Astronomy: Greek Astronomy

I give here a summary of the contributions of the Greek philosophers that we know about through writings that were translated to Arabic and eventually re-translated to other languages long after the original writings perished. This account is undoubtedly patchy, and, we must assume, biased in unknown ways. The Greeks nevertheless laid down patterns of thought that we follow to this day. The Greek universe was an Earth-centered one, and the sun, moon, stars, and planets moved around the earth on great spheres.

• Thales: The first of the Greek philosophers, c. 629-555 B.C. He thought the planets, stars, sun, moon, and earth were all composed of different forms of water. He appears to be the first to assert that celestial bodies were not gods, but were made of familiar substance. In those mystical times, this was a radical departure.
• Pythagoras of Samos (c. 580-500 B.C.): Showed that nature could be described by mathematics. Described the musical scale in terms of number ratios and thought that the "spheres that carry the planets" must be arranged in ratios that would be musically harmonious. (Hence the phrase "music of the spheres".)
• Aristotle (c. 384-322 B.C.): Called "The Philospher" by the learned of the European Middle Ages and Renaissance, his ideas were considered as reliable (and sacrosanct) as Bible passages. His chemistry consisted of four elements, earth, air, water, and fire, plus a fifth element (quintessence) for the stars because the stars were believed to be immutable and unchanging. His physics stated that the natural state of motion of any thing was to be at rest, and that elements possessed a natural affinity for themselves. This latter idea became Aristotle's theory of gravity: earth attracts earth, so all solid bodies fall to earth, and a body that is twice as heavy will fall twice as fast to the earth. Likewise, water seeks the ocean, and fire attempts to fly to its natural home above the atmosphere. In cosmology, the Earth was a stationary sphere surrounded by other spheres that carried the planets and a final sphere upon which the immutable stars were fixed.
• Aristarchus (c. 310-230 B.C.): boldly attempted to find the relative sizes and distances of the moon, earth, and sun. (1) The earth-moon relative size was obtained from the curvature of the earth's shadow cast on the moon during lunar eclipses. (2) The earth-moon distance was found from trigonometry to be 25 earth diameters. (3) The sun's distance was obtained by trying to find when the moon was at exactly first quarter phase. At that point the earth-moon-sun angle must be 90 degrees exactly, and Aristarchus thought that this position differed from where the moon should be halfway beteen new and full by 3 degrees. If 3 degrees is accepted, the sun is 20 times further than the moon, and (4) also 20 times larger in diameter. Beyond these amazing attempts, Aristarchus also proposed in a serious way a Sun-centered (heliocentric) cosmology, motivated by the observation that the sun is much larger than the earth. A prediction of this model is that the stars are enormously distanct, since they cannot be seen to shift as the Earth swings in its wide arc around the sun.
• Eratosthenes (c. 273-195) Measured the size of the Earth by observing shadows cast by sticks at Alexandria (where he worked) and Syene far to the south. Eratosthenes thus gave absolute scale to the measurements of Aristarchus, whose results were all relative to the size of the Earth.
• Hipparchus (c. 194-120 B.C.): The first great observational astronomer compiled a catalog of positions and brightnesses of 1080 stars as well as the sun, moon, and planets that remained unparalled until the late 1500s. He discovered the effect of precession by comparing to older star charts. With his data on planet positions, he realized that simple motion around a sphere was comletely inadequate to explain the loopy motion of the planets among the stars and invented the "epicycle", a smaller sphere centered on the main sphere, on which the planet moved so that it was periodically carried backwards with respect to its usual direction.
• Ptolemy (c. 100-170 A.D.): the Great Compiler. Note that Ptolemy lived 300 years after Hipparchus. Ptolemy compiled a 13-volume summary of Greek astronomy, including star charts, texts on trigonometry, complete information on the Sun and moon (motion, sizes, distances), and, most significantly for later astronomy, an elaborate model for predicting the positions of the planets (using deferents and epicycles) for any time in the future. All 13 volumes were translated into Arabic and are known collectively by their Arabic title "Almagest".

Ptolemy's model for planetary motion, with deferents (big spheres) and epicycles (small spheres).

Summary

Aristotle	Proposed elementary laws of motion. Affirmed spherical, stationary earth and heavenly spheres on which the sun, moon, and planets moved. Laid down much of the basis for other branches of philosophical thought (e.g. logic, ethics). Was influential in Europe in the middle ages.
Aristarchus	Measured or estimated the sizes of earth, moon, and sun. Measured relative earth-moon distance (25 earth diameters) and earth-sun distance (20 times the earth-moon distance). Proposed a sun-centered cosmology (which was ignored).
Eratosthenes	Measured the absolute size of the earth. This gave absolute scale to Aristarchus's size measurements.
Hipparchus	Discovered "precession of the equinoxes". Made position measurements of 1080 stars plus sun, moon, and planets. This catalog remained the best in existence until Tycho Brahe. Invented the geometric devices used by Ptolemy because he realized that spheres-plus-uniform-motion did not reproduce the planetary position very well: the deferent/eccentric, equant, and the epicycle.
Ptolemy	Compiled previous work into a 13-volume reference, the Almagest. Refined and used Hipparchus's planet model to make tables of the future positions of the planets with errors usually less than 5 degrees.