurred in the appropriate period.
Thus, in contrast to the geometrical and physical astronomy with which we shall be concerned in the following pages, Babylonian planetary theory, in its classic form, is represented merely by tables in which the motions, past and future, of the heavenly bodies are numerically formalized in complete abstraction from any physical or even geometrical conceptions of the cosmos.
It is unlikely, in fact, that the Babylonians ever arrived at any clear-cut, objective conception of the constitution of the universe as a whole, such as we encounter in Greek philosophy. It is possible, however, to piece together a composite picture of the cosmology which must have formed the background of Babylonian thought, at least throughout the historic period. The earth was conceived to be roughly circular in contour, rising toward the center to form a huge mountain, and resting upon a great ocean which girdled the land with a moat of sea; beyond this rose a circular mountain wall, forming the boundary of the world and supporting the hemispherical vault of heaven, or firmament. The heavenly bodies seem to have been regarded as moving freely through space.
The earliest extant Greek literature—the poems of Homer and Hesiod—presupposes a system of the world closely resembling that of the Babylonians. But Greek cosmology soon came to reflect the sharp contrast existing between Babylonian civilization and Greek society prior to the Alexandrian period. Even in their homeland the Greeks were never subject to a conservative and centralized priesthood; and in any case population pressure drove many of them to found colonies overseas, particularly in Asia Minor and in southern Italy. In the Ionian trading cities of the Asian coast there flourished, during the sixth and fifth centuries before Christ, the earliest known schools of philosophy. These schools established the conception of nature as an orderly system the constitution and phenomena of which were not to be attributed to supernatural agencies but were to be rationally deduced as consequences of the inherent properties of the one or more primary substances of which the entire universe was held to be composed. The Ionian philosophers, in fact, interpreted cosmic phenomena on the analogy of the natural processes with which they were familiar through the technical arts of their own largely industrial society. They elaborated a succession of naturalistic, if somewhat crude and speculative, cosmological systems, in which the earth generally figured as a disc, or a vortical condensation, floating at the center of the universe. The conception, attributed to Anaximenes of Miletus, of a rotating crystal sphere to which the stars are attached like silver studs persisted from the sixth century B.C. to the end of the sixteenth century A.D.
Meanwhile, at another outpost of Greek civilization in southern Italy, Pythagoras and his school were establishing a scheme of geometrical abstractions in terms of which celestial processes would continue to be conceived for upwards of two thousand years. To Pythagoras himself (sixth century B.C.) is credibly attributed the earliest declaration that the earth is a sphere resting at the center of a spherical universe, and not a floating disc, as the Ionians taught. Pythagoras seems also to have been aware (probably following the Babylonians in this) that the complicated apparent motion of the sun in the course of the year is, to all appearance, made up of two simple motions—(a) the motion common to all the heavenly bodies, whereby they appear to revolve about an axis through the earth once in a day, and (b) a motion peculiar to the sun, in a contrary direction to the first motion, and taking place about a different axis, in the space of one year. The first of these motions would account for the daily rising and setting of the sun; the second would account for the sun's annual circuit among the constellations and for the seasonal fluctuations in its rising and setting points and in its meridian altitude. This analysis seems also to have been extended by Pythagoras (although with less eligibility) to the apparent motions of the moon and planets; it gave rise to the idea that the complicated movements of the heavenly bodies could all be resolved into uniform circular motions. This doctrine was established by the authority of Plato, Aristotle, and Ptolemy; as we shall see, it still dominated astronomy in the time of Copernicus, two thousand years after Pythagoras, and it was first formally abandoned by Kepler at the beginning of the seventeenth century.
By the end of the fifth century B.C., the Pythagorean school had evolved the remarkable system of cosmology associated with the name of Philolaus. To this hypothesis Copernicus directed especial attention, for it was the earliest historic system to displace the earth from the center of the universe and to set it in revolution about the center like any other planet. According to Philolaus, the finite sphere within which the universe was contained had fire at its center and fire at its circumference. It was divided by concentric spheres into three layers. The outermost of these contained the stars. The intermediate layer contained the five planets, the sun, and the moon, in order of approach to the Central Fire, about which these bodies all revolved in circles in the several periods of their circuits round the heavens. Lastly, within the sphere which formed the core of the universe was the earth, which itself revolved daily about the Central Fire, turning toward it the hemisphere opposite to that inhabited by mankind, who, in consequence, could never behold the Fire. By making the earth revolve in a plane inclined to that in which the other planets moved, Philolaus was able to account not only for the risings and settings of the heavenly bodies but also for all seasonal phenomena now attributed to the inclination of the earth's equator to the ecliptic.
The somewhat fanciful system of Philolaus never established itself; but the primitive Pythagorean cosmology lived on in the natural philosophy of Plato (427–347 B.C.), who conceived the universe as a rotating sphere in the midst of a boundless void, and the earth as a stationary sphere in the midst of the universe. In picturesque allegories in the Timaeus and the Republic, it is easy to recognize Plato's expressions of the principle that the motion of each of the seven planets is compounded of two uniform revolutions about the earth which take place about different axes and in opposite senses, and one of which is common to all the heavenly bodies. It was obvious, however, that this simple Pythagorean conception of a planet's motion took no account of the recurrent retrogressions of the planets, to which we have already alluded, or of their departures from the ecliptic. It was probably with a view to remedying this defect in the current theory that, according to Simplicius, Plato set the astronomers of his time the general problem of adequately representing the observed movements of the heavenly bodies by combinations of uniform circular motions having a common center in the earth.
Excerpted from COPERNICUS and Modern Astronomy by ANGUS ARMITAGE. Copyright © 2004 Dover Publications, Inc.. Excerpted by permission of Dover Publications, Inc..
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