Sky spy logo

Monday, Feb. 29, is Leap Day, a day we add to the calendar every four years.

That’s because our defined year of exactly 365 days does not quite line up with the Earth’s going around the sun in 365.24219 days. This slight mismatch causes a drift of the seasons with the seasonal solstices and equinoxes gradually changing.

To keep the seasons in their traditional months and to keep the solstices and equinoxes near their usual dates, Julius Caesar had a revised Roman calendar introduced in 46 BC. It had 12 calendar months and a year of 365 days. It also included a leap year of 366 days every four years with the extra day added at the end of February. This “Julian calendar” worked quite well for more than a thousand years. By the 1500s there was a noticeable drift in seasons, and it was apparent further tweaking to the calendar was necessary.

Our present calendar was introduced by Pope Gregory XIII in 1582 to correct the minor problems in the Julian calendar. In the Gregorian calendar only those century years divisible by 400 are leap years. For example, 2000 was a leap year, but 1900 and 2100 are not leap years. The Gregorian calendar is accurate to about one day in 3,000 years.

On Monday we should tip our hats to Julius Caesar and Pope Gregory XIII and their scholars.