The Ecliptic and Celestial Equatorial Planes

The ecliptic (the path of the earth about the sun) is said to be in the ecliptic plane, which therefore contains the sun's and the earth's mass centers (Fig. 1.1). One also speaks of the celestial equatorial plane, which is the plane parallel to the earth's equatorial plane and through the sun's mass center. The ecliptic and celestial equatorial planes intersect, as illustrated in the figure, in a line referred to as the equinox line (because, when the earth on its annual path crosses this line, day and night have equal length). The crossing points are called the vernal equinox (the earth is at this point on about the 21st of March) and the autumnal equinox (about the 22nd of September). As discussed in Chapter 7, the vernal and autumnal equinox points play an important role in the operation of geosynchronous and other spacecraft. During two periods each year, centered around these points, and centered around local midnight, the sun will be eclipsed for these satellites. Special operational procedures are then needed to compensate for the lack of solar radiation.

The direction of the equinox line that points from the earth at the vernal equinox toward the sun is referred to, for historical reasons, as the first point of Aries and is often designated by the symbol T. (As the name suggests, the line was found by early astronomers some 2500 years ago to point toward the constellation Aries. But since it moves by about 0.8' per year, it is presently in Pisces, moving into Aquarius.)

The ecliptic plane is inclined to the equatorial plane at an angle ic called the obliquity of the ecliptic. At present it is 23.44°. (Primarily as a consequence of the gravitational attraction by the sun and the moon on the earth's equatorial bulge, the earth's spin axis, and hence also the equatorial plane, precesses relative to the most distant stars with a period of 25,920 years. The obliquity fluctuates between 21.5° and 24.5° with a period of about 41,000 years and is presently decreasing by about 0.5" per year. The eccentricity of the earth's orbit about the sun also has a small fluctuation, with a period of about 100,000 years. All these effects, however, are too small to be significant for most space operations.)

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