9. Conversion factors

Unlike modern astronomers, Ptolemy (c. AD 135) had the good fortune to access Babylonian observations without having to reconstitute the Babylonian calendar. Greeks had prolonged contact with the Ancient Near East, and the translation of foreign dates into a familiar calendar had been taken care of by Ptolemy's predecessors.

Ptolemy's astronomical work, the Almagest, is based on the Egyptian calendar made up of 12 months of 30 days plus 5 extra days. The year has exactly 365 days, which eliminates the hassle of sorting out leap years and leap months when totaling the number of days between astronomical events.