I have already warned you not to expect too much, even with the biggest
of telescopes; and just as a caution, I may, perhaps, tell you a story
I once heard of an astronomer who had a great telescope. It was a very
famous instrument, and people often came to the Observatory at night
to enjoy a look at the heavens. Sometimes these visitors were grave
philosophers, but frequently they were not very accomplished men of
science. One evening such a visitor came to the Observatory, and sent
in his name and an introduction to the astronomer, with a request
that he might enter the temple of mystery. The astronomer courteously
welcomed the stranger, and asked him what he specially desired to see.
“Oh!” said the visitor, “I have specially come to see the moon--that is
the object I am particularly interested about.”
“But,” said the astronomer, “my dear sir, I would show you the moon
with pleasure, if you were here at the proper time; but what brings you
here now? Look up; the evening is fine. There are the stars shining
brightly, but where is the moon? You see it is not up at present. In
fact, it won’t rise till about half-past two to-morrow morning, and it
is only nine o’clock now. Come back again in five or six hours, and you
shall observe the moon with the great telescope.”
But the visitor evidently thought the astronomer was merely trying to
get rid of him by a pretext. And he was equal to the occasion--he was
not going to be put off in that way.
“Of course, the moon is not up,” he replied; “any one can see that, and
that is the reason why I have come, for _if the moon had been up, I
could have seen it without your telescope at all_!”
Although no explorer can ever reach our satellite, yet it is hardly
an exaggeration to say that in some respects we know the geography of
the moon a good deal better than we know the geography of the earth.
Think of the continent of Africa. In that great country there are
mighty tracts, there are vast lakes and ranges of mountains, of which
we know but little. We could make a better map of Africa, so far at
least as its broad outlines are concerned, if it were fastened up on
our side of the moon than we actually possess at this moment. There
is no spot on the nearer side of the moon as large as an ordinary
parish in this country which has not been surveyed. There are maps
and charts of the moon showing every part of it, which is as big as a
good-sized field. Indeed, as there are no lunar clouds, the features
of its surface are never obscured whenever our own atmosphere will
permit us to make our observation. Artists have frequently sketched
the lunar features, and there is plenty of material for them to work
on. We have also had photographs taken of the moon, but there is a
special difficulty to be encountered in taking photographs of celestial
bodies which photographers of familiar objects on this earth do not
experience. For a photograph to be successful, everybody knows that
the first requisite is for the sitter to stay quiet while the plate
is being exposed. This is, unhappily, just what the moon cannot do. We
endeavor to obviate the difficulty by moving the telescope round so as
to follow the moon in its progress. This can be done with considerable
accuracy, but, unfortunately, there is another difficulty which lies
entirely beyond our control. As the rays of light from the moon perform
their journey through hundreds of miles of unsteady air, the rays are
bent hither and thither, so that the picture is more affected by the
atmosphere than in the case of a photographer’s portrait taken in the
studio. If we are merely _viewing_ the moon through the telescope, the
quivering effect on the rays of this long atmospheric voyage, though
rather inconvenient, does not prevent us from seeing the object, and we
can readily detect the true shape of each feature in spite of incessant
fluctuations. When, however, these rays fall not on the eye, but on
the photographic plate, they produce by their motion a picture which
cannot be much magnified without becoming very confused and wanting in
sharpness. Nevertheless, for the general outlines of our satellite’s
appearance and for the portraiture of its splendid features we have
derived the greatest assistance from photography.
[Illustration: FIG. 40.--The Full Moon.]
The adjoining picture (Fig. 40) gives a fair idea of what the full moon
looks like when viewed through a small telescope. I do not, however,
say that the lunar objects can then be observed under favorable
conditions; for when the moon is full is the very worst time for making
observations of our satellite. In fact, at this phase you can hardly
see anything except slight differences between the colors of different
parts. The best time for observing the moon is at the first quarter;
but even then you can only observe satisfactorily those objects which
happen to lie along the border between light and shade. To study the
moon properly you must, therefore, watch it during several different
phases, from the time when it presents a thin and delicate crescent
(just after new moon) until it has again waned to a thin and delicate
crescent (just before the next new moon). We want the relief given by
shadows to bring out the full beauty of lunar scenery.
On the map you will first notice the large dark-colored patches which
are so conspicuous on the moon’s face. They are, apparently, the empty
basins which great seas once filled. But if water was ever there it has
at all events now quite disappeared. These dark parts are, no doubt, a
good deal smoother than the rest of the surface; but we can see many
little irregularities which tell us that we are not looking at oceans.
The chief features I want you to observe are the curious rings which
you see in the figure; there is a very well-marked one a little below
the centre, and in the upper part many rings--large and small--are
crowded together. We call them lunar craters. You will see what they
are like from the model, of which a picture is shown in Fig. 42. But to
realize from this picture the proper scale of the object, you should
imagine it to be some miles in width. The cliffs which rise all round
to form the wall, as well as the mountain which adorns the centre, are
quite as high as any of the mountains in Great Britain.
[Illustration: FIG. 41.--View on the Moon.
(_By Lœwy and Puiseux, Paris Observatory._)