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North Stars II

July 25, 2010

There's one star in the night sky that you should never need help to find. In fact, it can help find you. It's Polaris, the North Star. It always stands due north -- every night of the year. And it's always at the same altitude in the sky -- an altitude that tells you your latitude on Earth. If you're at 30 degrees north, for example, Polaris is 30 degrees above the horizon.

Polaris won't always be the North Star, though. In a couple of millennia, that honor will go to a fainter star in the constellation Cepheus, the king.

Gamma Cephei is about 45 light-years away. It's a good bit bigger and brighter than the Sun, and about half-again as massive. It has a small, faint companion star, plus a giant planet.

Gamma Cephei will take over "North Star" honors because Earth wobbles on its axis like a spinning top. As it does so, the north pole aims at different stars. Right now, it aims at Polaris. But around the year 4100, it'll aim at Gamma Cephei. And around the year 7500, it'll aim at Alpha Cephei, the constellation's brightest star. It'll return to Polaris when Earth completes one full wobble -- in about 26,000 years.

Cepheus is to the right of Polaris as darkness falls. Its brightest stars form a pattern that looks like a child's drawing of a house, with Gamma Cephei at the peak. At nightfall, the house is turned on its side, so Gamma Cephei is the first star you come to you as you head away from Polaris.

Script by Damond Benningfield, Copyright 2010

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