Circumpolar Stars

Two well-known star patterns highlight the northern sky this evening. 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 the north-northwest at dawn. As that sequence tells us, both star patterns make a big circle around the sky during the night. They circle the North Star, Polaris – the hub of the sky. All the stars in the northern sky 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 – they’re 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 your latitude. From 30 degrees north, anything within 30 degrees of Polaris always remains above the horizon. From 50 degrees north, 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 to the north pole, all the stars are circumpolar – nothing ever rises or sets. Each star follows the same path across the sky night after night – circling Polaris, the hub of the northern sky. Polaris won’t keep that position; we’ll talk about its successor tomorrow. Script by Damond Benningfield