# Homework Help: Why is earth's rotation period < 24hrs

1. Oct 5, 2005

### dowjonez

Hey anyone who can give me a hand. Ive been racking my brain over this question my teacher told me to think about for next class. Its not worth any marks or anything but it might impress him. I would rather ask here then google it. Why do we use a 24hrs on our clocks if the rotational period of the earth is 23 hrs 56 minutes?

2. Oct 5, 2005

### CarlB

The difference has to do with whether you want to define the rotation of the earth with respect to the sun, or with respect to the distant stars. Maybe you should find out what one 365th of a day is, in minutes.

Carl

3. Oct 5, 2005

### dowjonez

its about 3.9xxx minutes thanks carl

4. Oct 6, 2005

### chroot

Staff Emeritus
It's much more convenient for us to use the Sun as a reference for keeping time, since our biological behavior is inextricably linked to it.

On the other hand, we're moving around the Sun about one degree per day, while we practically don't move at all with respect to the very distant stars. This means that every day, the earth has to turn about one degree further to return to the same position relative to the Sun as it does to return to the same position relative to the distant stars. That extra one degree of rotation takes about four minutes to complete, making the solar day about four minutes longer than the sidereal day.

- Warren