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Dueling Dippers

February 10, 2012

The Little Dipper twirls around the northern sky every night of the year. Tonight, the bowl dangles below the handle at nightfall, and stands above the handle at first light.

This pattern of stars might better be thought of as a spoon, because it seems to stir the sky around it. That’s because its handle is anchored by the North Star, Polaris. The star marks the projection of Earth’s north pole on the sky. As Earth spins on its axis, all the other stars seem to rotate around Polaris. Since the other stars of the Little Dipper are close by, their rotation around Polaris is pretty obvious.

And so is the rotation of the nearby Big Dipper, which is to the lower right of the Little Dipper at nightfall.

Because the stars of the Big Dipper are farther from Polaris, they draw bigger circles on the sky as they loop around it. The stars themselves aren’t moving, of course — they move across the sky because of Earth’s rotation. Still, if you watch the Big Dipper wheel around the sky each night, it’s easy to understand why it took thousands of years to understand that we’re the ones who are moving, and not the stars.

Because the Big Dipper is so prominent, it makes a good pointer for finding Polaris. Line up the stars at the outer edge of the Big Dipper’s bowl, and follow the line upward, away from the open bowl. The first moderately bright star you come to along that line is Polaris — the handle of the spoon that stirs the starry sky.


Script by Damond Benningfield, Copyright 2011


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