After the Presocratics, one of the earliest astronomers about whom we have any information is Meton, a contemporary of Socrates, who, through observations made at Athens, reconciled the solar and lunar calendars, noting that they realigned themselves every nineteen years. Such information was especially important in the regulation of the calendar. The Greeks mainly kept time according to the moon, but the agricultural year progressed according to the solar cycles (summer is summer, no matter what phase of the moon). Coordinating the solar and lunar cycles was a concern in many ancient societies.
Eudoxos of Knidos (390-340 b. c.e.) was the first Greek astronomer to construct a mathematical system to explain the apparent movements of the heavenly bodies: that of the Homocentric Spheres, discussed in a work entitled Peri Takhon. It was a great feat of mathematical evidence and, apparently, involved no observation whatsoever. Eudoxos believed that all heavenly movements were caused by the interactions of twenty-seven intertwined concentric spheres, with the Earth at the center. All the stars existed on the single, outermost sphere, and each of the planets made use of no fewer than four concentric spheres, which spun along different axes to produce the planets' apparently irregular courses. Likewise, the sun and moon were governed by three spheres each. All this complexity existed to explain the retrograde motion of the planets within a presumed geocentric system. Perhaps more influential were his descriptions of the constellations, including calendrical notices of their risings and settings.
Later, Eudoxos's theories were "corrected" by Callipus of Cyzicus (c. 330 b. c.e.), who added two more spheres for the sun and moon and one more for each of the planets. This did not simplify matters.
Following closely on the heels of Callipus was Aristarchus of Samos (c. 320 b. c.e.). Rather than further complicating the Homocentric Spheres theory, he derived a (semi-) heliocentric theory of astral movements, suggesting that the stars and sun were fixed in space and that the Earth revolved around the sun in a circle. A different means of explaining the "irrational" movements of the heavenly bodies was presented by the third-century mathematician Apollo-nios of Perge, who calculated the eccentric and epicyclical models of planetary motion. His work was to be of critical importance to the master of Greek astronomy, Ptolemy (see below).
Eratosthenes of Cyrene (276-197 b. c.e.), although best remembered for his work in mathematics (especially on the subject of prime numbers), is famous in astronomy/geometry for his extremely accurate measurement of the Earth's surface. To attain this, he compared the noon shadows in midsummer between Aswan and Alexandria in Egypt. He gave the Earth's circumference as 250,00 stadia, the distance to the sun as 804,000,000 stadia, and the distance to the moon as 780,000 stadia. Furthermore, he measured the tilt of the Earth's axis and attained a measurement of 23° 51'15. Finally, he compiled a catalogue of 675 stars. This is especially important because here, for the first time in centuries, the Greeks were learning the art of accurate observation from their eastern neighbors (fortunately, the Greek astronomers were willing to accept the perceptions of their senses).
The pinnacle of astronomy in ancient Greece was reached under Hipparchus of Nicaea (190-126 b. c.e.) and Claudius Ptolemy, who lived in Egypt in the mid-second century c. e. (although this late date rather puts him out of the scope of this book, he is included for the sake of completeness). Hipparchus was the first Greek to construct a full theory of the motion of the sun and moon that was properly based on observational data. He combined the mathematical system of Apollonios of Perge and his own observations with the Babylonian eclipse record (dating back to the eighth century b. c.e.), and thus extracted accurate estimates of the mean motions of the sun and moon, as well as the length of the tropical year (which he put at 365.26 minus 1/300th days). He is famous for his discovery of the procession of the equinoxes, which he did by comparing his own observation of the distance of the star Spica at the autumnal equinox with that recorded 160 years earlier. Hipparchus noted that, in its annual movement, it took the sun a little more time to reach the same zodiacal point than to reach the equator: The sidereal year would thus be different from the solar year. Hipparchus interpreted this phenomenon as a very slow movement of the sphere of the stars, from west to east (about 1 degree per century, although the actual value is 1 degree, 23 minutes, 30 seconds). Furthermore, he studied the problem of parallax, and thus came to devise the first practical method for determining the sizes of and distances to the sun and moon (the latter he actually calculated correctly).
The work of Claudius Ptolemy has survived in a tome entitled the Almagest, an Arabic translation of O Magiste, the "Greatest" (see chapter 3 on Arabic preservations of Greek scientific texts). Based on his own observations, Ptolemy compiled a list of stars, giving their ecliptic coordinates and magnitudes. He constructed a theory of motion for the five known planets, for which he used the established theories of eccentric and epistyle motion. He showed that the heavenly bodies did not move in perfect circles, as suggested by Aristotle (see below), but in ellipses. This made it possible to predict the position of all known heavenly bodies at any given moment, all the details of the eclipses, the retrograde motions of the planets, and the appearance and disappearance of the planets and stars.
The Greek study of astronomy was based primarily on geometry, with little or no attention paid to physics or chemistry since the days of Anaxagoras. The Greek astronomers could say what was there and where things would be later, but they could not explain the composition of the stars or why the planets moved as they did. This was to be the concern of Renaissance and modern research.
World History
