Sosigenes of Alexandria was an astronomer and philosopher who lived in the 1st century BCE. He was a follower of the Peripatetic school, which continued the philosophical tradition of Aristotle. He is historically significant for two main reasons: his practical role in calendar reform and his theoretical critiques of ancient astronomy.

According to the Roman author Pliny the Elder, Sosigenes was one of the experts consulted by Julius Caesar for his calendar reform. This project resulted in the Julian calendar, implemented in 46 BCE. His more technical work survives through later summaries. The 6th-century philosopher Simplicius records that Sosigenes wrote a critical work, On Revolving Spheres, which is now lost. In it, he argued against the astronomical models of Eudoxus, Callippus, and Aristotle. According to modern scholars, his main criticism was that their systems of concentric spheres could not explain observable phenomena like changes in planetary brightness and retrograde motion.

Sosigenes therefore represents an important link in the history of science. Practically, he contributed to a calendar system that was used for over 1,600 years. Theoretically, his preserved critiques highlight the recognized problems with older astronomical models within the Aristotelian tradition. Academics suggest his work helped pave the way for the more complex and accurate Ptolemaic system that would dominate astronomy for centuries.