Astronomy Essentials

Circumpolar stars stay up all night long

Circumpolar stars, concentric circles of white, blue and orange in sky over hillside.
A time-lapse photo creates these star trails. At the center of the concentric circles is the south celestial pole. The stars that never rise nor set are circumpolar stars. Image via Yuri Beletsky Nightscapes.

What are circumpolar stars?

Circumpolar stars always reside above the horizon, and for that reason, never rise nor set. All the stars at the Earth’s North Pole and South Pole are circumpolar. Meanwhile, no star is circumpolar at the equator.

Anyplace else has some circumpolar stars and some stars that rise and set daily. The closer you are to either the North or South Pole, the greater the circle of circumpolar stars. The closer you are to the equator, the smaller the circle.

From the Northern Hemisphere, all the stars in the sky go full circle around the north celestial pole once a day. Or, more precisely, go full circle every 23 hours and 56 minutes. And from the Southern Hemisphere, all the stars in the sky go full circle around the south celestial pole in 23 hours and 56 minutes.

Moving graphic shows two constellations swinging around central star.
The Big Dipper and the W-shaped constellation Cassiopeia circle around Polaris, the North Star. They do so in a period of 23 hours and 56 minutes. The Big Dipper is circumpolar at 41 degrees North latitude and all latitudes farther north. Image via Sazvežda – Page/ Ana Oriflame/ Pinterest.

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Polaris and the north celestial pole

We in the Northern Hemisphere are particularly lucky to have Polaris. It is a moderately bright star, closely marking the north celestial pole. The north celestial pole is the point of sky that’s at zenith (directly overhead) at the Earth’s North Pole.

At the equator (0 degrees latitude), the star Polaris – the stellar hub – sits right on the northern horizon. Therefore, no star can be circumpolar at the Earth’s equator. But at the North Pole (90 degrees), Polaris shines at zenith (directly overhead). So from the North Pole, every star in the sky stays above the horizon all day long every day of the year.

Your latitude determines the circle of circumpolar stars in your sky. For instance, at 30 degrees North latitude, the circle of stars within a radius of 30 degrees from Polaris is circumpolar. In the same vein, at 45 degrees or 60 degrees North latitude, the circle of stars within 45 degrees or 60 degrees of Polaris, respectively, would be circumpolar. Finally, at the North Pole, the circle of stars all the way to the horizon is circumpolar.

Trees in foreground with arcing lights circling in background, center of circle near horizon.
View at EarthSky Community Photos. | Basudeb Chakrabarti in Jhargram, West Bengal, India, created this blended image – showing star trails, or the movements of the stars over some hours — on February 6. He used 240 separate short takes, over several hours, to make the image. From his location at 22 degrees North latitude, the north celestial pole is near the horizon and few stars stay above the horizon all night long. That is, few are circumpolar. Thank you, Basudeb!
Short white dashes in concentric circles on a black background.
View at EarthSky Community Photos. | Ragini Chaturvedi took this image of the night sky on January 6, 2022. Ragini wrote: “First good star trail image I have taken at Death Valley, California, in the vast, open, clear sky of endless bright twinkling stars. Those concentric circles around the focused and fixed North Star, Polaris, look good.” Yes, they do! Thanks, Ragini! Learn how to create photos of star trails with long exposures.

The Big Dipper is circumpolar

At 41 degrees North latitude (the latitude of New York City) and all latitudes farther north, the famous Big Dipper asterism is circumpolar. That’s because the southernmost star of the Big Dipper, Alkaid – the star marking the end of the Big Dipper handle – is 41 degrees south of the north celestial pole (or 49 degrees north of the celestial equator).

Constellation in four positions around central star.
If you’re in the northern US, Canada or at a similar latitude, the Big Dipper is circumpolar for you. These images show the Dipper’s location at around midnight in each season. Just remember “spring up and fall down” for the Dipper’s appearance in our northern sky. It ascends in the northeast on spring evenings and descends in the northwest on fall evenings. Image via burro.astr.cwru.edu.

Bottom line: Circumpolar stars are those that never rise nor set from a certain location. At the poles, all stars are circumpolar, while at the equator, no star is.

Posted 
February 18, 2022
 in 
Astronomy Essentials

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