The plane of reference of this system is the equator of the Earth and the poles are at the points where the rotation axis of the Earth cuts the celestial sphere. The poles are called the NORTH AND SOUTH CELESTIAL POLES. The equatorial plane of the Earth cuts the celestial sphere at the CELESTIAL EQUATOR. The great circle that passes through the celestial poles and the zenith is called the MERIDIAN, and the great circle through the poles and the object is called the HOUR CIRCLE of the object. The hour circle and the meridian intersect the celestial equator at two points, the angle between which is called the HOUR ANGLE (HA) of the object. The celestial equator is divided not into degrees but into hours (24), minutes (60) and seconds (60). It is in these units that the hour angle is expressed. The angle between the intercept of the hour circle and the celestial equator, and a reference point (the vernal equinox) is called the RIGHT ASCENSION (RA) of the object. This is also measured in hours, and the direction is such that the right ascensions of objects crossing the meridian increase with time. This also means that if the hour angle of an object starts negative, it becomes zero when it crosses the meridian and thereafter is positive. The other coordinate in this system is the DECLINATION (Dec) of the object, measured in degrees. Declinations are positive when measured from the celestial equator along the hour circle towards the celestial north-pole and negative when measured towards the celestial south pole.
The right ascension of the meridian is the same as the local sidereal time and it is partly for this reason that right ascension is expressed in time units rather than degrees.
The reference point from which right ascension is measured is the VERNAL EQUINOX. This is one of the two points at which the plane of the Earth’s equator and the plane of the Earth’s orbit round the Sun (the ecliptic) intersect. The Sun passes through the vernal equinox in the spring (about March 21), and through the opposite point (the autumnal equinox) in the autumn (about September 22). The points on the celestial equator at right ascension of 6 hr and 18hr are the SUMMER AND WINTER SOLSTICES respectively. Because the equatorial system uses Earth’s equator and pole, the system is in motion relative to distant stars because Earth’s axis precesses. Consequently, the point at which the equator and the ecliptic cross slowly moves through the sky, taking about 26 000 years to make a full circuit.The vernal equinox is now in the constellation Pisces. Many years ago it was in Aries, and so it is sometimes called the first point of Aries.