What is the North Star?

The stars are not moving across the sky. We are turning beneath them. Earth spins on an axis that runs through the North and South Poles, and the northern end of that axis points at one spot in the sky. Polaris happens to sit very close to that spot.

So as the night goes on and Earth turns, the other stars swing round in circles, and Polaris stays almost exactly where it is. A long-exposure photograph of the northern sky shows every star drawn as an arc around one point, with Polaris making the smallest circle of all. It is not perfectly on the pole: it is a little more than the width of the full Moon away, so it traces a tiny circle of its own each night.

At the equator, Earth's spin carries the ground along at 0.4651 kilometres per second. Since you opened this page, the equator has turned 0 kilometres beneath the sky, and Polaris has hardly shifted.

Start with the Big Dipper, the seven bright stars that in Britain are called the Plough. From most of Europe and Canada it never sets, so you can find it on any clear night; farther south it dips low in autumn evenings. The two stars at the end of the Dipper's bowl, away from the handle, are called the Pointers. Draw a line through them, out from the top of the bowl, and keep going. The first fairly bright star you reach is Polaris.

Polaris is the last star in the handle of the Little Dipper, a fainter pattern that you may only see from a dark site.

How high should you look? Polaris stands as many degrees above your horizon as your latitude. To measure, use your hand: a fist held at arm's length covers about 10 degrees of sky. From the North Pole, Polaris is straight overhead. From the equator, it sits right on the horizon. From London or Copenhagen it is a little over halfway up.

Many people expect the North Star to be the brightest star in the sky. It isn't. On the magnitude scale, which runs backwards so that lower numbers are brighter, Polaris is 2.02. Depending on how you count close pairs of stars, between 47 and 50 stars outshine it. Sirius, the brightest, is -1.46.

Polaris is still easy to see, even from a town, because it is bright enough to cut through light pollution and there are few bright stars near it to confuse you. What makes it special is its place in the sky, not its glow.

Polaris looks modest because it is far away. It is about 52 times farther from us than Sirius. Measurements place it between 433 and 521 light-years away, and the next data from the Gaia spacecraft should help settle the question.

To the eye Polaris is one steady point. In fact it is at least three stars. The bright one is a yellow supergiant, a star that has swollen far larger than the Sun. It has a close companion that circles it, and a fainter one farther out.

The supergiant is also a Cepheid, a kind of star that slowly swells and shrinks, brightening and dimming in a regular rhythm. Polaris is the nearest Cepheid to us. Its pulses are so small that the eye can't see them, but they matter. Astronomers use Cepheids to measure the distances to other galaxies, because the rhythm of a Cepheid's pulse reveals how much light it gives out. Polaris, close enough to study in detail, could help check that method, once its own distance is pinned down.

Polaris has not always been the North Star, and it won't be forever. Earth's axis wobbles slowly, like a spinning top, and the point in the sky it aims at traces a wide circle. One full turn of that wobble takes about 26,000 years, about 5 times as long as all of recorded history.

When the first pyramids were being built in Egypt, the North Star was Thuban, a modest star in the constellation of the Dragon. Today it is Polaris, and the pole is still creeping closer to it, until around the end of this century. Then it will slowly draw away. About half a turn of the wobble from now, the pole will lie near Vega, one of the brightest stars in the northern sky.

Anyone who looks for the North Star is taking part in a very old human habit. The star changes; the looking does not.