EARTH SATELLITES

The positions of the moon and other earth satellites may be plotted on the Uniglobe using the same methods as for other celestial objects. Any earth satellite including the moon, revolves in an orbit defined by its APOGEE, the point at which it is furthest from the earth, and PERIGEE, the point at which it is closest to the earth and its inclination with respect to a reference circle. In the case of the moon, apogee is 253,000 mi. and perigee is 21,000 mi. The average distance is 238,000 mi. The period of the moon's orbit is approximately 29 1/2 days with respect to the sun. The orbit is inclined to the earth's ecliptic approximately 5°.

Artificial satellites are generally in orbits of 100 miles or higher above the earth and can assume any inclination to the earth's equator depending on the launch direction. Satellites launched from Vandenberg Air Forece Base, south across the Pacific Ocean assume an orbit at angles approaching 90° to the equator. Those launched from Cape Kennedy eastward over the Atlantic Ocean, generally have orbits lying in a band around the equator. The period of a 100 mile high orbiting satellite is generally about 90 minutes. Earth satellites in 2,200 mile high orbits have a period of 24 hours (stationary orbit).

These satellites are used for radio, television and telphone relays between continents and are usually equatorial orbits.

Referring to Figure 38 the moon's orbit is described by a descending node which is the point at which the moon passes the ecliptic on its way south and and ascending node 180° from the descending node. Eclipses of the moon appear when the moon is near a node and the sun is near the opposite node. Solar eclipses occur when the sun and the moon are both near either the descending or ascending node. The nodes are not fixed but move in a westerly direction along the ecliptic. In a period of 18.9 years the nodes will complete 1 cycle of 360°. Because of the effect of the sun on the moon's orbit it is not a true great circle but a spiral which approximates a great circle. See Figure 39.

To plot the moon's position, refer to the graphical ephemeris. When the moon is full it is directly opposite the sun's position. Consult the ephemeris for the sun's position for any particular date. When the moon is not full, it is somewhere else in its orbit depending upon the number of days from a full moon. The time between full moons is about 29 1/2 days, so 14 3/4 days froma full moon the moon is near the position of the sun and is called a new moon.

Page 21