Right ascension
 

We have seen that in the equatorial system, one of the coordinates of the star, namely the declination, is constant with time. The other, the hour angle, changes steadily with the passage of time and so is unsuitable for use in a catalogue of stellar positions.
The problem is solved in a manner analogous to the way in which places on the Earth’s surface are defined uniquely in position, although the Earth is rotating on its axis. Latitude is defined with respect to the terrestrial equator. In spherical astronomy, declination, referred to the celestial equator, carries out the same task in fixing the place of a celestial object. The longitude of a place on the Earth’s surface is defined with respect to a meridian through a particular geographical position, namely the Airy Transit Instrument at Greenwich, England, and the meridian through the place in question. The Greenwich meridian cannot be used for celestial position-fixing. Because of the Earth’s rotation under the celestial sphere, the projection of the Greenwich meridian sweeps round the sphere, passing through each star’s position in turn. Some other meridian must be chosen which is connected directly to the celestial sphere.
If a point, , fixed with respect to the stellar background, is chosen on the celestial equator, its angular distance from the intersection of a star’s meridian and the equator will not change, in contrast to the changing hour angle of that star. In general, then, all celestial objects may have their positions on the celestial sphere specified by their declinations and by the angles between their meridians and the meridian through . The point chosen is the vernal equinox, also referred to as the First Point of Aries, and the angle between it and the intersection of the meridian through a celestial object and the equator is called the right ascension (RA) of the object. Right ascension is measured from 0^h to 24^h or from 0^◦ to 360^◦ along the equator from eastwards, i.e. in the direction opposite to that in which the hour angle is measured. Like the definition of hour angle, this convention holds for observers in both northern and southern hemispheres. In drawing a celestial sphere it is advisable not only to mark the observer’s meridian heavily, inserting on the equator a westwards arrow with HA (hour angle) beside it but also to mark on the equator an eastwards arrow with RA (right ascension) beside it. We now show that the choice of as a reference point is closely connected with the Sun’s yearly journey round the stellar background of the celestial sphere.

Figure 1. The variation of the Sun’s declination through the year.