Did you know today is Leap Day? Once every four years or so, February gets a one-day extension that serves to sync the world’s most widely used civil calendar with the time it actually takes for the Earth to complete a full orbit around the Sun. If you ask anyone on the street how long a year is, you would probably get an answer of 365 days. In reality, though, it takes the Earth 365 days plus 5 hours, 48 minutes, and 45 seconds to make one trip around the Sun. Until the 16th century, the 365-day Julian calendar was commonly used, which simply added one day every four years to account for the discrepancy in timing. But a 5 hour, 48 minute, and 45 second surplus each year does not add up to exactly a quarter day; at 0.2421875 days, it’s just shy of that. So in 1582, Pope Gregory XIII introduced a modification—which would eventually be adopted globally—to realign everything. Dubbed the "Gregorian calendar,” it established these simple rules to determine if a year was a Leap Year: yes if the year can be evenly divided by four, and no if the year can be evenly divided by 100, unless the year is also evenly divisible by 400. So while the year 2000 was a Leap Year like 2020, the years 1800 and 1900 were not.
Photo: NASA/Terry Virts