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# We have leap year because there aren't exactly 365 days in a year, so we add an extra day every fourth year. But if you think about it, there aren't exactly 24 extra hours in 4 years either. So even with leap year, we're still slowly becoming off-sync with the earth's rotation and orbit. The system we use to keep track of time is not perfect, amirite?

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that is why we skip years divisible by 100 but not 400. I believe that there is another exception to those rules but as it is the time lost is very very small, so small that the calendar will probably be destroyed by the time it needs to be readjusted. The problem is that our calender uses two arbitrary and independent time measures (the time for the earth to rotate and the time for it to orbit) and so they do not divide evenly into each other. In fact seconds and hours do not go evenly into a day (there are leap seconds).

If I think about it too much it drives me insane.

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If Saigot's explanation doesn't it for you, CGP Grey does an amazing job. Mainly because he's one of the biggest badasses on Youtube.

Found it -

The earth's rotation isn't reliable, and a leap second is added every few years.

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@Simon Found it - http://discovermagazine.com/200...now-about-time The earth's rotation isn't...

Also back in the time of the dinosaurs, a day was 23 hours, that's pretty interesting.

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I don't get it :/ basically a year is pretty much 365 days and 6 hours, and 4 times 6 is 24 hours...

@Squirk I don't get it :/ basically a year is pretty much 365 days and 6 hours, and 4 times 6 is 24 hours...

But it's not EXACTLY 6 hours, so every leap year it gets off-set by a few more minutes

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@qwerty123 But it's not EXACTLY 6 hours, so every leap year it gets off-set by a few more minutes

Yeah so in a few thousand years we'll be about a day behind or something :D

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I read somewhere that every 400 years the add a couple more days to make up for this...but I'm probably wrong

@WinniethePooh I read somewhere that every 400 years the add a couple more days to make up for this...but I'm probably wrong

I read something similar as well, I'm trying to Google it, but apparently in Greenwich, or wherever the fuck they keep the absolute world clock, they add an extra X amount of seconds every X years, to account for the fact that a year isn't exactly 365.25 days.

Still trying to find the article.

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That's why we have leap seconds.

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The earths orbit is slowly, slowing. So that in several hundred years (I can't remember the exact number) we lose a grand total of 1second from the clock!

Time is a human invention.

@Albert_Einstein Time is a human invention.

No it's not - time exists, the ways and methods we use to measure time are human inventions, but if we didn't measure time, it would still exist, we'd still get older. If humans didn't exist the sun wound still set and rise, and animals and plants would still age and die.

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