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Why leap years exist [Random Knowledge #26]
posted March 17 2013 20:20.33 by Giorgos Lazaridis

Every now and then, or better, every four years something changes in our calendar. We all know that a year has 365 days, but once every four years it has 366. Why?

Of course the reason is not that the year gets longer... Or the Earth takes another route around the sun... Let's first define a year. One year is the time that Earth needs to orbit the sun. You may think that it takes 365 days to do so. But first let's define a day - A day is the time needed for the Earth to rotate around its axis.

Now lets put this all together - If one year had precisely 365 (or 366) days, it would mean that the time that the Earth needs to rotate around the sun and the time that the Earth needs to rotate around its axis are perfectly synchronized! Which.. as you understand they are not. Instead, one year takes 365 days and six hours. So, every four years these six hours add up to a complete day (24 hours). And thus, the leap year was born!

Now you may also think this: If the rotation of the Earth around the sun and around its axis are not synchronized, why the "six hours" should be precisely "six hours"? ... They are not! The difference is about one day every hundred years! Therefore, every century the leap year is skipped. So, 1800 and 1900 were not leap years.

But let me remember... 2000 WAS a leap year. Why? You guessed it... Skipping one day once every 100 years is not 100% precise, so once every 400 years the leap year is not skipped. Therefore, 1600 and 2000 were leap years!


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