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Sosigenes of Alexandria was named by Pliny the Elder as the astronomer consulted by Julius Caesar for the design of the Julian calendar. It appears that little or nothing is known about him apart from two references in Pliny's Natural History. Some web sources say that the calendar was designed by Aristarchus about 200 years earlier — it is not clear where this idea originates, although a similar reform of the Egyptian calendar was decreed by Ptolemy III Euergetes in 238 BC, but never implemented. The standard year of the Egyptian calendar had 365 days divided into 12 months plus five epagomenal days at the end of the year — the reform would have added a sixth epagomenal day.

He appears in Pliny 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."^[1]

In Pliny book 2, 8, Sosigenes is credited with work on the orbit of Mercury:

"Next upon it, but nothing of that bignesse and powerful efficacie, is the starre Mercurie, of some cleped Apollo: in an inferiour circle hee goeth, after the like manner, a swifter course by nine daies: shining sometimes before the sunne rising, otherwhiles after his setting, never farther distant from him than 23 degrees, as both the same Timæus and Sosigenes doe shew."^[2]

1. ^ translation from The Julian Calendar in Jesus and the Acceptable Year of the Lord by Ramond Soller
2. ^ a very old translation from C. Plinius Secundus The Historie of the World translated by Philemon Holland (1601)

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.