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Moon, the
Moon, the
The Moon (diameter 3,476km) is a natural satellite of the Earth and fifth largest
moon in the Solar System (see: Moons).
The Moon appears to
be almost trapped in time. While we are used to the surface of the Earth changing, nothing has changed on the Moon for perhaps thousands of millions of years. In this time some of the continents on the Earth’s surface have traveled halfway around the world!
The Moon and Earth orbit together in such a way that we only see 59% of the Moon’s surface: Before spacecraft were sent to photograph the “dark side” of the Moon in 1959, nobody had any idea what the far side looked like. (See also: Apollo project; Luna; Lunar module; Lunar orbiter; Pioneer; Surveyor.)
The Moon’s orbit
The Moon is about one-eightieth the mass of the Earth. The Moon and the Earth move together like a double
planet. The Moon is
tilted by just over 5° to
the plane in which the Earth moves around the Sun. This oval-shaped orbit takes the Moon as far away from the Earth as 407,000km and brings it as close as 356,000km.
The near side of the Moon
The Moon’s surface is covered with large craters. Many are incredibly old, perhaps over three and a half billion years old. When the meteorites that caused these
 Moon—Astronauts exploring one of the rocky parts of the Moon’s surface.
 Moon—The Moon has a thin crust. Most of the Moon is made of a rocky mantle enclosing a very small metallic core.
Outer crust Inner crust
Metallic core Rocky mantle
craters hit the Moon, there was still no life on Earth, and oceans were only just forming. The Moon has not changed since that time.
The Moon’s surface and structure
Although there is no life on the Moon, it has a structure made up of layers similar to those of the Earth. The core, however, is solid rock, which is why the Moon
has no magnetic field, and which is why the surface never suffers earthquakes. Moreover, there is
Sea of Tranquillity
no atmosphere surrounding the Moon; and, being much smaller than the Earth, its gravity is less than one-sixth of that on Earth.
The Moon’s rocks are not unlike some of those found on Earth. During the first half-billion years of the Moon’s history it was still hot enough for volcanoes to produce great lava flows. However, since then it has cooled, and all activity has died away. As a result there are no overturning movements—the reason the surface has been left undisturbed for so long.
There are two types of landscape on the Moon—a rough, bright mountainous land that covers
the majority of the surface, and a darker, flatter land that occupies less than a third of the surface. Early astronomers thought of them as “continents” and “oceans,” which is why the flat areas—even though they are totally dry—are called “seas” and “oceans” on lunar maps (see: Mare).
Craters cover the Moon’s surface. The largest (like the Sea
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