Sosigenes
Sosigenes
6 min read
Sosigenes of Alexandria was a Greek astronomer and mathematician of the 1st century BC. He advised Julius Caesar on the reform of the Roman calendar, which led to the Julian calendar in 46 BC. His work introduced the 365-day year with a leap day every four years.
Frequently asked questions
Key Facts
- Greek astronomer active in Alexandria in the 1st century BC
- Advised Julius Caesar on the reform of the Roman calendar
- Devised the Julian calendar, which came into effect in 46 BC
- Set the year at 365 days with an intercalary day every four years
- Is said to have written several treatises on celestial phenomena mentioned by Pliny the Elder
Works & Achievements
Designed at Caesar's request, a solar calendar of 365 days that founded our way of counting time for more than sixteen centuries.
A system of one day added every four years to make up for the quarter-day of the solar year, the ancestor of February 29th.
An estimate of the length of the solar year, remarkably close to the true figure, the mathematical basis of the entire reform.
According to Pliny, writings in which Sosigenes revised his own calculations, proof of a scientific approach grounded in self-criticism.
Works of astronomy and meteorology now lost, known only through the references in the “Natural History.”
Anecdotes
Sosigenes is known almost entirely from a single source: Pliny the Elder, who cites him three times in his *Natural History*. We do not know his dates of birth and death, nor even his face: he is a famous scholar of whom Antiquity preserved almost nothing but his work.
When Julius Caesar decided to reform the calendar in 46 BC, the Roman year was in such disorder that the seasons no longer fell in the right months. To make up for the accumulated lag, Caesar and Sosigenes had to add nearly 90 extra days to that single year: 46 BC lasted 445 days and was nicknamed “the year of confusion.”
Sosigenes's brilliant idea was to abandon the lunar calendar and follow the Sun alone, fixing the year at 365 days and a quarter. To handle that quarter of a day, he proposed adding one day every four years: this is the ancestor of our February 29th, the leap day.
Pliny reports that Sosigenes himself acknowledged that his calculations were not perfect and later published corrections. He was right to be cautious: his year was 11 minutes too long, a tiny error which, accumulated over centuries, forced Pope Gregory XIII to reform the calendar in 1582.
Sosigenes probably came from Alexandria in Egypt, then the greatest centre of learning in the world, with its famous Library. The Egyptians had long been using a solar year of 365 days: Sosigenes drew on this Eastern knowledge to reform the calendar of Rome.
Primary Sources
There were three principal schools: that of the Chaldeans, that of the Egyptians, and that of the Greeks. To these a fourth was added among us, in the time of the dictator Caesar, who brought the years, which had fallen out of step, back into agreement with the observation of the Sun, under the direction of the learned Sosigenes.
Sosigenes himself, though more accurate than the others, nevertheless went on later to correct his own calculation through three successive treatises, the error persisting all the same.
Caesar submitted the problem to the best philosophers and mathematicians, and, drawing on the methods already tried and tested, devised his own corrected way of setting the calendar right.
Key Places
Great learned city of Egypt, presumed homeland of Sosigenes and a hub of Greek astronomy thanks to its Museum and Library.
Research institution where Egyptian, Babylonian, and Greek astronomical knowledge was preserved — the very learning that inspired the calendar reform.
Capital of the Republic where Caesar, with the help of Sosigenes, put the Julian calendar into effect in 46–45 BC.
The political and religious heart of Rome, where Caesar, as high priest, proclaimed the calendar reform governing civic life.
