It seems so simple. A leapday is an extra day on the calendar every 4 years. And that is actually everything that people will have to know for the next 93 years. But then, something strange will happen. In 2100 there will be no leapday! There's going to be one in 2096 and one in 2104 but not in 2100. Why is that?
Well, the earth, as it goes once around the sun, rotates exactly 365.2422 times during that time (with respect to the sun). One rotation is one day.
Now, let us assume there was no leap day. Since a year is about 365.25 days long, we would be one day behind every four years. And that is exactly why we have leapdays. Unfortunatly nature didn't have the foresight to put the rotations of the earth in sync with our 365.25 calendars. So we have to correct things further. After every 400 years we would be three day ahead of the actual date. (400*0.0075) To correct that as well, the leapday rule was extended. There is no leapday if the number of the year can be divided by 100 but not 400 without leaving a rest. This is the official calendar, one year is 365.2525 long. That however also means, that in yet another 8000 years we will once again be ahead of the actual date by 3 days, but that should be enough time to learn the next, then even more complicated set of rules ... [ MORE ]
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