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 most circumpolar stars do not seem to engage in diurnal motion, at all.

The polstar and circumpolar stars

Such a definition implies that different stars can be defined as circumpolar at different Earth latitudes. For example, to an observer place 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.

On the northern hemisphere all circumpolar stars rotate around the North Star Polaris, which iself it 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.