Why the sky turns

Go out on a clear night, pick a star near a rooftop or a tree, and look again an hour later. It will have moved. All the stars will have moved together, keeping their patterns. The whole sky turns, slowly, all night long.

It is not the sky that is moving. It is you. The Earth spins round once a day, from west to east, and it carries you with it. So the Sun, the Moon and the stars all seem to go the other way. They rise in the east and set in the west.

A spinning ball has two points that stay put: its poles. The sky has two as well, straight above the Earth’s poles, and everything seems to turn round them. In the north a star happens to sit almost exactly on that point. It is called Polaris, or the pole star. It hardly moves all night, and it always marks north.

Stars close to the pole star go round it in small circles and never set. In Britain the Plough is one of these. You can see it on any clear night of the year, sometimes the right way up and sometimes on its head. Stars further from the pole rise and set.

How high the pole star stands depends on where you are. At the North Pole it is straight overhead. At the equator it sits on the horizon. In between, its height in degrees is the same as your latitude. Sailors found their way by it for centuries.

One more thing. The stars come round a little faster than the Sun does. The Earth turns once in 23 hours and 56 minutes, so each star rises about four minutes earlier every night. Over the months those minutes add up, and that is why winter and summer have different stars.