Features of the Moon
ATLAS OF THE UNIVERSE
Mare Imbrium and is a superb sight when the Sun is rising
or setting over it, catching the mountain-tops while the
floor is still in shadow and producing the appearance often
nicknamed the ‘Jewelled Handle’.
Most of the major maria form a connected system.
There is, however, one exception: the isolated, well-
formed Mare Crisium, near the Moon’s north-east limb,
which is easily visible as a separate object with the naked
eye. It appears elongated in a north–south direction, but
this is because of the effect of foreshortening; the
north–south diameter is 460 kilometres (285 miles), while
the east–west diameter is 590 kilometres (370 miles).
Maria still closer to the limb are so foreshortened that they
can be made out only under favourable conditions.
The whole lunar scene is dominated by the craters,
which range from vast enclosures such as Bailly, 293
kilometres (182 miles) in diameter, down to tiny pits. No
part of the Moon is free from them; they cluster thickly
in the uplands, but are also to be found on the floors of
the maria and on the flanks and crests of mountains.
They break into each other, sometimes distorting each
other so completely the original forms are hard to trace;
some have had their walls so reduced by lava flows that
they have become ‘ghosts’, and some craters have had
their seaward walls so breached that they have become
bays. Fracastorius, at the edge of the Mare Nectaris, is a
good example of this.
Riccioli, a Jesuit astronomer who drew a lunar map
in 1651, named the main craters after various person-
alities, usually scientists. His system has been followed
up to the present time, though it has been modified and
extended, and later astronomers such as Newton have
come off second-best. Some unexpected names are found.
Julius Caesar has his own crater, though this was for his
association with calendar reform rather than his military
prowess.
Central peaks, and groups of peaks, are common, and
the walls may be massive and terraced. Yet in profile a
crater is not in the least like a steep-sided mine-shaft.
The walls rise to only a modest height above the outer
surface, while the floor is sunken; the central peaks
never rise as high as the outer ramparts, so that in theory
a lid could be dropped over the crater! Some formations,
such as Plato in the region of the Alps and Grimaldi
near the western limb, have floors dark enough to make
them identifiable under any conditions of illumination;
Aristarchus, in the Oceanus Procellarum, is only 37
kilometres (23 miles) across, but has walls and central
peak so brilliant that when lit only by earthshine it has
sometimes been mistaken for a volcano in eruption. One
crater, Wargentin in the south-west limb area, has been
filled with lava to its brim, so that it has taken on the
form of a plateau. It is almost 90 kilometres (55 miles)
across.
The most striking of all the craters are Tycho, in the
southern uplands, and Copernicus in the Mare Nubium.
Under high light they are seen to be the centres of systems
of bright rays, which spread out for hundreds of kilo-
metres. They are surface features, casting no shadows, so
that they are well seen only when the Sun is reasonably
high over them; near Full Moon they are so prominent that
they drown most other features. Interestingly, the Tycho
rays do not come from the centre of the crater, but are
tangential to the walls. There are many other minor ray-
centres, such as Kepler in the Oceanus Procellarum and
Anaxagoras in the north polar area.
The main mountain ranges form the borders of
the regular maria; thus the Mare Imbrium is bordered byT
he Moon is much the most spectacular object in the sky
to the user of a small telescope. There is an immense
amount of detail to be seen, and the appearance changes
dramatically from one night to the next because of the
changing angle of solar illumination. A crater which is
imposing when close to the terminator, or boundary
between the daylight and night hemispheres, may be
almost impossible to identify near Full Moon, when there
are virtually no shadows.
The most obvious features are the wide dark plains
known as seas or maria. For centuries now it has been
known that there is no water in them (and never has
been!), but they retain their romantic names such as
the Mare Imbrium (Sea of Showers), Sinus Iridum (Bay
of Rainbows) and Oceanus Procellarum (Ocean of
Storms). They are of various types. Some, such as the
Mare Imbrium, are essentially circular in outline, with
mountainous borders; the diameter of the Mare Imbrium is
1300 kilometres (800 miles). Other seas, such as the vast
Oceanus Procellarum, are irregular and patchy, so that
they give the impression of being lava ‘overflows’. There
are bays, such as the Sinus Iridum which leads off the▼ The Alps. Part of the
Mare Imbrium; the craters
to the lower part of the
picture are Archimedes (left),
Aristillus and Autolycus. The
low-walled formation with
two interior craterlets is
Cassini. The Alpine Valley
can be seen cutting through
the Alpine range near the
top of the picture.B Atl of Univ Phil'03stp 31/3/03 4:05 pm Page 44