Eli5 what is the point in leap years ,like why do we have them?
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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.
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