> > > The earth rotates on it's axis once in 24 hours with respect to the sun. But with respect to the background stars, it is a little less. This is called a siderial day. Trusting my memory on that, I think it is about three minutes shorter than a solar day.
> >
> > The Earth's rotation is about 23 hours, 56 minutes, 4.091 seconds if I remember rightly. Is that spelt siderial or sidereal? I can never remember.
>
> Hang on a second. I was with this thread until right now. Now I'm wondering if I'm wrong, or I'm just misunderstanding someone.
>
> I thought that hours, minutes, and seconds were *defined* by the earth's rotation, such that the earth does, in fact, rotate 360 degrees in EXACTLY (or essentially) 24 hours. I thought the reason the stars appear in the same place in the sky four minutes earlier every night was because of the earth's *revolution* around the sun. Such that, after six months, the stars you saw at midnight are now hidden by the sun at noon.
>
> Now we're saying the earth's actual rotation is four minutes shy of 24 hours?
>
> My head is starting to spin. Somebody fix me.

"The Earth rotates once in a few minutes under a day (23 hours 56 minutes 04. 09053 seconds). This is called the sidereal period (which means the period relative to stars). The sidereal period is not exactly equal to a day because by the time the Earth has rotated once, it has also moved a little in its orbit around the Sun, so it has to keep rotating for about another 4 minutes before the Sun seems to be back in the same place in the sky that it was in exactly a day before."

-FP