2. Sequel to Aristotle's Astronomy
a) Drawbacks to the system. - Despite its undeniable ingenuity, the astronomical system devised by Eudoxus and his disciple Aristotle did not wholly succeed in saving the appearances. There were, it was soon realized, certain variations in the movement and appearance of celestial bodies that it could not account for. Specifically, the apparent variation in the diameter of the planets seemed to indicate a variation in their distance from the earth. Also, closer observation revealed certain regressions in the planetary orbits, and these regressive motions could scarcely be accommodated to the original theory. Subsequent astronomers coped with these problems.
b) Ptolemy and the astronomy of epicycles and eccentric rotations. - Foremost among the astronomers that followed the classical Greeks were Hipparchus (second century B.C.) and Ptolemy (second century A.D.), author of the famous Almagest, long to remain the standard reference on the subject of astronomy. They concluded, as did other astronomers of the time, that some of the Eudoxian postulates had to be given up, but not the fundamental of uniform, circular motions. These motions were therefore retained, but the earth was no longer considered the precise center of their rotations. Instead, a system of epicycles and eccentrics was devised, in substance as follows:
- the celestial bodies were thought to be carried each by a small circle whose center was fixed on the circumference of another moving circle, called the deferent: this was the system of epicycles.
- the celestial bodies continued to rotate around the earth, but the earth was no longer placed at their geometric center: this was the system of eccentrics or eccentric rotations.
These improvements made it possible to give a more satisfactory account of the irregularities in the planetary movements. They also ensured the continued acceptance of an astronomy based on uniform, circular motions, an acceptance that lasted into modern times. For the record, though, it should be mentioned that at the beginning of the medieval cultural renaissance allegiance wavers for a spell between Aristotle's version and Ptolemy's. St. Thomas bears witness to this state of mind; he knew both theories but sided with neither. From the end of the thirteenth century, however, the hesitancy gives way; Aristotle's system gradually yields to the mechanically superior Almagest.
3. Copernicus and Modern Astronomy
One of the principal differences between ancient and modern astronomy results from the substitution of heliocentricism for geocentricism. No longer is the earth the astronomical center around which the universe moves. Instead, the earth and all the planets revolve around the sun. Heliocentricism was not, however, a uniquely modern discovery. It had already been proposed among the Greeks by Aristarchus (third century B.c.); and even earlier Philolaus, a contemporary of Socrates, had thought of the earth as rotating, not indeed around the sun, but still around a central fire. Yet, there is no denying that until the Renaissance the theory which places the earth at rest at the center of the universe was almost universally upheld. How this centuried theory was finally supplanted is a study in itself. Perhaps the most complete presentation of this great scientific revolution is found in Pierre Duhem's monumental Le systeme du monde.8 According to Duhem the beginnings of the new astronomy are already discernible in the fourteenth century, among the Nominalists at the University of Paris. There, to mention the more notable, Albert of Saxony, John Buridan, and Nicholas Oresme laid the foundations of a system of mechanics altogether different from Aristotle's. Especially significant was their repudiation of the ancient theory of projectile-propulsion by surrounding air. For, instead of explaining, as heretofore, the motion of celestial and earthly bodies by different mechanical principles, it permitted them to account for both types of motion by a single system; which is to say they were able to combine into one what in the old theory had been two distinct systems of motion, celestial and terrestrial mechanics. Nicholas Oresme, moreover, clearly propounded the theory of the earth's diurnal movement.
Once begun, the new science moved ceaselessly on, its next phase looming up in the Italian Renaissance, thanks above all to names like Girolamo Cardano (Jerome Cardan: 1501-1576) and (as who doesn't know? ) Leonardo da Vinci (1452-1519). Still another phase dawns with Copernicus, first of that eminent line of astronomical scientists culminating in Issac Newton, founder himself of the system that was to rule virtually unaltered and unchallenged to the present time. That the physics of relativity has had to revise even Newton does not in the least detract from the grandeur of his achievement.
Truly, the development of modern astronomy is a renowned chapter in the history of science. Herewith, barely sketched, are the high lights and foremost representatives.
COPERNICUS ( 1472-1543 ) . - His De revolutionibus orbium caelestium (On the Revolutions of the Celestial Bodies) was published in the year of his death. In a preface marked by fine discretion he states that his astronomical theories should not be taken for more than they are, namely, a mathematical description. The earth, he says, rotates on its axis, and it also revolves around the sun, as do the other planets. But Copernicus still holds to uniform, circular motion, a circumstance that prevented his eliminating the system of eccentrics and epicycles.
TYCHO BRAHE (1546-1601 . - Brahe proposed a theory that incorporated some aspects of heliocentrism while retaining some features of the traditional astronomy. Thus, the earth is still at the center of the world, and the sun moves round the earth, but the other planets move round the sun. His real contribution, as a matter of fact, lay not so much in the discovery of new theories as in the wealth of his observations, which by their precision paved the way for future progress.
KEPLER (1571-1630) . - His principal discovery was the elliptical movement of the planet Mars. On the basis of this fact and after much computation he set down his three famous planetary laws: 1) A planet travels in an elliptical orbit, with the sun in one of its foci. 2) Its rate of travel is such that the "radius vector," the line joining it to the sun, covers equal areas in equal times. 3) The squares of the periods of planetary revolutions are in the same ratio as the cubes of their mean distances from the sun (the mean distance of a planet from the sun is half the major axis of its elliptical orbit).
GALILEO (1564-1642) . - His fame includes numerous works on the motion of bodies. He was also one of the first, if not the first, to construct a telescope, an invention that enabled him to discover Jupiter's satellites. In his Dialogo dei due massime sisteini del mondo (Dialogue on the Two Great World Systems) 4 he came to the defense of the Corpernican theory. In consequence of this - side issues in which he was not altogether blameless were also involved - he fell afoul of the Holy Office, incurring its condemnation in 1633.
ISAAC NEWTON (1642-1727) . - Newton's monument is his Philosophiae naturalis principia mathematica (Mathematical Principles of Natural Philosophy). Through the discovery of the law of universal gravitation Newton succeeded in organizing the new conceptions of the universe into a coherent system. The result was the Principia, which, interestingly, he still deems a study in "natural philosophy."