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(c.1st century bc) Egyptian astronomer

Nothing seems to be known about Sosigenes apart from his design of the Julian calendar. By the time of Julius Caesar the Roman calendar was hopelessly out of alignment with the seasons. The Romans had traditionally had a lunar 12-month calendar of 355 days. To bring it into phase with the solar year an intercalary (inserted) month of 27 days was supposed to be added every other year to a reduced February of 23 or 24 days. In theory this should have produced a year of 366¼ days, which would have proved inaccurate in the long run but should have been controllable by skipping an intercalation whenever the discrepancy became too uncomfortable. For whatever reason the practice had not been followed and to cure the confusion Caesar felt in need of expert foreign advice. He called in Sosigenes, an Alexandrian Greek from Egypt. To restore the situation to normal he introduced two intercalary months between November and December totaling 67 days and one of 27 days after February producing the famous year of ‘ultimate confusion’ of 445 days in 46 bc. To ensure that harmony would continue he introduced what was the basic Egyptian year of 365 days plus a leap year every four years. This is in fact eleven minutes too long but it is a tribute to Sosigenes that it lasted for 1500 years before being modified.

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