Circling round Earth once a month, the Moon is our planet’s only natural satellite. As the Moon orbits Earth, our view of it constantly changes, following a cycle known as the PHASES OF THE MOON. Like Earth, the Moon is made up of rock, but is only about a quarter as big across. Because it is so small, its gravity is low (about one-sixth of Earth’s), and it has no atmosphere.
Table 12. ESSENTIAL DATA
| Diameter at equator | 3,476 km (2,160 miles) |
| Average distance from Earth | 384,400 km (238,900 miles) |
| Orbital period | 27.32 days |
| Rotation period | 27.32 days |
| Time to go through phases | 29.3 days |
| Mass (Earth=1) | 0.01 |
| Gravity (Earth=1) | 0.17 |
| Average surface temperature | -20°C (-4°F) |
Two or three times a year the Moon enters the shadow Earth casts in space. This happens when the Sun, Earth, and Moon line up and is called a lunar eclipse. During a total eclipse, when the Sun, Earth, and Moon line up exactly, the Moon does not disappear but takes on a reddish hue as it is lit by light from the Sun that is bent by Earth’s atmosphere.
From Earth, we only ever see one side of the Moon – the near side. The dark areas are great dusty plains, called maria (seas). The bright areas are highlands hundreds of kilometres across and covered with craters. The hidden far side of the Moon is more heavily cratered, but has no large seas.
Over a month we see the Moon appear to change shape. These different shapes, or phases, occur because, as the Moon circles Earth, we see more or less of the half of its surface that is lit by the Sun. The Moon takes 29.53 days to go through its phases.
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