An Etymological Dictionary of Astronomy and Astrophysics
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A → solar calendar established by Julius Caesar in 46 B.C. to replace the → Roman calendar. It was inspired by the Egyptian calendar year of 365 days. The astronomer Sosigenes set up the months (January to December) and added an extra day in February every fourth year (→ leap year). This gave an average year of 365.25 days. The Julian calendar remained unchanged for 1,600 years, and was replaced by the → Gregorian calendar to correct its errors. The Roman calendar before the reform was running 80 days out of alignment with the seasons of a true year. Sosigenes fixed the calendar by having 445 days in the year 46 B.C., which brought the seasons in line with the calendar. The year 45 B.C. is known as the first leap year of the Julian calendar. However, Sosigenes' work was misinterpreted and they were placing leap years every 3 years instead of every 4 years.

Julian, adj. of L. Julius.