There were several historical figures called Sosigenes:
Sosigenes of Alexandria
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, each of 30 days, plus five epagomenal days at the end of the year - the reform would have added a sixth epagomenal day every fourth year.
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."
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."
Sosigenes the Peripatetic
Sosigenes the Peripatetic was a philosopher living at the end of the 2nd century AD. He was the tutor of Alexander of Aphrodisias and wrote a work On Revolving Spheres, from which some important extracts have been preserved in Simplicius's commentary on Aristotle's De Caelo.
He criticized both Aristotle and Eudoxus, and their theory of celestial spheres and epicycles, which he felt was inconsistent with Aristotle's philosophical postulates. He also pointed out that the planets varied markedly in brightness, and that eclipses of the sun are sometimes total and sometimes annular suggesting that the distances between the sun, moon and earth was not the same at different eclipses.
Sosigenes is perhaps called "the Peripatetic" only because of his connection with Alexander. Some ancient evidence may be taken to suggest that he was, in fact, a Stoic. As John Patrick Lynch has written:
The other two teachers of Alexander may actually have been the philosophers whom ancient sources called Stoics; in both cases, Herminos/Sosigenes "the Stoic" have been distinguished from Herminos/Sosigenes "the Peripatetic" only on the grounds that the two latter men were teachers of Alexander of Aphrodisias. But it is not improbable that Alexander of Aphrodisias studied with two Stoic teachers and that these two pairs of homonymous contemporaries are actually only two Stoic philosophers.
He is often confused with the Roman astronomer Sosigenes of Alexandria.