astronomer and mathematician employed by Julius Caesar to devise the Julian calendar. He is sometimes confused with Sosigenes the Peripatetic (fl. 2nd century AD), an Egyptian philosopher and tutor of the Greek philosopher Alexander of Aphrodisias.

At the conclusion of the Roman civil war (49–45 BC), Caesar (Caesar, Julius) set out to replace the multitude of inaccurate and diverse calendars of the Roman commonwealth with a single official calendar. At the suggestion of Sosigenes, on Jan. 1, 45 BC, he adopted the Julian calendar, with a 365-day year and an extra day every fourth year (leap year). With minor modifications this calendar is the same as the modern Gregorian calendar.

Sosigenes' astronomical writings, including Revolving Spheres, have all been lost, except for isolated fragments. One notable fragment mentions Sosigenes' belief that Mercury revolved around the Sun.