Brian's Biographies of Famous Astronomers

"Callipus of Cyzicus"

(Circa 370-300 BC)

Born at Cyzicus in Asia Minor, the dates given for his birth and death are guesses but having studied with Polemarchus, Eudoxus' pupil, he followed him to Athens in 330 BC, and dwelt with Aristotle(d, 322 BC). In Athens with Aristotle's help he corrected and completed the discoveries of Eudoxus. He made his astronomical observations on the shores of the Hellespont - this has been deduced from the observations themselves.

As a result of his observations of the the movements of the planets he attempted to use Eudoxus' scheme of connected spheres to account for their movements but he deduced that 27 spheres was insufficient to account for the observed movements. He introduced seven more spheres to make a total of 34, according to Aristotle's "Metaphysics" two were added for the sun, two for the moon and one each for Mercury, Venus, and Mars. The addition of these spheres over Eudoxus' 27 certainly increased the accuracy of the theory while still holding to the belief that the heavenly bodies had to follow 'perfect' circular paths. This would replace the hippopede of Eudoxus with an even more complex figure.

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He was probably the foremost astronomer of his day and formed what has become what is known as the Callippic period. Following up on the work of Meton of Athens, he carefully measured the lengths of the seasons and starting with the spring equinox he calculated them as 94, 92, 89 and 90 days. This fluctuation implied an apparent variation in the speed of the Sun, called the solar anomaly. From these workings he constructed an accurate lunisolar calendar using a 76 year cycle comprising 940 months to harmonise the solar and lunar years. The Metonic cycle has 19 tropical years and 235 synodic months in 6940 days. Callippus synchronized the lunar and solar years better than Meton by dropping 1 day after 4 Metonic cycles. This gave Callipus' 19 year cycle a length equal to 6439 3/4 days, or exactly 365 1/4 days per year, nearly 3 centuries before Julius Caesar. The month error of Meton was 1.9 minutes whereas that of Callipus was only 22 seconds.

His cycle was adopted in 330 BC and used by all later astronomers for dating observations. Calippus had his first cycle start at the summer solstice of 330 BC which was confirmed by Ptolemy in his "Almagest" saying that year 50 of the first cycle coincided with the 44th year following the death of Alexander. Although the system of concentric spheres eventually gave way to epicycles and eccentrics, Callippus' cycle became the standard for correlating observations accurately over many centuries, thus contributing to the accuracy of dating later astronomical observations. Calippus crater on the Moon is named for him.

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