The Far Side of the Moon
ATLAS OF THE UNIVERSE
There is a definite difference between the near and the
far sides, no doubt because the Moon’s rotation has been
synchronous since a fairly early stage in the evolution of
the Earth–Moon system; the crust is thickest on the far
side. One major sea, the Mare Orientale, lies mainly on
the hidden regions; only a small part of it can be seen
from Earth, and then only under conditions of favourable
libration. The spacecraft pictures have shown it to be
a vast, multi-ringed structure which is probably the
youngest of all the lunar seas. Otherwise there are no
large maria on the far side, and this is the main difference
between the two hemispheres.
One very interesting object is Tsiolkovskii, 240 kilo-
metres (150 miles) in diameter. It has a dark floor which
gives the impression of being shadowed in many photo-
graphs, though the real cause of the darkness is the hue of
the floor itself; there is no doubt that we are seeing a lake
of solidified lava, from which a central peak rises. In many
ways Tsiolkovskii seems to be a sort of link between a
crater and a mare. It intrudes into a larger but less regular
basin, Fermi, which has the usual light-coloured interior.
Many of the familiar types of features are seen on the
far side, and the distribution of the craters is equally non-
random; when one formation breaks into another, it is
always the smaller crater which is the intruder. Valleys,
peaks and rays systems exist. Though the Moon has no
overall magnetic field that we can detect, there are regions
of localized magnetism here and there; one of these lies
near the rather irregular far-side crater Van de Graaff. It
has been suggested that the Moon used to have a definite
magnetic field which has now died away.
On the original Lunik 3 picture a long, bright feature
running for hundreds of kilometres was shown, and was
thought to be a mountain range which was promptly
named in honour of the Soviet Union. Alas, it was later
found that the feature is nothing more than a surface ray,
and the Soviet Mountains were tactfully deleted from
the maps. However, it was surely right to name the
most imposing far-side feature in honour of Konstantin
Eduardovich Tsiolkovskii, the great pioneer who was
writing about spaceflight almost a hundred years ago.1
4
Apogee3
2
Perigee
96 ̊
84 ̊84 ̊96 ̊ EarthXXX XMoon
at its maximum height
above the horizon
Moon
risingMoon
settingEarthADiurnal libration.We are
observing from the Earth’s
surface at A, not its centre,
so that we can see a little
way alternately round the
northern and southern limbs. Van de Graaff is a
large but rather irregular
formation, perhaps
compound, notable because
of the amount of remnant
magnetism in and near it.
Its floor contains several
smaller craters. The wall
is broken (top right-hand
corner) by Birkeland, a
well-formed crater with
a prominent central peak.▲Libration in longitude.
X is the centre of the Moon’s
disk, as seen from Earth.
At position 1 the Moon is at
perigee. After a quarter of its
orbit it has reached position
2; but since it has travelled
from perigee it has moved
slightly faster than its mean
rate, and has covered 96°
instead of 90°. As seen from
Earth, X lies slightly east of
the apparent centre of the
disk, and a small portion
of the far side has come
into view in the west. After
a further quarter-month the
Moon has reached position- It is now at apogee, and
X is again central. A further
84° is covered between
positions 3 and 4, and
X is displaced towards the
west, so that an area beyond
the mean eastern limb is
uncovered. At the end of one
orbit the Moon has arrived
back at 1, and X is once
more central on the Moon’s
disk as seen from Earth.
L
ook at the Moon, even with the naked eye, and you will
see the obvious features such as the principal maria.
The positions of these features on the disk are always much
the same, because of the synchronous rotation. Yet there
are slight shifts, due to the effects known as librations. All
in all we can examine a grand total of 59 per cent of the
lunar surface, and only 41 per cent is permanently averted,
though of course we can never see more than 50 per cent.
The most important libration – the libration in longi-
tude – is due to the fact that the Moon’s path round the
Earth is elliptical rather than circular, and it moves at its
fastest when closest to us (perigee). However, the rate of
axial rotation does not change, so that the position in
orbit and the amount of axial spin become periodically ‘out
of step’; we can see a little way round alternate mean
limbs. There is also a libration in latitude, because the
Moon’s orbit is inclined by over 5 degrees, and we can see
for some distance beyond the northern and southern limbs.
Finally there is a diurnal or daily libration, because we are
observing from the surface, not the centre of the globe.
All these effects mean that the ‘libration regions’ are
carried in and out of view. They are so foreshortened that
it is often difficult to distinguish between a crater and
a ridge, and before 1959 our maps of them were very
imperfect. About the permanently hidden regions nothing
definite was known. It was reasonable to assume that they
were basically similar to the familiar areas – though some
strange ideas had been put forward from time to time. The
last-century Danish astronomer Andreas Hansen once
proposed that all the Moon’s air and water had been drawn
round to the far side, which might well be inhabited! The
first pictures of the far side were obtained in October
1959 by the Russian space probe Lunik 3 (also known as
Luna 3). It went right round the Moon, taking pictures of
the far side and later sending them back by television
techniques. The pictures are very blurred and lacking
in detail, but they were good enough to show that, as
expected, the far side is just as barren and just as crater-
scarred as the areas we have always known. Later space-
craft, both manned and automatic, have enabled us to
draw up very complete maps of the entire lunar surface.B Atl of Univ Phil'03stp 31/3/03 4:05 pm Page 48