Sosigenes

Sosigenes of Alexandria was an ancient Greek astronomer who is traditionally identified as an adviser to Julius Caesar on the reform of the Roman calendar. Labeled as coming from Alexandria, he is described in some sources as a leading astronomer of the era, though the details of his life are not well documented.

Sosigenes is credited with proposing a solar calendar in which the year would consist of 365 days, with a leap day added at four-year intervals to approximate the solar year. According to Roman historians, Caesar entrusted Sosigenes with the task of producing a calendar that would align the calendar year with the seasons, moving away from the previous lunar-based system.

The reforms resulting from Sosigenes’ proposal were implemented in the Julian calendar, introduced in 46 BCE and taking effect in 45 BCE. The system used a 365-day year with a leap year every four years, producing an average year of 365.25 days. The exact influence of Sosigenes versus other advisers is a matter of scholarly debate, and early sources differ on the extent of his contribution. Some antiquarian accounts emphasize his central role, while others describe a broader Caesarian consultation process.

The Julian calendar remained in use throughout the Roman world and beyond for many centuries, eventually falling short of the true solar year. In 1582, the Gregorian reform adjusted the leap year pattern to correct the drift over long periods. Sosigenes’ legacy lies in his association with the transition from a lunar- to a solar-based calendar in ancient Rome and, by extension, in the long-term development of calendar systems in the Western world.