Those who are in charge of an observatory are often visited by persons
who, coming to see the wonders of the heavens, and finding instruments
of such great proportions, not unnaturally expect the views they are to
obtain of the celestial bodies shall be of corresponding magnificence.
So they are, no doubt, but then it frequently happens that the pictures
which even the greatest telescope can display will fall far short of
the ideal pictures which the visitors have conjured up in their own
imaginations, so that they are often sadly disappointed. Especially is
this true with regard to the moon. I have seen people who, when they
had a view of the moon through a great telescope, were surprised not
to find vast ranges of mountains which looked to them as big as the
Alps, or mighty deserts, over which the eye could roam for thousands of
miles. They have sometimes expected to behold stupendous volcanoes that
not only were, but that looked to be as big as Vesuvius. Others seem
to have thought they ought to see the moon with such clearness that
the fields were to be quite visible, and some would not have been much
astonished if they had observed houses and farmyards, and, perhaps,
even cocks and hens.
There are different ways of estimating the apparent dimensions of
an object, but the size the moon appears to me to have in a great
telescope may be illustrated by taking an orange in your hand and
looking at the innumerable little marks and spots on its surface. The
amount of detail that the eye will show on the orange is about equal
to the amount of detail that a good telescope will show on the moon. A
desert on the moon, which really is a hundred miles across, will then
correspond to a mark about an eighth of an inch in diameter on the
orange. Some of you may ask what is gained by the use of a telescope,
for the moon looks to us as large as a plate with the unaided eye, and
now we hear it only looks as big as an orange in the telescope. But
where is the plate with which you compare your moon supposed to be
held? It is surely not in your hand. It is imagined to be up in the
sky, a very long way off. Though an orange is much smaller than a
plate, yet you will be able to see many more details in the orange by
taking it in your hand than you could see on a plate which was at the
other side of the street.
[Illustration: FIG. 39.--The Advantage of using a Telescope.]
I sometimes find that people will not believe how much the telescope
that they are using is magnifying the moon until they use both eyes
together, of which one is applied to the telescope, while the other
is directed to the moon. It will then be seen, even with a very small
instrument, that the telescopic moon is as big as the larger of the two
crescents in the adjoining figure (Fig. 39), while the naked-eye moon
is like the smaller.
The greatest telescopes are capable of reducing the apparent distance
of an object to about one-thousandth part of its actual amount. If,
therefore, a body were a thousand miles away, it would, when viewed by
one of these mighty instruments, be seen as large as our unaided vision
would show it, were the body only a single mile distant. No doubt this
is a large accession to our power, but it often falls far short of what
the astronomer would desire. The distances of the stars are all so
great that even when divided by one thousand, they are still enormous.
If you have a number expressed by 100,000,000,000,000, then dividing it
by a thousand merely means taking off three of the ciphers, and there
is still a large number left. We are, however, at present concerned
with the moon, and, as its distance is about 240,000 miles, the effect
of the best telescope is to reduce this distance apparently to 240
miles. Here, then, we find a limit to what the best of all telescopes
can do. It can never show us the moon better than, hardly indeed so
well as, we could see it with our unaided eye were it only 240 miles
over our heads. We cannot expect the most powerful instruments to
reveal any object on the moon unless that object were big enough to be
seen by the unaided eye when 240 miles away. What could we expect to
see at a distance of 240 miles?
Here is a little experiment which I made to study this point. I marked
a round black dot on a sheet of white paper. The dot was a quarter of
an inch in diameter, and then I fastened this on a door in the garden,
and walked backwards until the dot ceased to be visible. I found this
distance to be about thirty-six yards. I tried a little boy of eight
years old, and it appeared that the dot became invisible to him about
the same time as it did to me. “What has this to do with the moon?” you
will say. Well, we shall soon see. In thirty-six yards there are 5184
quarters of an inch, and as it is unnecessary to be very particular
about the figures, we may say, in round numbers, that the distance when
we ceased to be able to distinguish the dot was about five thousand
times as great as the width of the dot itself. You need not, therefore,
expect to see anything on the moon or on anything else which is not at
least as wide as the five-thousandth part of the distance from which
we are viewing it. The great telescope practically places the moon at
a distance of 240 miles, and the five-thousandth part of that is about
eighty yards; consequently a round object on the moon about eighty
yards in diameter would be just glimpsed as the merest dot in the most
powerful telescope. To attract attention, a lunar object should be
much larger than this. If St. Paul’s Cathedral stood on a lunar plain,
it would be visible in our great telescopes. It is true that we could
not see any details. We should not be able to distinguish between a
Cathedral and a Town-hall. There would just be something visible, so
that the artist who was making a sketch of that part would put down a
mark with his pencil to show that something was there. This will show
us that we need not expect to see objects on the moon, even with the
mightiest of telescopes, unless they are of great size.