Sidereal time is an astronomical time unit measured in relation to the Earth's rotation in relation to distant stars, rather than the Sun. In a sidereal day, the Earth makes one rotation around its axis. However, at the same time, it has also travelled around 1° along its orbit around the Sun. After a sidereal day has passed, therefore, Earth still needs to rotate for nearly four minutes longer before the Sun reaches local noon according to solar time. A mean solar day is, therefore, nearly 4 minutes longer than a sidereal day.
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