Callippus of Cyzicus
Callippus (or Calippus) (circa 370 B.C.–circa 300 B.C.) was a Greek astronomer.
He was born at Cyzicus, and studied under Eudoxus. 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 26 spheres was insufficient to account for the planetary movements, and so he added eight more for a total of 34.
He also made careful measurements of the lengths of the seasons, and used this to build a 76-year cycle to synchronize the lunar and solar years. This was adopted in 330 BC and subsequently used by later astronomers.
In More detail
Callippus of Cyzicus ( c.370 BC – c.310 BC) was a Greek philosopher and a pupil of Eudoxus’ school of astronomy. His main achievement was an extension of Eudoxus’ homocentric sphere model which attempted to explain the motions of the Sun, Moon, planets and fixed stars in terms of a system of spheres centered on the Earth.
By introducing 7 new spheres to the 27 required in Eudoxus’ model, Callippus was able to achieve a level of accuracy for the planetary orbits that was as good as the available observations! The additional spheres added to the Sun accounted for the varying velocity of the Sun during the year – a notable failing of the earlier model.
The table below summarises the number of spheres required in Callippus’ model.
Callippus also made accurate measurements of the lengths of the seasons, and was able to bring the solar and lunar years into alignment through a Callippic period of 76 years. His cycle combined 441 × 29-day months (12,789 days) and 499 × 30-day months (14,970 days), giving a total of 27759 days. Dividing this by 76 years gave an estimate of the tropical year (the length of time between successive equinoxes) of 365.25 days, a duration that has been used ever since.
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