Why do the sidereal and the solar day differ in length?
A sidereal day is one rotation of the Earth on it’s axis: and this takes about 23 hours and 56 minutes. This is less than the 24 hour length of the solar day because the solar day is how long it takes for the sun to go from noon to the next noon. As the earth turns, it is also going around the Sun. If we start at noon and wait for the earth to turn once (23 hours and 56 minutes), the sun will not be in the noon position because the earth also went around the sun 23 hours and 56 minutes worth of distance. The earth must turn a little past, something like four minutes past, where it was the day before so that the sun will again be in the noon position. 4. Why isn’t the solar day always exactly 24 hours long? As discussed in number 2, the solar day is a combination of two motions. The Earth’s 23 hour 56 minute rotation coupled to the Earth’s annual motion around the Sun. The Earth’s rotation is almost perfectly constant but it’s motion around the sun is not. The Earth’s orbital motion is
