We always see the same side of the moon. The moon and Earth are tidally-locked
in this position. In a few more billion years, the same face of the Earth will
always face the moon. The moon will never rise or set from one point on the
Earth, but future astronomy students will still have to learn about its phases!
Terminator and Limb
The dividing line between day and night on the moon is called
the terminator. The strong shadows mark the best place
to observe craters, mountains and other surface features.
The moon has innumerable craters, some incredibly large. Very large dark patches are seen on it's surface, called maria, after the Latin word for sea. There is no water on the Moon. Instead, the mare are smooth, low-lying surface regions, filled with ancient lava flows.
A Dead Moon
While the maria suggest active volcanism, they formed billions of years ago,
though late enough to escape heavy bombardment (and show few craters).
Sometimes the maria are seen to fill in older craters.
We can use the density of craters seen on a surface to give us an
estimate of the age of that surface, if we know the rate of cratering
with time. Such cratering studies reveal absolute ages of
the moon surfaces, which can be checked using radio-active
dating of moon rocks brought back by the astronauts.
Sinuous rilles, channels seen to cut or channelled into the smooth
freshly formed maria, were formed from volcanic flows long ago.
Today, the Moon is entirely cold, dry and lifeless.
