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.
With the Moon directly between Sun and Earth, the side facing us
is dark. We call it a New Moon. As the Moon moves on, we see more and more of
its face lit up, until we see it all at Full Moon. Afterwards, we see less and
less until it disappears at the next New Moon.
