How it works

Astronomers still talk about the sky as the ancients saw it: a great hollow ball with the stars fixed to the inside, turning round us once a day. Nobody believes in the ball any more, but it is a very handy way of saying where things are. It is called the celestial sphere.

On the Earth	In the sky
North and south poles	The two celestial poles, straight above them
The equator	The celestial equator, a circle half way between the poles
Straight up, wherever you stand	The zenith

Reading the photograph. In half an hour the Earth makes one forty-eighth of a turn, which is seven and a half degrees (worked out here: 360 degrees divided by 48). So every streak in the picture is a seven and a half degree piece of a circle. The ones near the pole are short because their circles are small. The ones far from it are long.

How fast you are going. The Earth is 40,075 kilometres round at the equator, and it turns once in just under 24 hours. Someone standing on the equator is carried east at about 1,670 kilometres an hour. Further from the equator the circle you travel is smaller. At the latitude of Edinburgh, 56 degrees north, it comes to about 940 kilometres an hour. Both figures were worked out here. You feel nothing because the speed is steady and everything round you shares it.

The pole star. Polaris is not exactly at the pole. It is about two thirds of a degree away, a little more than the width of the full Moon, so it does go round in a very small circle of its own. That it is there at all is luck. The Earth’s axis slowly wobbles like a spinning top, taking about 26,000 years to go round once, and as it does the pole of the sky drifts among the stars. A few thousand years ago there was no bright star near it, and a few thousand years from now there will not be one again.

The southern pole has no bright star near it now, as the photograph shows.

What you see from where.

Where you stand	Where the pole of the sky is	What the stars do
The North Pole	Straight overhead	They circle level with the horizon. None rises and none sets.
Edinburgh, 56 degrees north	56 degrees up, in the north	Those within 56 degrees of the pole never set. The rest rise and set at a slant.
Cairo, 30 degrees north	30 degrees up	Fewer never set, and more of the southern sky comes into view.
The equator	On the horizon	Every star rises straight up and sets straight down. You can see the whole sky over a year.

The rule is simple. The height of the pole above your horizon, in degrees, equals your latitude. A navigator who measured the height of Polaris had the ship’s latitude to within a degree.

Two kinds of day. Measured against the stars, the Earth turns once in 23 hours, 56 minutes and 4 seconds. That is called the sidereal day. But in that time the Earth has also moved on round its orbit, by about one degree. The Sun is no longer in quite the same direction, and the Earth has to turn for nearly four minutes more to bring it back to the same place in the sky. That makes the 24 hours on the clock.

Four minutes a day is two hours a month and 24 hours a year. So after exactly one year the stars are back where they began.