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Circumpolar Stars

October 6, 2019

Two well-known star patterns highlight the northern sky tonight. The Big Dipper is low in the north-northwest at nightfall, and in the northeast at first light tomorrow. And W-shaped Cassiopeia is just the opposite — in the northeast at nightfall, and north-northwest at first light.

As that sequence suggests, both patterns make a big circle around the sky during the night. They circle the North Star, Polaris — the hub of the northern sky. All the stars in the northern hemisphere appear to move around Polaris — the result of Earth turning on its axis.

For much of the United States, the stars of the Big Dipper and Cassiopeia never set at all. They appear close enough to Polaris that they never drop below the horizon. So they’re in the sky every day and night of the year, endlessly circling the North Star.

Such stars are called “circumpolar.” The number of such stars from any given location depends on latitude. From 30 degrees north — the latitude of Austin or New Orleans — anything within 30 degrees of Polaris always remains above the horizon. From 50 degrees north — roughly Paris or Vancouver — it’s anything within 50 degrees of Polaris. So as you go farther north, more stars are circumpolar.

And if you go all the way north — to the north pole — all the stars are circumpolar — nothing ever rises or sets. Instead, each star follows the same path across the sky night after night, circling the hub of the northern sky: Polaris.

Script by Damond Benningfield

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