The Earth's Moon

The Earth's one natural satellite, the Moon, is more than one quarter the size of Earth itself (3,474 km diameter). Because of its smaller size, the Moon's gravity is one-sixth of the Earth's gravity, as we saw demonstrated by the giant leaps of the Apollo astronauts.

While there are only two basic types of regions on the Moon's surface, there are many interesting surface features such as craters, mountain ranges, rilles, and lava plains. The structure of the Moon's interior is more difficult to study. The Moon's top layer is a rocky solid, perhaps 800 km thick. Beneath this layer is a partially molten zone. Although it is not known for certain, many lunar geologists believe the Moon may have a small iron core, even though the Moon has no magnetic field. By studying the Moon's surface and interior, geologists can learn about the Moon's geological history and its formation.
The footprints left by Apollo astronauts will last for centuries because there is no wind on the Moon. The Moon does not possess any atmosphere, so there is no weather as we are used to on Earth. Because there is no atmosphere to trap heat, the temperatures on the Moon are extreme, ranging from 100° C at noon to -173° C at night.
The Moon doesn't produce its own light, but looks bright because it reflects light from the Sun. Think of the Sun as a light bulb, and the Moon as a mirror, reflecting light from the light bulb. The lunar phase changes as the Moon orbits the Earth and different portions of its surface are illuminated by the Sun.