Astronomical Co-ordinate System

The position of a source in the sky can be specified in various spherical coordinates systems in astronomy, differing from each other by the position of the origin and orientation of the axis. The position of the sources are specified using the azimuth and elevation angles in these coordinate systems. In the Equatorial Co-ordinate system the source position is specified by the Declination ($\delta$) which is the elevation of the source from the normal to the celestial equator and the Right Ascension ($RA$), which is the azimuthal anlge from a reference position (``the first point of Aries''). The reference direction for $RA$ is line of intersection of the equatorial and Ecliptic planes. The position of the source in the sky, in this coordinate system, remains constant as earth rotates. The azimuth and elevation of the antennas, which rotate with earth, are constantly adjusted to track a point in the sky specified by ($RA,\delta$) coordinates. The changing position of the sources in the sky, as seen by the observer on the surface of earth is specified by replacing $RA$ by Hour Angle ($HA$), which is the azimuth of the source measured in units of time, with respect to the local meridian of the source with $HA=-6^h$ pointing due East.