The Moon is the earth's only natural satellite (hence its name
the Moon) and it is an abnormaly large moon for a planet the size of the Earth - its diameter is about 1/4
the size of Earth's. Consequently, it is sometimes known as a double planet - not to the same
extent as Pluto, whose moon Charon has a diameter just half Pluto's size, but enough
to warrant the term's use.
As the Moon is the only celestial object close enough to be able to make out its details with the naked eye, as
well as being by far the closest such body, we know more about the Moon than anything else in the Solar System (save the Planet Earth of course).
Countless telescopes and spacecraft have scrutinised it in great detail, and men have even walked on its surface. And yet
remarkably, we still know very little about its far side, which is forever facing away from the Earth and untouched by terran eyes (all we know about it
is from the Luna 3 probe which passed by the far side in 1959). This is all because the length of the Moon's orbit of the
Earth and the length of its day are identical - it is thus said to rotate synchronously with the Earth.
Broadly speaking, there are two main types of Lunar terrain - the brighter, higher 'terrae' and the darker 'maria'. The latin words
are in fact rather descriptive; the terrae are packed full of craters and basins, often overlapping, which are almost all thought to
be caused by meteoric impacts. This terrain therefore, whose top few kilometres of crust has been repeatedly broken, crushed and remoulded
bythese impacts, bears more resemblance to the rugged earth land, than do the calmer, darker, lower maria (in latin = seas).
Rather unsurprisingly, of the 16% of Lunar land which is covered by the maria, most of it is on the side facing the Earth, as
this side has been less exposed to meteoric impacts. It is thought that the maria as we see them today are substantially
more recent than the terrae, and have therefore been subject to less bombardments. It is also important to point out that
different such 'seas' on the Moon can be quite geologically different from each other.
The Formation of the Moon
The existence of the Moon has long posed some challenging questions to Astronomers. After all, only 2 of the 4 inner planets, Earth and Mars,
have moons, and Mars' two moons are clearly mere asteroids, captured by Mars' gravitational pull.
Analysis of the Lunar rocks returned by the Apollo astronauts has shown that none of the previous theories of the
Moon's formation could be totally correct. These theories included the idea that the Moon formed at the same time as
the Earth from the same cloud of dust and gas, an idea which was shown to be wrong as the Lunar composition was
shown to be slightly different to the Earth's. Another idea was that the Moon was captured by the Earth's gravity, but
calculations show the Earth would not have had a strong enough gravitational pull to capture a fast-moving Moon sized object.
However, in the mid 1970s, a new theory was proposed, which has since recieved enthusiastic support from many astronomers, and
this si the 'giant impact' theory. The idea is that a large object hit a young, hot Earth 4,500 million years ago in an off-centre collision,
causing large quantities of both the Earth and of the object's material to be ejected from the planet and into the Earth's orbit. This material eventually coalesced
to form the Moon. Not only does this hypothesis sound plausible, but it would also explain the Earth's fast spin rate, and the orientation of the Moon's orbit.
The 'lunar crust' was formed near the start of
the moon's history by it's outer layers melting to form a layer of magma which covered the
whole moon and then subsequently cooling. Although in its
history it has had some volcanic activity, this has now ceased and the Moon is
sometimes referred to as a 'fossil planet'. This isn't entirely true though as earthquakes
have been recorded deep below the Moon's crust.