Institute of Astronomy

Length of a day

Published on 08/02/2012 
Question: 

Hello my name is Sam and I am 11. I have recently got into Science and I done some research and showed my teacher that there wasnt 24 hours in a day. My teacher said that I was wrong and there was 24 hour in a day. could you help me and tell me the real answer to how many hours, minutes and second there are in a day please?

The answer is that you are both right, the problem is what you mean by 'day', and there are two ways of thinking about it.
One way is the length of time between the Sun appearing in the same place in the sky (overhead for example), this is what people usually think of as a 'day' and is what our clocks measure.  For this way of defining the length of a day there are exactly 24 hours in a day, and in scientific terms this is called a 'solar day'.
The other way of thinking about the length of a day is the time it takes for the Earth to rotate once about it's axis.  This is slightly shorter at only 23 hours 56 minutes and 4 seconds and is called a 'sidereal day'.

You can see why there is a difference between the two in the diagram below (which I admit we borrowed from Wikipedia!).  As the Earth rotates it is also moving around the Sun, so if you are living where the little red arrow is on the diagram by the time the Earth has rotated once (the curved arrows next to Earth show the direction of rotation) it has also moved along its orbit from position 1 to position 2, and although the Sun was directly above the red arrow at position 1 it isn't quite overhead at position 2.  For the Sun to be directly overhead again you have to wait until the Earth has moved and rotated a little further, to position 3, so the usual way people think of a 'day' is slightly longer than the time it takes Earth to spin once.

Page last updated: 17 February 2013 at 12:54