Happy Leap Day!
It’s Feb. 29, the day the Gregorian calendar adds (almost) every four years to bring the calendar in line with the apparent motion of the Sun across the heavens.
Our calendar normally has 365 days in it, but the purpose of the civil calendar is partly to keep the equinoxes and solstices on the same days each year. Trouble is, the Earth takes a little more than 365 days to orbit the Sun, so over time the calendar “runs slow,” losing almost a day every four years. Since we can’t slow down the Earth in its orbit, the Gregorian calendar adds a day every four years to keep it in step with the Earth.
It’s not precise, actually. Earth refuses to observe our petty attempts at scheduling the solstices and equinoxes, and actually takes about 365.2422 days to orbit the Sun, instead of the more convenient 365.25 days. So the additional leap day every four years, over the course of 400 years, means the calendar would be about 3 days ahead of the Earth’s motion. The Gregorian calendar fixes that little detail by adjusting the add-a-day rule.
The rule is a bit obscure. It goes like this (sing along if you know the melody). Years that are divisible by 100 are also divisible by 4, so they would normally be leap years, as they were in the old Julian calendar. But we only apply the leap-day fix on those 00 years that are also divisible by 400. So, 1600 and 2000 were leap years, but 1700, 1800 and 1900 were not. So in the 400 years after 1600, the calendar “lost” 3 days — the right amount to keep it and the Sun in line with one another.
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