<P> The currently used standard epoch is J2000. 0, which is January 1, 2000 at 12: 00 TT . The prefix "J" indicates that it is a Julian epoch . Prior to J2000. 0, astronomers used the successive Besselian Epochs B1875. 0, B1900. 0, and B1950. 0 . </P> <P> A star's direction remains nearly fixed due to its vast distance, but its right ascension and declination do change gradually due to precession of the equinoxes and proper motion, and cyclically due to annual parallax . The declinations of Solar System objects change very rapidly compared to those of stars, due to orbital motion and close proximity . </P> <P> As seen from locations in the Earth's Northern Hemisphere, celestial objects with declinations greater than 90 ° − φ (where φ = observer's latitude) appear to circle daily around the celestial pole without dipping below the horizon, and are therefore called circumpolar stars . This similarly occurs in the Southern Hemisphere for objects with declinations less (i.e. more negative) than − 90 ° − φ (where φ is always a negative number for southern latitudes). An extreme example is the pole star which has a declination near to + 90 °, so is circumpolar as seen from anywhere in the Northern Hemisphere except very close to the equator . </P> <P> Circumpolar stars never dip below the horizon . Conversely, there are other stars that never rise above the horizon, as seen from any given point on the Earth's surface (except extremely close to the equator . On flat terrain, the distance has to be within about 2km, but this varies based on observer altitude and surrounding terrain). Generally, if a star whose declination is δ is circumpolar for some observer (where δ is either positive or negative), then a star whose declination is − δ never rises above the horizon, as seen by the same observer . (This neglects the effect of atmospheric refraction .) Likewise, if a star is circumpolar for an observer at latitude φ, then it never rises above the horizon as seen by an observer at latitude − φ . </P>

The altitude of this star changes as we change latitude