2.1 Coordinate Systems

Several coordinate systems are used in the study of the motions of the Earth and other celestial bodies. The heliocentric-ecliptic coordinate system, illustrated in Fig. 2.1, has its origin at the center of the sun. ^[1] The X _? Y _? fundamental plane coincides with the ecliptic plane, which is the plane of the Earth's revolution around the sun. The line of intersection of the ecliptic plane and the Earth's equatorial plane defines the direction of the X _? axis. On the first day of spring, a line joining the center of the Earth and the center of the sun points in the direction of the positive X _? axis. This is called the vernal equinox direction and is denoted by the symbol of a ram's head by astronomers because it points in the general direction of the constellation Aries. All Earth locations experience identical durations of daylight and darkness. The Earth's spin axis wobbles slightly and shifts in direction slowly over centuries. This effect is known as precession, and it causes the line of intersection of the Earth's equatorial plane and the ecliptic plane to shift slowly. As a result, the heliocentric-ecliptic system is not an inertial reference frame. Where extreme precision is required, it is necessary to specify the coordinates of an object based on the vernal equinox direction of a particular year or epoch.

Figure 2.1: Heliocentric ecliptic coordinate system (from Ref. ^[1]).