over the period of one rotation. This does not happen in the case of the Moon,
which keeps the same face turned towards us all the time, a situation known
as captured or synchronous rotation. Almost all the major satellites of major
planets behave in the same way. Tidal friction over the ages is responsible.
There is, however, one important qualification. The Moon’s path around
the Earth is not perfectly circular and this means its orbital speed varies
slightly, quickest when nearest the Earth (perigee) and slowest when
furthest away (apogee). This means that over one orbit the amount of spin
becomes out of step and we can see alternately first round one limb then
round the other. The Moon’s orbit is also inclined by a small amount, allowing
us to see a little beyond both poles at different times. This means that all in
all we can examine a total of 59% of the entire surface though no more than
50% at any one time.When the Moon is exactly opposite the Sun in the sky the entire day side
is turned towards us and the Moon is full. The phases are repeated then in
reverse order. Gibbous, half again (last quarter), crescent and back to new.
Now suppose the Moon did not rotate. Over a month we would see the
whole surface instead of only a fraction more than 50%.
The best example we can give is to imagine someone (the observer)
sitting on a chair in the middle of the room and a companion walking round;
as the walker moves, if he does not rotate he will face the same wall of the
room the whole time and the observer will see the walker from every angleJust as an example, Copernicus has a diameter of 56 miles, high
continuous walls and a lofty central mountain mass. It is also the centre of
a system of bright streaks or rays. Tycho, 54 miles across, in the southern
uplands, is of the same basic type but is considerably foreshortened with an
even greater system of rays. They become evident under high lighting. Near
full Moon the rays from Tycho and Copernicus dominate the entire Moon.
Tycho is often mistaken for the Moon’s polar crater but this is not the case. It
is some way from the Moon’s south geographical pole.The areas on the edge of the Moon that appear and disappear as the
Moon rocks and rolls its way across the sky are the so-called libration zones.
PM spent more than 30 years trying to compile charts using telescopes
of various kinds, including the 15-inch in his own observatory. Now that
spacecraft have been round the Moon and mapped the entire surface, he
can look back and see how accurate his efforts were. On the whole they are
pretty good.Lunar Craters
Craters cover the entire Moon. Some are regular, with high walls and a
central peak. Others have lower, flatter walls and no peak. Also the craters
are of very different ages and many have been damaged by later impacts.
For years there was a fierce argument about whether they were volcanic
structures or formed by impact. PM "admits" he was on the wrong side of the
argument, convinced they had volcanic origin, and only when the evidence
for impact became overwhelming did he have to admit his mistake.
Note, too, that the craters are not shaped like steep-sided mineshafts.
Seen in profile they are much more like saucers (not flying saucers,
please note), some of them of immense size, well over a hundred miles in
diameter. One of the most prominent on the Moon’s nearside is Clavius, with
a diameter of 144 miles and a string of craterlets on its floor. Basically it is
circular but craters near the limb are often so foreshortened that they appear
as ellipses and in some case it is not easy to tell a crater from a ridge. It was
this kind of thing that made mapping the libration areas such a big problem
before space photography.546Astronomer Book