According to Pliny the Elder's Natural History 18.210–212, Julius Caesar consulted him while he was designing the Julian calendar.
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 brought the separate years back into conformity with the course of the sun.
[3]Sosigenes is credited with work on the orbit of Mercury, which is described by Pliny in book 2, Natural History: 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.
The misattribution may be from the fusion of two pieces of news about the reform of the Roman calendar at the initiative of Julius Caesar, namely: the indication by Appian,[6] Cassius Dio[7] and Macrobius[8] that the reform was based on "Egyptian teachings" together with the news from Pliny that Sosigenes was one of the experts consulted by Caesar for the adaptation of the calendar.