Google says that “On Earth, a sidereal day is almost exactly 23 hours and 56 minutes.” Meaning that every year we lose 4×365:60≈24h so one day of time. And leap year is supposed to compensate for that but it only happens every 4 years not every year. Can someone explain me why is that so?
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The sidereal day is not related to the length of the year. When measuring the length of the year we are really only concerned about solar days; the Earth takes 365.25(ish) solar days to complete an orbit, so every four years the .25(ish) adds up to one extra day we need to insert.
What matters for most purposes of daily life is the solar day, which is 24 hours by definition. This is the time it takes for the Earth to fully rotate with respect to the sun. A sidereal day is how long it takes the Earth to rotate with respect to more distant stars. This is shorter because it also includes the motion of Earth around the sun (which is why you found the accumulated yearly difference to be exactly one day!)
Leap years are to correct for the fact that a solar *year* is not exactly 365 solar days, but rather close to 365.25. Therefore, every 4 years,* we’re one day further around the sun than we were, so we add a day to compensate.
*It’s not *exactly* every 4 years because a solar year isn’t *exactly* 365.25 days. On years divisible by 100 (except those also divisible by 400), we skip a leap year.
That is not what a leap year compensates for. Leap years compensate for the fact that one year is not exactly 365 days, it is ~365.24 days. There rotation of the Earth and the orbit of the Earth around the sun are independent, so one is never going to be a whole number multiple of the other.
The reason a sidereal day (the actual time it takes the Earth to rotate once) and a solar day (the time it takes the sun to appear to circle the Earth) are different is because the Earth is both rotating and revolving.
Imagine you are walking in circles around a lamp. When you are east of the lamp, the west side of you is lit up. But when you are west of it, your east side is lit up. So facing the same direction means different times of “day” depending on where you are in your “orbit”, and it will add up to the difference of one rotation.
A year is 365 days…and a little bit more
366 days is just too many, over several years your dates start drifting away from your seasons.
Or more specifically the dates designated for the solstice and equinoxes are no longer accurate.
So we deal with it by “adding” one day to the year every four years. However, every 4 years is in fact too often. Over many years we start start drifting. So every century we don’t have a leap year.
It’s still approximate, but close enough.
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