The Moon The Earth's moon is not the largest satellite in the solar system. However, it is closer in size to its planet than any other satellite (with the possible exception of Pluto's Charon). The moon's diameter is 3476 kilometers - slightly over ¼ the Earth's diameter. The moon's density is about 3.3 g/cm3 (less than Earth's) and it's mass is only about 1/80th Earth's.

The moon has gravity! It's gravity is only about 1/6 that of Earth's and it's escape velocity is only 1/5 that of Earth's. This means that it's easier to launch a spacecraft from the moon than it is from Earth. YES! Is there gravity on the moon?

The moon's period of rotation is about the same as its period of revolution, about 27.3 days. This means that the moon turns just once around its axis while it goes once around the Earth. The effect is that the same side of the moon always faces the Earth. (There is no "dark side of the moon" however)

When you look at the moon, most people see a pattern of light areas and dark areas. Telescopes reveal details within the patterns. The light areas are called lunar highlands. They are rugged mountains pockmarked with craters. The dark areas are great basins and level plains.

Galileo had thought the dark areas were filled with water and had named them MARIA, the Latin word for seas. The moon's front side is nearly half highlands and half maria. The back of the moon is very different. Photos taken by lunar probes show the back half to be mostly highlands and craters. There are only a few small maria.

The Lunar Maria Although Galileo thought the dark areas on the moon's surface were seas, the lunar maria contain no water and there is no sign that they ever did. The maria are smooth plains with huge circular basins.

2 - Mare Tranquillitatis 1 - Mare Serenitatis 3 - Mare Foecunditatis 4- Mare Nectaris 8 - Mare Nubium Sea of Serenity Sea of Tranquility

Maria has been discovered to be made of a rock known as basalt. A dark, volcanic rock similar to that seen in Hawaiian and Icelandic volcanoes.

Rilles are long, deep clefts or cracks running through maria bedrock. The best known is Hadley Rille on the floor of Mare Imbrium. Scientists believe that this rille may have formed when the roof of a lava tunnel caved in.

The Lunar Highlands These are the lighter areas on the moon. They appear lighter mostly because their rocks are lighter in color and they reflect more sunlight.

Within the lunar highlands are a few mountain ranges and many craters. Most of the mountain ranges are at the edges of maria.

Two kinds of rock were returned from the lunar highlands. One is a light-colored, coarsely crystalline igneous rock. The composition of this rock lies somewhere between Earth's gabbro and anorthosite.

Other specimens brought back from the lunar highlands are lunar breccias. Breccias are rocks made of angular fragments cemented together with fine material.

Lunar Craters Lunar craters are hollows on the moon's surface. The smallest craters are microscopic pits. The largest, Clavius, is about 240 kilometers across. Most craters were formed by the impact of meteorites.

Craters are roughly circular. Their rims are rugged cliffs. In large craters, the rims may be thousands of meters above the plains. Most crater floors are themselves dotted with many small peaks and craters. Like the highlands, these peaks reflect enough sunlight to look bright.

Lunar Soil Lunar soil is not really soil. Scientists prefer to call it regolith which means loose rock material. Regolith is a grayish-brown mixture of small rock pieces and fine particles that range in size from sand gains to fine dust. Unlike Earth soil, regolith contains no water or organic material. It was made by smashing impace of meteoroids of all sizes. When large meteoroids explode, they mix rock fragments all over broad areas.

The Moon's Motions and Phases Moon's Orbit The moon revolves around the Earth from west to east in an elliptical orbit. It's period of revolution is 27 1/3 days. Its average distance from the Earth is about 386,000 kilometers. When the moon is nearest Earth, it is said to be at perigee. When farthest from Earth, it is at apogee.