Astronomy – Why Is There One Fewer Solar Day Per Year Than Sidereal Days?

astronomyorbital-motionreference framessolar systemtime

solar day = time between solar noons

sidereal day = period of Earth's spin

Wikipedia says "relative to the stars, the Sun appears to move around Earth once per year. Therefore, there is one fewer solar day per year than there are sidereal days."

Shouldn't it be relative to the Earth instead of the relative to the stars? I'm having trouble following this argument. Can someone please explain it in more detail?

Best Answer

Shouldn't it be relative to the Earth instead of the relative to the stars?

We need some reference background to plot the "movement" of the Sun. If we could see the stars during the day, and we were to go to a fixed point on the equator and mark the location of the Sun each day at noon on a star chart, this point would move in a circle through the stars once per year. The Sun rotates around the Earth more slowly than the stars do, so the number of solar rotations is one fewer than the number of sidereal rotations.

Imagine walking counterclockwise around a circular track, facing North the whole time. Suppose there's a light in the middle of the track. If you start out in the Eastern part of the track, the light will start out on your left. Once you get to the Northern part of the track, the light will be at your back. When you get to the Western part, it will be on your right. At the Southern part, it will be in front of you. So the light will appear to rotate around you counterclockwise.

So if the Earth didn't rotate at all, the Sun would appear to rise and set once over the course of the year. This one circuit due to the revolution around the Sun cancels out one of the 366 circuits due to the rotation of the Earth, leaving only 365 solar cycles.

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