Our friendly Moon
the Earth-Moon system, in 27.3 Earth days. This is also the period taken by
the Moon to rotate on its axis. The result is that we always see the same side
of the Moon, while the opposite side cannot be seen at all. Some find this
hard to accept. We have even seen books saying the Moon does not rotate
at all.
Imagine a model with the Earth in the centre, the Moon moving round
it and the Sun off in the distance. As the angle between the Moon, Sun and
Earth changes, the Moon shows phases or apparent changes of shape and
these make what we call the lunation.
If the Moon lies virtually between the Sun and the Earth, its night side
is turned towards the Earth and we have a new Moon. In this position we
cannot see it at all except when the alignment is exact and we see the glory
of a total solar eclipse.
As the Moon moves along in its orbit, a little of its sunlit side turns in
our direction and the Moon appears as a crescent in the evening sky,
thickening up until half the lighted side is turned towards us. Confusingly,
this is named first quarter – the Moon has completed one quarter of its
monthly journey. As the movement continues we see more and more of
the sunlit side, and the Moon appears gibbous, meaning between half
and full.There can be no breath of wind nor weather. Since the surface is
unprotected, the temperatures are extreme, ranging from very hot to very
cold! The maria cover wide areas, and those on the Earth-turned side
make up essentially a once-connected system, though some, notably Mare
Crisium, are disconnected from the rest.
There is one very important point to be borne in mind. The Moon goes
round the Earth, or to be more pedantic the barycentre (or centre of mass) of
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