Nearly every four years, we add an extra day to the calendar in the form of February 29, also known as Leap Day. Put simply, these additional 24 hours are built into the calendar to ensure that it stays in line with the Earth’s movement around the Sun. While the modern calendar contains 365 days, the actual time it takes for Earth to orbit its star is slightly longer—roughly 365.2421 days. The difference might seem negligible, but over decades and centuries that missing quarter of a day per year can add up. To ensure consistency with the true astronomical year, it is necessary to periodically add in an extra day to make up the lost time and get the calendar back in synch with the heavens.
The Egyptians were among the first to calculate the need for a leap year, but the practice didn’t arrive in Europe until the reign of the Roman dictator Julius Caesar. Before then, the Roman calendar had operated on a muddled lunar model that regularly required adding an extra month to maintain celestial consistency. Finally, in 46 B.C., Caesar and the astronomer Sosigenes revamped the Roman calendar to include 12 months and 365 days. This “Julian Calendar” also accounted for the slightly longer solar year by adding a leap day every four years.
Caesar’s model helped realign the Roman calendar, but it had one small problem. Since the solar year is only .242 days longer than the calendar year and not an even .25, adding a leap year every four years actually leaves an annual surplus of roughly 11 minutes. This minute discrepancy meant that the Julian Calendar drifted off course by one day every 128 years, and by the 14th century it had strayed 10 days off the solar year. To fix the glitch, Pope Gregory XIII instituted a revised “Gregorian Calendar” in 1582. In this model, leap years occur ever four years except for years evenly divisible by 100 and not by 400. For example, the year 1900 was not a leap year because it was divisible by 100, but not 400. The Pope’s updated calendar remains in use to this day, but it’s still not perfect—experts note that the remaining discrepancies will need to be addressed in around 10,000 years.