Callippic cycle
About the Callippic cycle
Callippus of Cyzicus improved the Metonic cycle by taking four 19 year cycles, 76 years, and dropping one day, giving 940 months and 27,759 days exactly. Ptolemy dated observations by it, counting from 330 BCE. Its mean month of 29.530851 days errs by only about half a minute, remarkable for the 4th century BCE.
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