E

called declination and is measured in degrees from -90° to +90° with zero at the equator. The celestial longitude (Greek alpha, a) is called right ascension and is measured from west to east in units of time, hr-min-sec from 0 h to 24 h (equivalent to 0° to 360°), with zero at the sun's position when it crosses the celestial equator on March 21. This position (a = 0, 8 = 0) on the celestial sphere is called the vernal equinox. The directions of increasing a and 8 define a "right handed" coordinate system.

These two coordinates, a and 8, are angles subtended from the center of the earth; they are spherical coordinates. Most stars are so far from the solar system that it would not matter if the origin of the system were the center of the earth, the top of Palomar Mountain, or the barycenter (center of mass) of the solar system. However, for closer stars and planets, the motion of the earth about the sun leads to varying apparent positions on the celestial sphere, e.g., the parallax discussed above. Thus, as noted, current precise catalogs of stellar positions report the positions in a barycentric coordinate system wherein the motion of the earth is removed.

The equatorial coordinates of a star change slowly year by year due to the slow precession (period 25 770 years) of the earth. The earth's axis precesses slowly about an average direction perpendicular to the orbital plane of the sun-earth system, much

Table 3.1. Examples of celestial equatorial coordinates

Object

Epoch

Right asc. (a) h m

Declination (5)

° /

Sirius (a CMa)

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