HUNTSVILLE, Ala. — It seems the extra day is somewhat legendary. Once every four years, we add an extra day to February and that day just so happens to be this Saturday. This Saturday will be February 29, 2020. Why do we do this? The answer seems quite simple at first, but it's actually a bit more complicated and involves a little math.
At its most basic we have a leap day because the calendar year is 365 days, but it actually takes 365.2421 days for the earth to make a complete revolution around the sun. We often round this to 365.25 days.
If we didn't have a leap day at all the calendar would be off 24 days every 100 years. This means Spring would be pushed forward by almost a month and start in February as opposed to March.
The concept of making up this quarter day we essentially lost was the brainchild of Julius Caeser. Julius loved the Egyptian method of basing the calendar off the solar year. To make up for the quarter day lost in each year he decided to add one extra day to February every four years. This happened in 45 B.C.
After a while, it was noticed that while the rounding up from 365.2421 to 365.25 was close the math was still off and the solar year was overestimated by 11 minutes. This means that every 100 years the calendar is still off by 18 hours. Not perfect, and still creating some issues.
This messed with the Catholic Church since Easter wasn't falling where it should. To fix this Pope Gregory XIII decided that we could keep the leap year except at the start of centuries that were NOT divisible by 4. Hence the Gregorian Calendar was born.
Ready for this? The Gregorian calendar is STILL not perfect, but it was determined it's one day off every 3,000 so they decided we as a human race can live with that because it really won't be our problem.
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