Wired: Julius Caesar Takes the Leap
45 B.C.: Roman dictator-for-life Julius Caesar, alarmed that the calendar is growing out of whack with the seasons, adds an extra day to the month of February every four years.
Caesar was reforming a calendar based on 364 days, with an occasional extra leap month. But the Roman religious officials in charge of minding the calendar had been asleep at the switch, chronologically speaking. Caesar consulted with Egypt’s top astronomers, who told him the year was 365¼ days long. While he was making the fix, Julius also decided to give his name to the month of July.
Although Caesar decreed the new calendar in 46 B.C., that year had 15 months to make up for the accumulated discrepancy. The first add-a-day leap year was 45 B.C.
Though the Julian Calendar was more accurate than what preceded it, it wasn’t really as accurate as it needed to be. That’s because an Earth year is about 11 minutes short of 365¼ days: It’s 365 days, 5 hours, 48 minutes, 46 seconds. This was known, more or less, since the second century A.D., but by 1582, the calendar was 10 days out of whack, and Easter was falling too late in the real spring. So Pope Gregory XIII tweaked the Julian Calendar by subtracting three leap years in every 400 (years ending in 00, unless they are divisible by 400).
