At some point in elementary school, you most likely learned that there are 365 days in a year, since that’s how long it takes for Earth to complete one full rotation around the sun. What you might not have learned, though, is that a year isn’t exactly 365 days…it’s actually more like 365.2421 days.
So, to ensure our calendar year begins correctly as Earth begins a new revolution around the sun, about an extra quarter of a day each year has to be accounted for, or 1 day every 4 years. Way back in the day, the Egyptians had already been doing that for a while before Europeans finally caught on in 46 BCE, when astronomer Sosigenes aided Roman emperor Julius Caesar in devising what we now know as the Julian calendar, with its 365 days, grouped into 12 months, and an additional “leap day” (February 29th) every four years.
But .25 isn’t 0.242 (obviously), and trying to round that 0.242 up each year created its own problem, since it didn’t quite add up to a full day every four years. And that little discrepancy meant that, after 128 years, the calendar year ended up starting a day before the Earth had made its full rotation around the sun. And by the 14th century, the calendar year was starting a whole 10 days before Earth had completed its orbit!
So, in 1582 Pope Gregory XIII decided he was going to nip this in the bud by encouraging everybody to just skip a leap day every so often. And while that sounds flippant on the surface, it’s actually Pope Gregory XIII’s Gregorian calendar that we still use today. And it omits the leap day during years evenly divisible by 100 but not by 400. For instance, the year 2000 included a leap day because it’s divisible by both 100 & 400. Meanwhile, the year 2100 will not include a leap day, since, while it is evenly divisible by 100, it’s not also divisible by 400.
Truth be told, though, Pope Gregory XIII’s correction to Julius Caesar’s over-correction was, itself, still a bit of an under-correction. So, our leap day protocol is most likely going to have to be readdressed…in about 10,000 years.
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