Sosigenes

Sosigenes 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 various reforms had been proposed around this time for the Egyptian administrative calendar. Like the Julian calendar, this had a standard year of 365 days divided into 12 months, but lacked a leap year.

In Pliny book 18, 210-212, appears (translation from [1]):

"... 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, appears (a very old translation from [1]):

"Next upon it, but nothing of that bignesse and powerfull 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."

I.e., Sosigenes is credited with work on the orbit of Mercury.

According to the 1911 Encyclopedia Britannica, there was another Sosigenes who was a Peripatetic philosopher living at the end of the 2nd century A.D. 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.