The accurate positions frequently needed in astronomy must be measured relative to a standard reference frame. To understand the coordinate systems used in astronomy, it is helpful to review first the familiar latitude-longitude system. It is convenient to measure positions on Earth in terms of LATITUDE and LONGITUDE. These two coordinates are determined relative to a standard reference frame or COORDINATE SYSTEM that, in this case, is based on*the equator of the Earth as the PLANE OF REFERENCE (and the angular distance measured along the equator from the zero point of longitude). The zero point is defined as the longitude of the line perpendicular to the equator passing through Greenwich, England.
It is convenient to use a variety of astronomical coordinate systems depending on the class of object which is being observed. Because coordinate systems are intended to allow the positions of objects to be specified with very great accuracy, it is inevitable that their definitions appear to be pedantic. We will find it helpful to use the concept of a great circle. If we imagine any plane which passes through the centre of the Earth, then the line on the surface of the Earth which this plane makes is called a GREAT CIRCLE. If we now imagine this plane to extend out into space, the imaginary line it marks on the sky (or CELESTIAL SPHERE) is also a great circle. The celestial sphere is an imaginary spherical surface at a great distance from the Earth with the Earth at its centre.