Circumpolar star: Difference between revisions
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'''Circumpolar stars''' are those stars which are located near the [[celestial pole]]s of the [[celestial sphere]], i.e. the poles in the [[equatorial coordinate system]]. As the Earth rotates, the sky appears to rotate; and most stars will be hidden below the [[horizon]] at some point in their circular paths. If, from a certain location, a star is near enough to the [[celestial pole]] that it never appears to go "under the horizon"; it will therefore be visible (from said location) for the entire night, on every day of the year. Some of the circumpolar stars nearest the poles do not seem to engage in [[diurnal motion]] at all. For instance, a stargazar in |
'''Circumpolar stars''' are those stars which are located near the [[celestial pole]]s of the [[celestial sphere]], i.e. the poles in the [[equatorial coordinate system]]. As the Earth rotates, the sky appears to rotate; and most stars will be hidden below the [[horizon]] at some point in their circular paths. If, from a certain location, a star is near enough to the [[celestial pole]] that it never appears to go "under the horizon"; it will therefore be visible (from said location) for the entire night, on every day of the year. Some of the circumpolar stars nearest the poles do not seem to engage in [[diurnal motion]] at all. For instance, a stargazar in Ohio will never be able to see a star far enough south of the celestial equator to be circumpolar.[[Image:Circumpolar_stars.jpg|thumb|The North Star and circumpolar stars]]. |
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Such a definition implies that different stars can be defined as circumpolar at different Earth latitudes. For example, to an observer placed right at the [[Earth]]'s [[North Pole|North]] or [[South Pole]], virtually all the stars are circumpolars. For an observer exactly on the [[equator]], no star can be defined circumpolar, as the pole itself is on the horizon. At different latitudes, an intermediate situation makes some stars circumpolars and others not. |
Such a definition implies that different stars can be defined as circumpolar at different Earth latitudes. For example, to an observer placed right at the [[Earth]]'s [[North Pole|North]] or [[South Pole]], virtually all the stars are circumpolars. For an observer exactly on the [[equator]], no star can be defined circumpolar, as the pole itself is on the horizon. At different latitudes, an intermediate situation makes some stars circumpolars and others not. Some astronomers define a circumpolar star as any star having a [[declination]] of 55 or more degree from the celestial equator; i.e., a declination between 55° and 90° on the celestial sphere. From the vantage point of stargazers outside of the tropics, most stars with declinations between 55° and 90° never set below the horizon. Stars of extreme northern constellations, such as [[Cassiopeia (constellation)|Cassiopeia]], [[Cepheus (constellation)|Cepheus]], [[Ursa Major (constellation)|Ursa Major]], and [[Ursa Minor (constellation)|Ursa Minor]] are well within this zone. Stars of extreme southern constellations, such as [[Crux (constellation)|Crux]], and [[Carina (constellation)|Crux]] are also well within this zone. |
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On the northern hemisphere all circumpolar stars rotate around the [[North Star]] [[Polaris]], which itself is almost stationary, always at the north (i.e., the [[azimuth]] is 0°), and always at the same [[altitude]] (angle from the horizon), equal to the [[latitude]] of the point of observation on Earth. |
On the northern hemisphere all circumpolar stars rotate around the [[North Star]] [[Polaris]], which itself is almost stationary, always at the north (i.e., the [[azimuth]] is 0°), and always at the same [[altitude]] (angle from the horizon), equal to the [[latitude]] of the point of observation on Earth. |
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Revision as of 05:10, 10 January 2007
Circumpolar stars are those stars which are located near the celestial poles of the celestial sphere, i.e. the poles in the equatorial coordinate system. As the Earth rotates, the sky appears to rotate; and most stars will be hidden below the horizon at some point in their circular paths. If, from a certain location, a star is near enough to the celestial pole that it never appears to go "under the horizon"; it will therefore be visible (from said location) for the entire night, on every day of the year. Some of the circumpolar stars nearest the poles do not seem to engage in diurnal motion at all. For instance, a stargazar in Ohio will never be able to see a star far enough south of the celestial equator to be circumpolar.
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Such a definition implies that different stars can be defined as circumpolar at different Earth latitudes. For example, to an observer placed right at the Earth's North or South Pole, virtually all the stars are circumpolars. For an observer exactly on the equator, no star can be defined circumpolar, as the pole itself is on the horizon. At different latitudes, an intermediate situation makes some stars circumpolars and others not. Some astronomers define a circumpolar star as any star having a declination of 55 or more degree from the celestial equator; i.e., a declination between 55° and 90° on the celestial sphere. From the vantage point of stargazers outside of the tropics, most stars with declinations between 55° and 90° never set below the horizon. Stars of extreme northern constellations, such as Cassiopeia, Cepheus, Ursa Major, and Ursa Minor are well within this zone. Stars of extreme southern constellations, such as Crux, and Crux are also well within this zone.
On the northern hemisphere all circumpolar stars rotate around the North Star Polaris, which itself is almost stationary, always at the north (i.e., the azimuth is 0°), and always at the same altitude (angle from the horizon), equal to the latitude of the point of observation on Earth.