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June 14, 2015

The stars that mark the outer edge of the Little Dipper are known as the Guardians of the Pole. That’s because they’re not far from the Pole Star, Polaris. They circle around Polaris all night, every night — like guards on patrol.

But a couple of thousand years ago, one of those stars didn’t just guard the pole — it was the pole. Kochab was closer to the celestial pole than any other bright star beginning in about 1500 BC and continuing for two millennia.

The pole star changes because of an effect known as precession. Earth wobbles on its axis like a spinning top that’s running down a bit. It takes 26,000 years to complete a single wobble. During that time, Earth’s axis draws a big circle on the northern sky, so different stars take turns marking the pole.

Polaris is less than a degree away from the true celestial pole, which makes it a great pole star. Kochab was a good bit farther from the pole, so it wasn’t as good a navigational marker. Still, it got the job done for 2,000 years.

And you can find Kochab in the northern sky tonight. It stands almost directly above Polaris as night falls. It’s only a bit fainter than Polaris, so if you can find the current Pole Star, you shouldn’t have any trouble picking out its predecessor. Pherkad is to its upper right, at the opposite corner of the dipper’s bowl. They continue their tight march around the north pole throughout the night.


Script by Damond Benningfield, Copyright 2015

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