Sosigenes of Alexandria was named by Pliny the Elder as the astronomer consulted by Julius Caesar for the design of the Julian calendar.
Little is known about him apart from Pliny's Natural History. Sosigenes appears in Book 18, 210-212:
- "... There were three main schools, the Chaldaean, the Egyptian, and the Greek; and to these a fourth was added in our country by Caesar during his dictatorship, who with the assistance of the learned astronomer Sosigenes (Sosigene perito scientiae eius adhibito) brought the separate years back into conformity with the course of the sun."
In Book 2, chapter 6, Sosigenes is credited with work on the orbit of Mercury:
- "The star next to Venus is Mercury, by some called Apollo; it has a similar orbit, but is by no means similar in magnitude or power. It travels in a lower circle, with a revolution nine days quicker, shining sometimes before sunrise and sometimes after sunset, but according to Cidenas and Sosigenes never more than 22 degrees away from the sun."
Some sources state that the Julian calendar was designed by Aristarchus of Samos; it is not clear where this originates, although a reform of the Egyptian calendar was decreed by Ptolemy III Euergetes in 238 BC, but never implemented.
Sosigenes was portrayed by Hume Cronyn in the 1963 movie Cleopatra.