The Earth doesn't take exactly 365 days to orbit the sun, it's a bit closer to 365.25 days. So after four years, we have basically banked an extra day, and a leap year includes that extra day in the calendar to keep our calendar on track.

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A leap year is a year that has an extra day. This extra day is added at the end of February: February 29th. It's added every four years because Earth does not actually take exactly 365 days to orbit the Sun, but closer to 365 1/4 days. If not for leap years, we'd gradually shift our calendar relative to Earth's orbit.

The Earth takes a few minutes less than 365-1/4 days (365.24219) to go completely around the Sun. Once every four years we add a day to the Gregorian calendar — February 29 — to make up for most of this difference. That’s a leap year.

However, because the difference is a few minutes short of 1/4 day, any year that is evenly divided by 100 would not be a leap year unless it is evenly divided by 400. Thus 1600, 2000, and 2400 are leap years, but 1700, 1800, 1900, 2100, 2200, and 2300 are common years, even though they are all divisible by 4.

The exceptions are years multiple by 400. Those we do not make leap years even though they are divisible by 4. And we do that because it is actually closer to 365.246 days to a year, and that makes it pretty much consistent over the course of even thousands of years.

The Earth is dong two types of movement in space - it's rotating on its axis, and it's revolving around the sun. Rotating on its axis gives us day and night - because sometimes we're facing the sun, and sometimes we're not. Revolving around the sun gives us our seasons, because of the Earth's tilt relative to the axis of the sun.

But here's the thing - those two movements don't quite match up. It would be really cool if they did, but they're just a little off. So today is November 30th, and it would be awesome if next year on this day (Nov 30, 2023), the Earth was in the exact same place relative to the sun. But it's not - it's about a quarter of a day behind. It's not a big difference, but over the years, that difference will add up. We're used to the winter solstice being around December 21, for example, but if we didn't adjust our calendar, it would start creeping up sooner and sooner. Which isn't a bad thing, but we're creatures of habit, and it's nice to have these things be more consistent.

So for consistency, we just mess with our calendar so that the year's movement and the day's rotations keep on track with each other. That maths out to roughly one day added every 4 years to keep us on track - we also skip leap years every century (except every 4th century) to keep that on track even closer to the true value. It's just a necessity because our day measurements and our year measurements don't exactly correspond with each other as neatly as we'd like.

The opposite - years divisible by 400 are leap years, but other years divisible by 100 but not 400 are not leap years. 2000 was a leap year and it's divisible by 400.

My bad and thank you for the correction.

You deserve a bunch of upvotes for civility.

Are they not teaching this stuff in school anymore? I don't mean to be snarky, just curious.

So how was skipping leap years every century work in practice? Like when was the last time that happened and when will it happen next? I’m trying to work it out in my head but I havent had enough coffee yet 😂

>A leap year is a year that has an extra day. This extra day is added at the end of February: February 29th. It's added every four years because Earth does not actually take exactly 365 days to orbit the Sun, but closer to 365 1/4 days. If not for leap years, we'd gradually shift our calendar relative to Earth's orbit.

FYI: In the beginning, the leap day was actually added in between days in February and all patron saints coming after that leap day moved one day over.
Later, the leap day was indeed added at the end of February.

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You skip every century (cause all century years like 1700, 1800 are also leap years), except for the multiples of 400. So in 2000 (which is a multiple of 400) we didn’t skip the leap year, we had it as expected. In 2100, there will be no Feb 29th, because it’ll skip.