Callippus

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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 made careful measurements of the lengths of the seasons, finding them (starting with the summer solstice) 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 also followed up on the work done by Meton of Athens to measure the length of the year and construct an accurate lunisolar calendar. The Metonic cycle has 19 tropical years and 235 synodic months in 6940 days. The Callippic cycle synchronizes the lunar and solar years better than the Metonic cycle does by dropping 1 day after 4 Metonic cycles, and therefore has a duration of 76 years. Calippus had his first cycle start at the summer solstice of 330 BC (28 June in the proleptic Julian calendar). These cycles were used by later astronomers for dating observations.

Calippus crater on the Moon is named for him.