The sidereal day is the time it takes the Earth to rotate once on its axis.

It is shorter than the solar day we use to measure time due to the Earth moving in its orbit so the Sun will appear to have moved and a little more time is needed for it to make it back to the meridian.

Jeeeeeeezzzz.... I thought I'm pretty educated in such things.
But you've just opened whole new world for me!😯

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