Circumpolar star
From Wikipedia, the free encyclopedia
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 stargazer in Ohio will never be able to see a star far enough south of the celestial equator to be circumpolar.
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 degrees from the celestial equator; i.e., a declination between 55° and 90° on the celestial sphere. In other words, circumpolar stars are generally within 35° of the north or south celestial pole. These astronomers propose this idea because, from the vantange point of stargazers outside of the tropics, most stars with declinations between 55° and 90° never rise or set on the horizon in their respective hemispheres. Stars of extreme northern constellations, such as Cassiopeia, Cepheus, Ursa Major, and Ursa Minor are well within this zone. Observers living in mid-northern latitudes always see the stars of these constellations above the horizon, never rising or setting. Stars of extreme southern constellations, such as Crux, Carina, and Hydrus are also well within this zone of circumpolar stars. Observers living in mid-southern latitudes, in countries such as Australia, South Africa, and Argentina, always see the stars of these constellations above the horizon. From their vantange point, the diurnal circles of these extreme southern stars do not cross below the horizon; therefore, they never rise or set. Stars and constellations that are circumpolar (within 35° of the celestial pole) in one hemisphere are virtually invisible, never rising above the horizon, in the middle and high latitudes of the opposite hemisphere. For example, the southern circumpolar star Acrux is invisible from most the Continental United States, likewise, the seven stars of the northern circumpolar Big Dipper asterism are invisible from most of the Patagonia region of South America.
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.
