Everything You'll Ever Need to Know About Leap Day

“People think the length of the year is to regulate the seasons. That’s totally untrue. It’s based upon religious considerations.”
  1. The solar year is calculated as the amount of time that elapses between successive vernal equinoxes—the date in spring when day and night are of equal length.
  2. This date is important to Catholics, and some other Christians, because they celebrate Easter on the first Sunday after the first full moon after the vernal equinox.
  3. The current pattern of leap years was designed to keep March 21st, the ecclesiastical date of the vernal equinox, aligned with the astronomical event.
  4. The solar year is actually 365.242374 days though, and the calendar year and seasons would eventually diverge. A 13th month was occasionally inserted to close the gap.
  5. By 46 B.C., the calendar had gotten out of hand.
    In 46 B.C. or so, the solar and calendar years had drifted apart by about 90 days, and Julius Caesar decided to introduce a calendar that would more closely resemble the solar year.
  6. Instead of leap months, the new scheme would include leap days.
    But before rolling out his new calendar, Caesar had to realign the calendar and solar years, and to do that, the first year of his calendar reform included 445 days.
  7. That giant year became known as the Year of Confusion.
    Afterward, the Julian calendar included 365 days plus a leap year with 366 days every fourth year.
  8. It was a good idea, but there was a problem: The average length of a year with this scheme was slightly longer than the solar year.
  9. To fix the problem, Pope Gregory XIII reformed the calendar again in 1582 by dropping a few leap years from the mix.
    Instead of every fourth year, the Gregorian calendar included a leap year every four years except in century years, which are only leap years when they are evenly divisible by 400. So, 2000 was a leap year, but 2100, 2200 and 2300 won’t be.
  10. But there is one other wrinkle: Calendar reform is a two-step process.
    “You have to correct the old calendar,” said Anthony Aveni author of “Empires of Time: Calendars, Clocks, and Cultures.” “To do that, you add or drop days to make it coincide with the solar calendar. Then you have to put in a program to correct things in future.”
  11. To realign his calendar with the vernal equinox, Pope Gregory removed 10 days from the middle of October.
    This also realigned the seasons, but the primary goal was to calculate the date of Easter, and not everyone in the world was on board.
  12. Britain stuck with the Julian calendar until 1752, and when it finally made the switch, the kingdom and its colonies had to skip forward 11 days.
  13. That’s why George Washington’s Birthday, which was recorded originally as Feb. 11, was later changed to Feb. 22: He was born before the switch.
  14. There is roughly 1 in 1,461 chance of being born naturally on a leap day.
  15. People born on Feb. 29 are called “leaplings” or “leapers."
    Happy birthday leapers!