I’ve talked here before about sidereal days and synodic
days. What exactly is the difference between the two?
It all comes down to one thing: the object that you are
using as a reference. For example, the sidereal day of the Earth is equal to 23
hours, 56 minutes, and 4 seconds. The sidereal day uses a distant star (which
is basically any star that is not the Sun) to measure against. You would
determine how long a star would take to reach the same location in the sky on
consecutive days. Another way to look at it, is the length of time it takes the
Earth to rotate 360°.
However, the synodic day is 24 hours. The best way to think
of the synodic day is to just determine the length of time between two
consecutive noons. A synodic day means the Earth rotates more than 360° to get
the Sun in the same location in the sky. The reason this happens is because as
the Earth rotates on its axis, it is also moving around the Sun. Granted, it is
only moving a little less than a degree in its orbit (since the Earth year is
about 365.25 days), but that is long enough that the Earth has to rotate just a
tad more to get the Sun back to noon.
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