onth. But while
we turn on our axis every twenty-four hours, thus causing the
alternation of light and darkness--day and night--the Moon takes a month
to revolve on hers, so that she always presents the same, or very nearly
the same, surface to us.

Seeing her as we do, not like the Sun and Stars, by light of her own,
but by the reflected light of the Sun, her form appears to change,
because the side upon which the Sun shines is not always that which we
see. Hence the "phases" of the Moon, which add so much to her beauty
and interest.

Who is there who has not watched them with admiration? "We first see her
as an exquisite crescent of pale light in the western sky after sunset.
Night after night she moves further and further to the east, until she
becomes full, and rises about the same time that the Sun sets. From the
time of full moon the disc of light begins to diminish, until the last
quarter is reached. Then it is that the Moon is seen high in the heavens
in the morning. As the days pass by, the crescent shape is again
assumed. The crescent wanes thinner and thinner as the Moon draws closer
to the Sun. Finally, she becomes lost in the overpowering light of the
Sun, again to emerge as the new moon, and again to go through the same
cycle of changes."[66]

But although she is so small the Moon is not only, next to the Sun, by
far the most beautiful, but also for us the most important, of the
heavenly bodies. Her attraction, aided by that of the Sun, causes the
tides, which are of such essential service to navigation. They carry
our vessels in and out of port, and, indeed, but for them many of our
ports would themselves cease to exist, being silted up by the rivers
running into them. The Moon is also of invaluable service to sailors by
enabling them to determine where they are, and guiding them over the
pathless waters.

The geography of the Moon, so far as concerns the side turned towards
us, has been carefully mapped and studied, and may almost be said to be
as well known as that of our own earth. The scenery is in a high degree
weird and rugged; it is a great wilderness of extinct volcanoes, and,
seen with even a very moderate telescope, is a most beautiful object.
The mountains are of great size. Our loftiest mountain, Mount Everest,
is generally stated as about 29,000 feet in height. The mountains of the
Moon reach an altitude of over 42,000, but this reckons to the lowest
depression, and it must be remembered