And the celestial equator passes through the east and west points of the horizon regardless of an observer's latitude. Coordinates in the equatorial system are called right ascension and declination. Declination is the angular distance of an object north or south of the celestial equator measured along an hour circle. The other coordinate is called right ascension and is measured eastward along the celestial equator from the vernal equinox to the foot of the hour circle passing through the object. The vernal equinox, so named because the Sun moves through this point on or about March 21, is the point of intersection of the celestial equator with the annual apparent path of the Sun on the celestial sphere, the ecliptic.
Right ascension is measured in units of time rather than degrees of arc.
Since a 360 o rotation by the Earth corresponds to 24 h , then 1 h equals 15 o of arc, or 1 o equals 4 m , and so on. Finally, the angular distance of the hour circle from the observer's celestial meridian is called the hour angle of the star. It is obvious that the right ascension and declination of a celestial object remain fixed as the sky rotates. This makes it possible to catalog astronomical objects by right ascensions and declinations in the same way in which places on Earth are cataloged by longitudes and latitudes. Thus over a number of years, a star's right ascension and declination change so that cataloged positions are not accurate indefinitely and must be updated periodically to correct for precession.