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A sidereal day is a period of the Earth's rotation with respect to the celestial sphere, the time it takes for a star to appear at the same angle in the sky from one day to the next. A solar day is a period of the Earth's rotation with respect the Sun, the time it takes for the Sun to appear at the same angle in the sky from one day to the next. The solar day is 4 minutes longer than the sidereal day. You can see this if you realize that the Earth is spinning as it orbits the Sun, so it takes an extra little bit of time for the Earth to rotate to the point where the Sun is overhead in the sky compared to the celestial sphere. This extra motion corresponds to the distance the Earth travels in its orbit of the Sun in one day, about one degree. One degree on the rotating Earth is 1/360 of the motion. 24 hours divided by 360 is 4 minutes, so the solar day is 4 minutes longer than the sidereal day.

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