ABSTRACT
Astrometry is the branch of astronomy that deals with the positions and motions of heavenly objects. The positions are specified in terms of their distances and angular positions on the sky. We need to choose a suitable reference or coordinate system for this purpose. For example, the entire surface of the Earth is mapped by longitudes and latitudes. The latitude and longitude of any city give us its angular position or coordinates. We similarly imagine a grand celestial sphere with all the stars and planets scattered on its surface. We are interested in defining a suitable angular grid to specify angular positions on this sphere. Besides the two angular coordinates, we also need the third coordinate, namely the distance of the object from some chosen point, called the origin of the coordinate system. The origin may be taken to be the position of the Earth or the Sun.
