# 24 hours in a day?

1. Jan 28, 2005

### darkar

2. Jan 28, 2005

### franznietzsche

A wizard did it.

3. Jan 28, 2005

### BobG

If you watch the stars a lot, you'll notice their location shifts a little each night as the Earth orbits the Sun. There's approximately 360 nights a year, so you could say the amount that each star shifts per night is some unit about 1/360 th of a complete rotation. The Babylonians built a base 60 numbering system based on tracking the stars, hence the 60 minutes per degree and the 60 seconds per minute (and the approximation of 360 days per year - it would be hard to build a numbering system from 365).

The stars also seem to rotate during the night, so the only way you can really use them for navigation is to know what time it is. It only makes sense to make your time units compatible with your position units, hence the 60 minutes per hour, the 60 seconds per minute. Of course, it only takes one day for the Earth to rotate, so the length of the day winds up being 24 hours instead of something more compatible with a base 60 numbering system, but it works well enough.

Edit: Actually, that doesn't really explain why 24 and not some other number since the length of a second didn't have to be the current length. Ideally, the day's segments would be easy to fit back into a circle. The easiest angles are angles like pi/2, pi/3, pi/4, pi/6. pi/12 isn't that much harder than pi/6, and I guess they felt pi/6 was just too long for one hour.

Last edited: Jan 28, 2005
4. Sep 13, 2006

### EL

But then it has to turn a little bit more to keep up with the motion around the sun -> 24 hours in a (mean solar) day.

5. Sep 13, 2006

### EL

6. Sep 13, 2006

### EL

If it was like you're saying (i.e. that one day is 23 hours 56 minutes), then half the year we would have daylight during ordinary nighttime...

What we want is that the sun should be at its highest point in the sky about the same time (12.00) every day. That's why we make sure there is 24 hours between two such events.

(Ok, then we could have day-light saving time and so on, but that's really another story. Also, one hour is not defined as 1/24 of a solar day anymore (it's defined as a certain amount of oscillations in a cesium atom), but the deviation is really tiny, and it's enough to put in some extra second now and then.)

Last edited: Sep 13, 2006
7. Sep 13, 2006

### EL

Compared to what?

Compared to a siderial day, yes!
But not compared to a solar day (i.e. an ordinary day)!

Last edited: Sep 13, 2006
8. Sep 13, 2006

### johnchao

You are right. I am confused.
One day = 24 hours = Daily cycle of the sun.
Daily cycle of star = 23.93 hours.
I get wrong information from some web pages.

9. Sep 13, 2006

### johnchao

Last edited by a moderator: May 2, 2017
10. Sep 13, 2006

Actually the days are getting longer. The moon's gravity is slowly tugging at the earth, slowing its rotation ever so slightly each year. Billions of years ago the days(rotation of the earth) was only 18 hours long. Eventually the earth will stop spinning altogether but by that time the sun will have long burned out.

11. Sep 13, 2006

### Cyrus

He tutors me on my HW.

12. Sep 13, 2006

### franznietzsche

That was an 8-bit theater joke, incidently.

I can't make the same necroposting-necrophilia-morrowind joke twice in the the same month.

13. Sep 13, 2006

### Cyrus

You're no fun.

14. Sep 13, 2006

### franznietzsche

Yeah but looks aren't everything.