Sosigenes of Alexandria

Greek astronomer and mathematician, probably from Alexandria, employed by Julius Caesar to devise the Julian calendar. He is known as the main astronomer who helped Julius Caesar with his reform of the Roman lunar calendar, although his role in this reform is not very clear.  Plutarch simply states, without mentioning any names, that Caesar consulted the best philosophers and mathematicians before making an improved calendar of his own. The Romans had traditionally had a lunar 12-month calendar of 355 days. To bring it into phase with the solar year an intercalary (inserted) month of 27 days was supposed to be added every other year to a reduced February of 23 or 24 days. In theory, this should have produced a year of 366¼ days, which would have proved inaccurate in the long run but should have been controllable by skipping intercalation whenever the discrepancy became too uncomfortable. Caesar’s adoption of the 365–l/4–day a solar year may have been one result of Sosigenes’advice, and the stateman’s seasonal calendar another. The 365–1/4–day year could even have been borrowed directly from Callippus at the suggestion of Sosigenes. All that Pliny says in this connection, however, is that during Caesar’s dictatorship Sosigenes helped him to bring the years back into conformity with the sun (Naturalis Historia 18.211). He adds (Naturalis Historia 18.212) that Sosigenes wrote three treatises, including corrections of his own statements.

Sosigenes is a lunar impact crater on the west edge of Mare Tranquillitatis. Its diameter is 17 km. It was named after ancient Greek astronomer Sosigenes of Alexandria.^[1] It lies to the east of the large walled plain Julius Caesar. The crater rim has a high albedo, making it relatively bright. It has a small central rise at the midpoint of the floor.

To the east on the mare is a formation of parallel rilles designated the Rimae Sosigenes. These follow a course to the north and have a length of about 150 kilometers. The small, bowl-shaped crater Sosignes A lies across one of these rilles.