Lunar Perigee

The Moon is the Earth's only satellite. It is the only astronomical body that humans have visited. It is also the only body beyond the Earth that scientists have samples of known origin that can be analyzed and dated in terrestrial laboratories. The moon is large enough to pass through the stages of planetary evolution.

The Moon has a low mass and thus has a low escape velocity. It therefore cannot hold an atmosphere (which is why it holds no water). Because it has no atmosphere there are no winds. Thus, erosion cannot occur because neither wind nor water is present. So the impact craters that meteors impress on the moon are not worn away. Larger meteorites scatter pulverized rock called ejecta over large surface areas. This is very apparent where this debris forms rays, white streamers radiating from craters such as Copernicus and Kepler.

The Moon has no plates and has a small core. The lunar terrain is very sandy and divides into two markedly different regions, the lowlands and the highlands. The lowlands are called maria (singular mare), meaning seas, and are smooth dark plains with circular outlines and few craters. They got their name because the first astronomers to examine the moon with telescopes thought they were oceans. Maria are covered by a thick layer of pulverized lava. The lunar highlands are the lighter-colored heavily cratered regions that lie 3 km higher than the lowlands. They were not flooded by the lava that formed the maria because of their height, and so represent and earlier stage in the moon's history.

Like the Earth, the moon formed in four stages. In order, they were differentiation, cratering, flooding of the basins with lava, and surface evolution. The moon is now a dead body stuck somewhere in between the third and fourth stages.

The now excepted hypothesis about the origin of the moon is the large-impact hypothesis. The large-impact hypothesis states that the moon and the Earth resulted from the collision of two very large planetesimals.

Like the other planets, the moon has two periods of revolution, the sidereal period and the synodic period. The sidereal month, or period between two successive full moons, is 29 days and 12 hours. The synodic month, or average period in time for the moon's orbit using the background stars as a frame of reference is 27 days and 7 hours. We on Earth observe the moon's sidereal month.

The moon goes through several phases. The new moon is the phase of the moon when it is out during the day. Because the dark side phases us we cannot see it. Each day it rises and sets about 50 minutes later. When a new moon begins it grows, or waxes in size. It starts from a crescent, to a quarter moon, then a gibbous, and finally a full moon. During the full moon the moon's bright side faces the Earth. Then it shrinks, or wanes, to a gibbous, a quarter moon, a crescent, and finally a new moon again. The moon's period of revolution is used as the basis of the Hebrew/Jewish calendar, with each new moon starting off a new month.

The moon revolves around the Earth in an elliptical orbit. The moon has a mean distance of 238,866 miles from the Earth. At perogee, or its closes approach to the Earth, the moon is 228,000 miles away. At apogee, or its furthest approach to the Earth, the moon is 252,000 miles away.

About twice a year a lunar eclipse occurs. This takes place during the full moon, when the Earth is between the Sun and the moon. But it only occurs if the moon falls under the Earth's shadow. If it falls under the Earth's umbra, or the totally shaded region, than it is a total lunar eclipse. If the moon falls only under part of the Earth's umbra or only falls under the penumbra, or the partially shaded region, than it is a partial lunar eclipse.

During a total lunar eclipse, it takes about an hour for the moon to reach the umbra. Totality can last up to an hour and 45 minutes. At this time the moon is not completely dark but glows a bright coppery red. The light seen is the Sun's light refracted from the Earth. It can only be seen with a telescope, though. A total lunar eclipse is not as spectacular as a total solar eclipse, but it can be seen from anywhere on the ground as long as the observer is on the side of the Earth which faces the moon during the eclipse.

Bibliography:

	Muirden, James. The Amateur Astronomer's Handbook. Copyright 1974. Thomas Y. Crowell Company. New York.
	Seeds, Michael A. Horizons. Copyright 1991. Wadsworth Publishing Company. Belmont, California.