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Spica by John Chumack
A circumpolar star is a star that never sets when viewed from from a particular latitude here on Earth(never disappears below the horizon), because of its proximity to one of the celestial poles, so these stars are visible for the entire night, every night of the year. They would also be visible 24 hours of the day if the Sun was not brighter.
As Earth spins on its axis, the stars appear to rotate in circular paths around one of the celestial poles. Stars far from a celestial pole appear to rotate in large circles; those close to a celestial pole rotate in small circles and may not seem to move at all. The circumpolar stars are close enough to the celestial pole to remain continuously above the horizon.
The circumpolar stars appear to be in a circle centered at the celestial pole and tangential to the horizon. At the North Pole all stars that are visible are circumpolar. As you travel south, the north celestial pole moves towards the northern horizon, so more stars begin to disappear below the horizon for some portion of the day. At the equator there are effectively no circumpolar stars at all.
When you travel south of the equator the opposite happens. The south celestial pole appears increasingly high in the sky, and all the stars lying within an increasingly large circle centered on that pole become circumpolar. This continues until you reach the South Pole where all visible stars are circumpolar.
Basically, your position on Earth determines whether a star appears to be circumpolar. The star itself does not move, but it position appears to change based on the location of the observer.
We have written many articles about circumpolar stars for Universe Today. Here’s an article about the celestial sphere, and here’s an article about Canopus.
If you’d like more information on stars, check out Hubblesite’s News Releases about Stars, and here’s the stars and galaxies homepage.
We’ve done many episodes of Astronomy Cast about stars. Listen here, Episode 12: Where Do Baby Stars Come From?
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