Other effects of the moon's revolution about the earth are solar and lunar eclipses, which occur when the sun, the earth, and the moon are in line. The earth and the moon both cast shadows into space, in a direction away from the sun. A solar eclipse occurs whenever the shadow of the moon falls on a part of the surface of the earth, blocking light from the sun. As determined by the alignment of the three bodies, an observer on the earth may witness a total eclipse, or only a partial eclipse if part of the disc of the sun is visible. A solar eclipse is annular when the moon's distance from the earth is so great to permit a narrow ring of sunlight to appear around the moon. A lunar eclipse Occurs when the moon passes through the shadow of the earth; it, too, may be partial or total.
The earth is, in effect, a gigantic gyroscope, and is subject to the laws of gyroscopic motion. However, it is not a perfect sphere and has a bulge about its equator which is inclined at 23.5° to the plane of its orbit.
The moon and sun exert gravitational forces on the earth, and these forces tend to make the polar axis perpendicular to the plane of its orbit. Due to its rotation the earth these strong forces react like a gyroscope when external force is applied. It precesses in a direction which is at right angles to the direction of the external force.
This precession causes a slow rotation of the earth's axis about an axis projected outward at right angles to the plane of its orbit, slowly tracing a circle on the celestial sphere. The period of precession is about 25,800 years. As noted before, the apparent location of the sun among the stars when its declination is 0° is termed an equinox; this occurs each spring and fall. The gyroscopic action of the earth causes a precession of the equinoxes; this is at a rate of about 50 seconds of arc per year and is in a westerly direction-that is, clockwise from the north. This is the opposite direction to both the earth's rotation and its revolution. The period of the earth's precession is not uniform due to the varying positions of the moon relative to the earth's equator and some small effects of other bodies; this slight variation is termed nutation.
In addition to the major motions described above, there are several motions of the earth of minor importance. Two of the more significant in navigation are the wandering of the terrestrial poles and the variations in speed of rotation of the earth.
The north and south terrestrial poles, or the points where the earth's axis of rotation pierces the earth's surface, are not stationary. They wander slightly in somewhat circular paths. The movement is believed to be caused by meteorological effects. Each pole wanders in an area smaller than a baseball diamond, and neither has been known to move more than 40 feet from its average position. The phenomenon is also called "variation in latitude."
The rotational speed of the earth on its axis is steadily decreasing by a small amount, causing the length of the day to increase at the rate of about 0.001 second a century. There are also small irregular changes in the rotational period, the causes of which are uncertain. With the introduction of atomic time standards which keep absolute time, variations in the speed of rotation of the earth-which affect its rotational position and astronomical observations are of interest to the navigator.
Navigational astronomy is that part of astronomy in general! which is of interest and use to a navigator. It is concerned primarly with the apparent motion of celestial bodies. These apparent motions are relative motions caused by the actual movements of the bodies as seen from the earth. Their apparent positions in space are tabulated in almanacs and are used by a navigator in solving the navigational triangle to determine his position.
