# The Naked Scientists Forum

### Author Topic: How do clocks catch up with the earth?  (Read 1528 times)

#### Elliott

• Guest
##### How do clocks catch up with the earth?
« on: 26/04/2010 23:30:02 »

I know that the Earth's rotation is only 23 hours 59 minutes and 56 seconds long, which means that every 24 hours (1 clock day), our clocks are 4 seconds behind the Earth's spin.

Now, this also means, that every 10,800 days, our day would be dark and our night would be light, and every 21,600 days, our clocks would be 1 day behind the Earth's spin.

I also know, that this does not happen, so where does this day go?

What do you think?
« Last Edit: 26/04/2010 23:30:02 by _system »

#### wolfekeeper

• Neilep Level Member
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##### How do clocks catch up with the earth?
« Reply #1 on: 27/04/2010 02:55:58 »
Nope the Earth's rotation (the 'sidereal day') is just off 23 hours 56 minutes long, not 23 hours 59 minutes.

If you do the maths, it adds up to 1 full day per year!

The reason is... the Earth moves around the sun once per year... and this means that the sunrise/sunsets are delayed slightly each day by the motion of the Earth and over time this adds up to 365.25 days being in the year, rather than 366.25 which is what it would theoretically be if the Earth wasn't moving relative to the Sun.

All clocks are built already running just *slightly* slow to make up for this.

Handy that!

#### Geezer

• Neilep Level Member
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• "Vive la résistance!"
##### How do clocks catch up with the earth?
« Reply #2 on: 27/04/2010 05:03:48 »
There are 365.25 24 hour days in a year, but the earth actually rotates on its axis 366.25 times in a year.

The moon provides a clue. It does not obviously spin on its axis, but it still rotates around its axis once every time it orbits the Earth

#### The Naked Scientists Forum

##### How do clocks catch up with the earth?
« Reply #2 on: 27/04/2010 05:03:48 »