Acoreus

Acoreus was a wise man consulted by Julius Caesar, according to the Roman writer Lucan, asking him many questions about ancient Egypt’s history and its calendar. Caesar learned that the Egyptians based their year on the solar year, that is, on the apparent motion of the Sun through all of the zodiacal constellations, and that the Egyptians knew that such a year averaged 365 ¹/₄ days.

The Egyptians were the first people to base their calendar on the solar year, though in a form that caused it to wander through all the seasons, based on the annual flooding of the Nile. Their calendar year had just 365 days, organized into 12 months of 30 days each with five epagomenal days added to the end of each year. Although they realized that their year wandered, they did not attempt to stop it until 238 BC, when the pharaoh Ptolemy III attempted to add an extra day to every fourth year, but these leap days were apparently never implemented. He may have based his intercalation frequency on the Callipic cycle, which consisted of 76 solar years averaging 365¼ days each, invented about a century earlier by the Greek astronomer Callippus. Although that cycle was never used in any calendar, knowledge of it would have resided in the great library at Alexandria, established by Ptolemy II several decades earlier.

Julius Caesar based his calendar on this Egyptian knowledge of the average solar year supplied by Acoreus and Sosigenes.

ee also

*Julian calendar