What Can You See in the Night Sky This Week? Polaris!

8 March 2012 by Anonymous (not verified)

Polaris is best known for its position in the night sky: it always appears almost directly North. This is why it is commonly known as both the North Star and the Pole star. To find Polaris, draw a line from the two stars in The Plough which are furthest from its handle (see picture) and keep going; the next prominent star you see will be Polaris, writes David Sinclair.

Left image: How to locate Polaris. Draw a line from the two stars in Ursa Major / The Plough, Polaris is located in the constellation Ursa Minor. Right images: Polaris is actually made up of three stars, Polaris A, Polaris B and Polaris Ab. Polaris A and Ab are roughly the same distance from one another as the Sun and Uranus, Polaris B is much further away.

The Earth is constantly moving, rotating on its axis whilst it orbits the Sun, so why does Polaris always appear in the North? As the Earth spins, the sky appears to rotate above our heads. This rotation is centred on the point in the sky directly above the North Pole. It just so happens that Polaris is located near to this point. That means that no matter where you are in the Northern Hemisphere, you will always see the stars rotating around Polaris.

Over the course of a year the Earth moves around the Sun, and yet Polaris’ position appears fixed. This is due to an effect called parallax. Think about when you’re riding in a car in the countryside. Objects close to you, like street lamps, appear to whizz by in a flash; however far away objects, like mountains, seem to move much more slowly. You have to drive a long way along the road before you notice it move at all. Now think how little that object would appear to move if it were as far away as the stars are (about 25 trillion miles to the nearest one, Proxima Centauri). Although you cannot see this very small difference in position over a year with your eye, you can if you use a large telescope which can detect distant background stars whose parallax is incredibly small. This is a reliable method of measuring the distance to nearby stars.

However, in a thousand years, Polaris will no longer be positioned directly North. This is because the Earth’s rotation axis ‘wobbles’, just like that of a spinning top. This wobble, called precession, traces out a circle pointing the North Pole at different stars. Over time, Polaris will move away from the pole and another star, Gamma Cephei, will take its place. The period of this wobble is incredibly slow, taking 26,000 years to complete one revolution. Indeed, during the time of the Ancient Greeks, no bright star lay directly above the North Pole.

Earth's Precession: This image illustrates the Earth's precession. It takes 26,000 years to complete one revolution

Polaris is about 430 light years away from Earth and is in fact a triple star system, as the picture from the Hubble Space Telescope shows. These three stars are imaginatively called Polaris A, Polaris Ab and Polaris B! We see them as one star because they appear positioned very close together in the sky. Whilst Polaris A and B are in reality a large distance from one another (240 billion miles), Polaris A and Polaris Ab are about the same distance from one another as Uranus and the Sun (2 billion miles). The picture, taken in 2006, was the first time astronomers were able to distinguish Polaris Ab from Polaris A. Up until then, we had to use the gravitational attraction between the stars to realise Polaris Ab existed.

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