On this day in 45 BCE, the Julian Calendar came into effect in the Roman Empire by order of Julius Caesar. Previously, the Empire had relied on the Roman calendar: a system that accounted for 355 days and sometimes had a 23 day “intercalary” month inserted around March, based on the whims of the pontifices (the high priest of Rome, a political appointee). While travelling in Egypt, Caesar – who was well aware of the deficiencies of the Roman calendar – was reminded of the findings of Eudoxus, who had apparently figured out that a year lasted 365 ¼ days. Upon his return to Rome, Caesar began reforming the calendar with the help of Sosigenes and other great minds from around the Empire. Initially, leap years were every three years, but this was eventually changed to four; and the unreliable intercalary system was abolished. Additionally, 2 or 3 days were added to several of the months and, later on, “Quintilis” became “Iulius” (or, July, after Julius).
Much of the world adopted the Julian calendar, and it was used widely until the the emergence of the Gregorian Calendar proposed by Pope Gregory XIII in 1582. The new calendar – which forms the basis for our modern-day understanding of the yearly cycle – was a minor correction, shortening the year by 0.0075 days. Today, the Julian calendar, which is behind the Gregorian one by about 13 days, is still in use by some elements of the Eastern Orthodox Church.