Callippus

Callippus (or Calippus) (c. 370 BC–c. 300 BC) was a Greek astronomer and mathematician.

He was born at Cyzicus, and studied under Eudoxus of Cnidus at the Academy of Plato. He also worked with Aristotle at the Lyceum, which means that he was active in Athens prior to Aristotle's death in 322. He observed the movements of the planets and attempted to use Eudoxus' scheme of connected spheres to account for their movements. However he found that 27 spheres was insufficient to account for the planetary movements, and so he added seven more for a total of 34. According to the description in Aristotle's Metaphysics (XII.8), he added two spheres for the Sun, two for the Moon, and one each for Mercury, Venus, and Mars.

He also followed up on the work done by Meton of Athens to measure the length of the year and construct an accurate lunisolar calendar. He also made careful measurements of the lengths of the seasons, finding them to be 94 days, 92 days, 89 days, and 90 days. This variation in the seasons implies a variation in the speed of the Sun, called the solar anomaly. He used this to improve the Metonic cycle in which 19 tropical years = 235 synodic months. The Callippic cycle is a 76-year eclipse cycle which synchronizes the lunar and solar years. This was adopted in 330 B.C. and subsequently used by later astronomers.

Calippus crater on the Moon is named for him.

Reference

Kieffer, John S. "Callippus." Dictionary of Scientific Biography 3:21-22.

External links

John J. O'Connor and Edmund F. Robertson. [] at the MacTutor History of Mathematics archive.

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