Earth revolves around its own axis in approximately 23 hours 56 minutes 4 seconds. If we measure the length of a day by 24 hours we hour saving 180+56=234 seconds a day. In a year that is 234 * 365 = 85, 410 seconds. In 4 years that is 341,640 seconds. Now a say has 86400 seconds, so we are the end of 4 year we have 341,640/86400 = 3.95 days. But we add 1 day to February after 4 years. Why is this?
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The Earth rotates 360 degrees in 23 hours, 56 minutes, 4 seconds. But at the same time, Earth moves just a little bit further in its orbit around the Sun, so that it takes a little bit more time (approximately 3 minutes, 56 seconds) so that the Sun appears over the same geographical point on the Earth’s surface. So Earth has to rotate *just a little bit more* to make a complete “day”, as relative to the Sun’s position over a consistent point on the Earth’s surface.
Leap years are an additional small correction every four years (minus an addition correction of the correction every century) to make sure our calendar matches nature. Because nature doesn’t care about human math or human calendars.
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