The Earth–Moon System
ATLAS OF THE UNIVERSE
T
he Moon is officially classed as the Earth’s satellite, but
in many ways it may be better to regard the Earth–
Moon system as a double planet; the mass ratio is 81 to 1,
whereas for example Titan, the largest satellite of Saturn,
has a mass only 1/4150 that of Saturn itself – even though
Titan is considerably larger than our Moon.
We are by no means certain about the origin of the
Moon. The attractive old theory according to which it
simply broke away from the Earth, leaving the hollow
now filled by the Pacific Ocean, has long been discount-
ed. It may be that the Earth and the Moon were formed
together from the solar nebula, but there is increasing sup-
port for the idea that the origin of the Moon was due to a
collision between the Earth and a large wandering body,
so that the cores of the Earth and the impactor merged,
and debris from the Earth’s mantle, ejected during the col-
lision, formed a temporary ring round the Earth from
which the Moon subsequently built up. The Earth’s man-
tle is much less massive than its core, and this theory
would explain why the Moon is not so dense as the Earth;
moreover, analyses of the lunar rocks show that the Moon
and the Earth are of about the same age.It is often said that ‘the Moon goes round the Earth’.
In a way this is true. To be strictly accurate the two bodies
move together round their common centre of gravity, or
barycentre; however, since the barycentre lies deep inside
the Earth’s globe, the simple statement is good enough for
most purposes.
The orbital period is 27.3 days, and everyone is familiar
with the phases, or apparent changes of shape, from new to
full. When the Moon is in the crescent stage, the ‘dark’ side
may often be seen shining faintly. There is no mystery
about this; it is due to light reflected on to the Moon from
the Earth, and is therefore known as earthshine. It can
be quite conspicuous. Note, incidentally, that because the
Earth and the Moon are moving together round the Sun,
the synodic period (that is to say, the interval between one
New Moon and the next) is not 27.3 days, but 29.5 days.
The Moon’s axial rotation period is equal to its orbital
period. This is due to tidal friction over the ages. During its
early history, the Moon was much closer to the Earth than
it is now, and the Earth’s rotation period was shorter; even
today the ‘day’ is becoming longer, while the Moon is
being driven outwards from the Earth. However, these
effects are very slight. The Moon’s distance is increasing at
a rate of less than 4 centimetres (1.5 inches) per year.
The captured or ‘synchronous’ rotation means that
there is a part of the Moon which is always turned away
from us, so that until 1959, when the Russians sent their
probe Lunik 3 on a ‘round trip’, we knew nothing definiteThe New Moon (1 and 9)
occurs when the Moon is
closest to the Sun. In the
Crescent Moon (2), Mare
Crisium is prominent
between the eastern
limb and the terminator.
Earthshine is often seen.The Half Moon, First
Quarter(3) reveals Mare
Serenitatis with the great
chain of craters near the
central meridian. Since
the Sun is still low over the
area that can be seen, the
features are well defined.The Gibbous Moon(4)
reveals the great ray-craters
Tycho and Copernicus.
Although the craters are
well illuminated and readily
identifiable, their spectacular
rays are not yet as striking
as they will soon become.The Full Moon(5). There are
no shadows, and the rays
from Tycho and Copernicus
are so prominent that crater
identification becomes
difficult. The lunar maria
take on a decidedly dark
hue against the brilliant rays.The Waning Moon(6).
This is not as brilliant as
the waxing Gibbous Moon.
More of the dark maria
which were once thought
to be seas are illuminated.
They are, in fact, gigantic
plains of volcanic lava.sB Atl of Univ Phil'03stp 31/3/03 4:04 pm Page 42