Kochab

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 it 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 gyroscope that’s running down. 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 marker. Still, it got the job done for 2,000 years.

You can find Kochab in the northern sky tonight. It stands almost directly to the left of Polaris as night falls, well above the bowl of the Big Dipper. It’s only a bit fainter than Polaris, so if you can find the current Pole Star, you shouldn’t have any trouble finding its predecessor. The other guardian, Pherkad, is to Kochab’s upper left, at the opposite corner of the Little Dipper’s bowl. They march around the north pole throughout the night.

Script by Damond Benningfield