Jimmy Westlake:
"Have you ever wondered why February has only 28 days most years but occasionally has 29 days, as it does this year? This whole leap year thing started back in the days of the Roman Empire under the reign of Julius Caesar. Even that long ago, astronomers realized that there weren’t a whole number of days in a year.
The Earth rotates 365 and one-quarter times in a year. Because it wouldn’t make any sense to have the last day of the year be only six hours long, Julius Caesar decreed in 44 B.C. that we would let the quarter-days days accumulate and then add in one whole day — a leap day — every fourth year. This would take up the slack between our calendar and Earth’s actual orbit around the sun. This calendar reform by Caesar assumed that the year was exactly 365.25 days long, but it isn’t. It’s really 365.2422 days long, about 11 minutes and 14 seconds shorter than Caesar assumed. So, using Caesar’s method, we were adding in too many leap days throughout the years. By the 1500s, all those 11 minutes and 14 seconds had added up to a full 10 days on the calendar, causing the vernal equinox, or first day of spring, to shift from March 21 to March 11.
If that error continued, we’d eventually be celebrating the first day of spring in December..."
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