Eli5 what is the point in leap years ,like why do we have them?
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The earth doesn’t travel around the sun in exactly 365 days. It’s closer to 365 and a quarter days. If we didn’t add a day every 4 years, slowly but surely the axial tilt of the earth relative to the sun would make our seasons shift forward so that February would start to feel like March and March would start to feel like April, etc.
Some years also have a leap second added because adding a day every 4 years still doesn’t keep it exact.
The Earth takes about 365.25 days to revolve around the sun, so every four years that .25 adds up to an extra day. We put it in the calendar bc otherwise after a few years it would shift and be off by a lot
The orbit of the Earth around the Sun isn’t an integral number of days. This was ignored for a while, during the dark ages, and the calendar got out of sync with the seasons.
Leap Year is a solution to a problem of solar system geometry, and we have to have them or we’ll get the dates wrong.
We have two things we’re dealing with here: years and days.
A day is the amount of time that it takes the Earth to rotate exactly one time. A year, on the other hand, is the amount of time that it takes the Earth to complete one revolution around the sun.
The issue is that a year takes ~356.25 days. That is, the Earth rotates 365.25 times for every full circle around the Sun. This presents issues. If we just set a year as 365 days, the seasons would shift a quarter of a day per year. Over time, January would end up in warm weather, because each year the “1st of January” would be happening later in the rotation around the Sun.
Leap days account for this by adding a day every 4 years. This keeps the 1st of January in roughly the same place in the circle around the Sun.
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A year is supposed to be the time it takes the earth to go all the way around the sun. That’s *about* 365 days but not exactly. It’s really more like 365.25 days. So, if you don’t do something, every 4 years or so you’ll end up being “off” by a day. By “off” I mean the position you expect the sun to be on January 1 now happens on January 2.
This matters because, if you do it long enough, your seasons get totally out of whack. You end up having Christmas in the middle of the summer and skiing in August (in the northern hemisphere). This wreaks havoc on seasonal industries that work by calendar, schools and other intermittent schedules, etc.
So every 4 years we stick in an extra day to keep everything mostly lined up.
We occasionally do “leap seconds” to account for the rest. You never really notice because it’s so small and most of our devices are synchronized by the GPS network now anyway.
The time it takes for the Earth to spin around on its axis is a day.
The time it takes for the Earth to go around the sun is a year.
Problem is, there aren’t a whole number of days in a year. That is, the Earth spins just a little over 365 times (days) in the time it takes to go around the sun (year).
Now, this really isn’t a problem, but if you build a calendar out of whole numbers of days, it’s going to gradually shift because your calendar year will be slightly shorter than a real year.
Why is this actually a problem? Seasons.
Seasons are quite important because how and when you grow food depends heavily on them. So knowing when seasons begin and end is a fairly good piece of information to have. But if your calendar year keeps shifting with respect to the real year then it will shift with respect to the seasons as well, because the seasons don’t care what calendar you use. That means every year, they’ll gradually be later and later in the year because your calendar year keeps ending too early. This makes tracking them all the harder.
So, to compensate, we add in leap days. The calendar year is about one quarter of a day shorter than the real year, so after four calendar years we are about a day short, so we add in a day. A leap day.
But it’s not *quite* a full day, and this actually puts us slightly ahead of the real year. So every one hundred years, when it would normally be a leap year, we actually skip that leap year to get us closer on track.
But this, again, puts is *very very* slightly behind the real year ago. So every four hundred years, when we would skip that leap year, we actually put it back in.
This still doesn’t get us exactly aligned with the real year, but it’ll keep us good and well for about a millennia or so. Let them worry about that.
The time it takes for the Earth to go around the Sun isn’t a whole number of days, it’s around 365.242. This means that for any year length you pick (say 365 days), when you reach midnight on New Years, the Earth will be slightly behind in its orbit compared to the last time around.
For a few years this isn’t much of an issue, but over long periods of time this leads to the calendar and seasons ‘drifting’ apart, we’d really prefer for them to stay synchronized. So every 4 years we insert an extra day, this makes the average year length 365.25 days, this slightly overshoots 365.242 so every 100 years we skip a leap year (so 1900 wasn’t a leap year, despite 1896 and 1904 being ones). This correction actually slightly undershoots so every 400 years we have a leap year despite the 100 years rule (so 2000 was a leap year, though 1900 and 2100 aren’t).
Long story short, we have Leap Years because our “year” is based off Earth’s revolution around the Sun, which takes roughly 365 days. Our “day” is based on Earth’s rotation around it’s axis, its “spinning”. I said the Year is roughly 365 days, it’s actually 365*.25* days. That 0.25 days is a problem because our seasons are created by Earth’s position as it’s moves around the Sun. So after 100 years we’d be 25 days-worth-of-motion *out of position*, meaning *out of season*. So over long periods of time our seasons would be moving with relation to our months, and that just sucks.
So we invented Leap Year to “catch us up” a whole day every 4 years (0.25 x 4 =1 ) to reset the calendar back on track.
The Earth takes longer than 365 days to go around the Sun. Currently the 365-day estimate is a little over six hours off. If we just left the year at 365 days, eventually the year would drift away from the cycle of seasons, and eventually January would be in the springtime (or summer, or fall, if you go long enough). But if we went for 366 days each year, the calendar would drift in the opposite direction.
Different calendars handle this differently. Some calendars insert a whole extra month in some years to keep things from drifting too far. The Roman calendar firat tackled this by inserting an extra day every four years, and to this day, most people still think that’s how it works.
But a few hundred years later, we discovered that a day every four years is *slightly* too many: the calendar still drifts. It’s about 3 days too many every 400 years. So they made an exception to the one-every-four rule -years divisible by 100 are not leap years- and an exception to *that* rule: years divisible by 400 *are* leap years. This is why 2000 was a leap year, but more importantly, it brings us to 97 leap days every 400 years, which is very close to the amount we actually need.
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