Why is one day 23 hours and 56 minutes?
by 
queenellen
ANSWERS: 2
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That is the time it takes for the earth to complete one revolution. This is also called a sidereal day (a day measured with respect to the stars). While it would be more accurate to use sidereal days to mark time, it's more convenient to use 24 even hours.
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A day is 24 hours - this is the average time between noon and noon the next day averaged over the whole year. It takes 23 hours and 56 minutes for the Earth to rotate so that the stars return to the same fixed position. This is a sidereal day. (i.e. relative to the stars). It's shorter than 24 hours, because in one sidereal day the Earth also moves around the sun. The Earth and the stars are in the same relative alignment, but (relative to the Earth), the Sun moved! The Earth has to rotate a little bit more in order to get the Sun in the same place. The Earth moves about 1/365.25 of an orbit in a day, so it needs to rotate an extra 1/365.25 of a day to get the sun in the same place. = 86400/365.25 seconds = 236.5 seconds = an extra 4 minutes approx to get noon at the same time.
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