The sidereal day Main Index

The sidereal day

As well as spinning on its axis the Earth is going round the Sun. This means that the time from zenith to zenith is actually very slightly longer than the time for it to rotate on its axis. The actual time for the Earth to rotate on its axis is about 23 hours 56 minutes. This is a sidereal day (from the Latin siderus a star), because after a sidereal day the stars will appear in the same positions in the sky. This is more fully discussed on the Page on The Length of the day.

If we use an ordinary clock and calendar the positions of the stars in the sky will vary night by night. You can buy special star maps which allow you to set your position and the date and time so that you can work out what stars will be visible to you. Some national and local newspapers also publish sky maps on the first or last day of each month.

The change in the positions of the stars each night is more fully discussed in another part of this web site, on the Ancient Egyptian calendar - to see it click here scarab

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