Callippus (/kəˈlɪp.əs/;Ancient Greek:Κάλλιππος,romanized: Kallippos; c. 370 BC – c. 300 BC) was aGreek astronomer andmathematician.
Callippus was born atCyzicus, and studied underEudoxus of Cnidus at theAcademy ofPlato. He also worked withAristotle at theLyceum, which means that he was active inAthens prior to Aristotle's death in 322 BC. 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 were insufficient to account for the planetary movements, and so he added seven more for a total of 34. According to the description in Aristotle'sMetaphysics (XII.8), he added two spheres for the Sun, two for the Moon, and one each for Mercury, Venus, and Mars.
Callippus made careful measurements of the lengths of the seasons, finding them (starting with the springequinox) 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 byMeton of Athens to measure the length of the year and construct an accuratelunisolar calendar. TheMetonic cycle has 19tropical years and 235synodic months in 6,940 days. TheCallippic cycle synchronizes days per orbit and rotations per orbit within the Metonic cycle, noting the difference of one after 4 Metonic cycles, a duration of 76 years. Distinguishing rotations and days infers knowledge of the precession cycle.
Callippus started his observation cycle on the summer solstice in 330 BC (28 June in theproleptic Julian calendar). The cycle's begin position, the stellar position and sidereal hour timing the eclipse, are used by later astronomers for calibrating their observations in relation to subsequent eclipses. The Callippic cycle of 76 years appears to be used in theAntikythera mechanism, an ancient astronomical mechanical clock and observational aide of the 2nd century BC (discovered in Mediterranean waters off Greece). The mechanism has a dial for the Callippic cycle and the 76 years are mentioned in the Greek text of the manual of this old device. The craterCalippus on theMoon is named after him.
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