The Moon is the only satellite orbiting the Earth that is natural. It was called Luna by the Romans and the Greeks had two names for it, Selene and Artemis. Many other cultures had names for the Moon in their mythologies. After the Sun, the Moon is the second brightest object in the sky. The Moon does not produce its own light. It reflects sunlight from its surface. The Moon orbits around the Earth once per month, and as it does, the angle between the Earth and the Moon and the Sun changes. These changes that we see are the cycle of the Moon's phases. The time between new moons is 29.5 days. This is slightly different from the Moon's orbital period around the Earth since the Earth moves a significant distance in its orbit around the Sun in that time. The Moon is sometimes classified as a terrestrial planet because of its size and composition.
The Soviet spacecraft Luna 2 was the first to visit the Moon in 1959. The Moon is the only celestial body to have been visited by humans. The first landing on the Moon was on July 20, 1969. The last time humans set foot on the Moon was December 1972. The Moon is also the only extraterrestrial body from which samples have been brought back to Earth. A small spacecraft named Clementine did some very extensive mapping of the Moon in 1994. The Moon was mapped again by Lunar Prospector in 1999.
The gravitational force between the Earth and the Moon causes some interesting effects here on Earth. The gravitational attraction of the Moon is stronger on the side of the Earth nearest to the Moon and weaker on the opposite side. Since the oceans on Earth are not perfectly rigid, it is stretched out along the line toward the Moon. From the prospective of the Earth's surface, we see two small bulges. One bulge is in the direction of the Moon and the other one is directly opposite. When looking at ocean water, the effect is much stronger than in the solid crust. Thus, the bulges in ocean water are much higher. Also, because the Earth rotates much faster than the Moon moves in it's orbit, the bulges move around the Earth about once a day giving the Earth two high tides per day.
The Moon's rotation always keeps the same face toward the Earth. When the Moon goes through its phases, we are still seeing the same side. This special rotation is called synchronous rotation. This means that the Moon takes exactly as long to rotate on its axis as it does to make one orbit around the Earth. This also means that there is no "dark" side of the moon. All parts of the Moon get sunlight half of the time. At times, one can see slightly more than half of the Moon's surface. This is because the Moon appears to "wobble" slightly over the course of its orbit. It seems as though the Moon is rocking back and forth around its north-south axis and to nod up and down in a north-south direction. This phenomenon is called libration and allows viewers of the moon to see 59% of its surface. The Moon is not really wobbling. We are able to see more of the Moon's surface because its orbit around the Earth is slightly elliptical. As a result, the Moon's orbital motion is not able to keep pace with it's rotation at all points around the orbit. In addition, because the Moon's rotation axis is not exactly perpendicular to the plane of its orbit, the Moon appears to nod up and down.
There is no atmosphere on the Moon. However, the spacecraft Clementine has produced evidence that suggests that there may be water ice in some of the deep craters near the Moon's south pole which are permanently shaded. The spacecraft Lunar Prospector has produced supporting evidence of this possible ice as well as evidence that there may be ice at the north pole also. The Moon is too small of a planet to have an atmosphere. The surface gravity on the Moon is low. Its gravity is only about one-sixth as large as the gravity here on Earth. This low gravity would not allow gas molecules to stay near the Moon's surface. The gas molecules would easily escape into space. With there being no atmosphere on the Moon, there is nothing to scatter the sunlight, so the daytime sky on the Moon is as black as the nighttime sky on Earth.
The surface of the Moon is broken into two primary types of terrain. There is the heavily-cratered and very old highlands terrain. There is also the relatively smooth and younger terrain called
. The maria makes up about 16% of the Moon's surface. The maria terrain has huge impact craters that were later flooded by molten lava. A mixture of fine dust and rocky debris produced by meteor impacts called regolith, covers most of the surface. The maria are the large dark areas on the Moon's surface
. (See Figure 1
) They form a pattern that sort of looks like a human face. This is what began the phrase "the man on the moon." For reasons yet unknown, the maria are concentrated on the near side. Virtually all of the craters on the Moon are a result of the Moon being bombarded meteors. No matter how big or small the crater, they are all called impact craters. Most craters have a circular shape because when a meteor collided with the Moon, it created a shock wave in the Moon's surface at the point of impact. This shock wave usually produced a circular crater no matter what direction the meteor was moving. It is believed that meteors and asteroids, some as large as tens of kilometers across, struck the Moon forming basins. These basins would then flood with lava which would flow from the Moon's interior through the cracks in Moon's crust. The hardened, solidified lava formed the maria that we see today. Surprisingly, when the Moon's far side was photographed by the Soviet spacecraft Luna 3 in 1959, there was almost no maria there. Scientists presume that the Moon's crust is thicker on the far side. It is believed that even the most massive impact from a meteoroid would not be able to crack through the crust and let lava flood onto the surface. Thus, the far side of the moon has very little maria.
(image available in print form)
Image Courtesy of NASA