[Illustration: FIG. 42.--Our Model of a Lunar Crater.]
You may desire to know how we are able to measure the heights of
mountains on the moon. That is what I am now going to show you; and for
this purpose we shall look at our imitation lunar crater. Here is the
great ring, or circular enclosure, surrounded by cliffs, and here is a
sharp mountain peak rising in the centre. I shall ask to have the beam
from the electric lamp turned on our model. You see how prettily it
is lighted up. I have placed the lamp so that the beams are sloping;
and I have done this with the express object of making the shadows
long. In fact, as we look at a lunar crater, which lies on the border
between light and shade, the sun illuminates the object under the same
conditions as those shown in the figure. I dare say you have often
noticed what long shadows are cast at sunset. Similar shadows are made
to teach the astronomer the altitudes of the lunar mountains; for he
measures the length of the shadow, and then by a little calculation he
can find the height of the object by which that shadow has been cast.
I shall suppose that we want to measure the height of a flagstaff
(Fig. 43). It is quite possible to do this by merely measuring the
length of the shadow which that flagstaff casts at noon. It would not
be correct to say that the height of the flagstaff is the length of
its shadow. This will, indeed, be the case if you are fortunate enough
to make your measurement at or near London on either the 6th of
April or the 5th of September. On all other days in the year a little
calculation must be made, which I need not now mention, but which the
astronomer, with the aid of his Nautical Almanac, can do in a very few
minutes. In a similar manner, by measuring the lengths of the shadows
on the moon, and by finding the number of miles in the shadow, we are
able to calculate the altitudes of the lunar mountains and of the
ranges of cliffs by which the walled plains are surrounded.
[Illustration: FIG. 43.--How we found the Height of the Flagstaff.]