Standard Time 1B.4. Calendars To locate an object on the sky, astronomers use several different coordinate systems which are similar in concept to the system for plotting a point in a plane. The two most frequently used coordinate systems are the horizon system and the equatorial system, and they are analogous in principle to the grid lines of geographic longitude and latitude. On the celestial sphere, distances are measured in degrees along the arc of a great circle. A great circle, formed by passing a plane through the center of a sphere, divides the sphere into two equal halves.
Several features are common to all astronomical coordinate systems.
Each has a principal axis, or polar axis, about which the system rotates. The points of intersection of this axis and the celestial sphere are the poles of the system. Perpendicular to the principal axis is a great circle, which is the principal reference circle along which one coordinate is measured. Finally, there is an infinite number of secondary reference circles that are great circles perpendicular to the principal reference circle, and which meet at the poles of the principal axis, as shown in Figure A2.1. The most natural and the easiest coordinate system to visualize is the equatorial system.