Earth rotates daily on its axis, the stars appear to move in circular paths around one of the
celestial poles (the north celestial pole for observers in the
Northern Hemisphere, or the south celestial pole for observers in the
Southern Hemisphere). Stars far from a celestial pole appear to rotate in large circles; stars located very close to a celestial pole rotate in small circles and hence hardly seem to engage in any
diurnal motion at all. Depending on the observer's latitude on Earth, some stars – the circumpolar ones – are close enough to the celestial pole to remain continuously above the horizon, while other stars dip below the
horizon for some portion of their daily circular path (and others remain permanently below the horizon).
The circumpolar stars appear to lie within a circle that is centered at the celestial pole and
tangential to the horizon. At the Earth's
North Pole, the north celestial pole is directly overhead, and all stars that are visible at all (that is, all stars in the
Northern Celestial Hemisphere) are circumpolar. As one travels south, the north celestial pole moves towards the northern horizon. More and more stars that are at a distance from it begin to disappear below the horizon for some portion of their daily "orbit", and the circle containing the remaining circumpolar stars becomes increasingly small. At the
Equator, this circle
vanishes to a single point – the celestial pole itself – which lies on the horizon, and there are thus effectively no circumpolar stars at all.
As one travels 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 about it. This continues until one reaches the Earth's
South Pole where, once again, all visible stars are circumpolar.
The north celestial pole is located very close to the
pole star (Polaris or North Star), so from the Northern Hemisphere, all circumpolar stars appear to move around Polaris. Polaris itself remains almost stationary, always at the north (i.e.
azimuth of 0°), and always at the same
altitude (angle from the horizon), equal to the observer's
latitude. These are then classified into quadrants.