Glossary term: Circumpolar Stars

Description: In most locations on Earth, either the celestial North Pole or the celestial South Pole is visible in the sky at some distance above the horizon. For an observer at such a location, the stars appear to rotate around the celestial pole as time passes: Each star traces a circle in the sky, with the circle centered on the celestial pole to which Earth's axis points. At the two points where a circle crosses the observer's horizon, the star in question will rise and set, respectively. For stars that are close enough to the celestial pole, the traced-out circle will be completely above the horizon. Our observer will never see those stars either rise or set. Those never-setting stars are called circumpolar stars.

Which stars are circumpolar depends on the observer's geographic latitude and on the star's declination – the latter is the angle between the star's location and the celestial equator. In the northern hemisphere, a star is circumpolar if its declination is larger than 90° minus the observer's latitude. In the southern hemisphere, we need to take into account that both southern latitudes on Earth and southern declination values have a minus sign. Taking these signs into account, on the southern hemisphere, a star is circumpolar if its declination is smaller than –90° minus the observer's latitude.

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Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

The OAE Multilingual Glossary is a project of the IAU Office of Astronomy for Education (OAE) in collaboration with the IAU Office of Astronomy Outreach (OAO). The terms and definitions were chosen, written and reviewed by a collective effort from the OAE, the OAE Centers and Nodes, the OAE National Astronomy Education Coordinators (NAECs) and other volunteers. You can find a full list of credits here. All glossary terms and their definitions are released under a Creative Commons CC BY-4.0 license and should be credited to "IAU OAE".

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Tags: History , Geography , Celestial navigation
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