The moon is the attendant, or the satellite of the earth, ministering
to the wants of the earth by mitigating the darkness of our nights. The
earth goes around the sun in its annual journey of 365 days. The moon
revolves around the earth once every twenty-seven days. The motion of
the moon is thus a very complicated one, for it is, in fact, moving
round a body which is itself in constant motion (Fig. 33).
You will see by your almanacs every year that certain eclipses are to
take place; and after what we have said about the sun and the moon, it
will be easy to understand how eclipses arise. There are two different
kinds. You will sometimes see an eclipse of the moon, and sometimes
those eclipses of the sun of which we have spoken in the last Lecture.
You may be surprised to find with what accuracy the eclipses can be
predicted. We can tell not only those that will occur this year and
next year, but we could also foretell the eclipses that will appear
in a hundred or a thousand years to come; or we can, with equal ease,
calculate backwards, so as to find the circumstances of eclipses that
happened thousands of years ago. This shows how well we have learned
the way the moon moves.
[Illustration: FIG. 33.--To show how the Earth goes round the Sun and
the Moon round the Earth.]
[Illustration: FIG. 34.--A Total Eclipse to the Girl and a Partial
Eclipse to the Boy.]
[Illustration:
Partial. Annular.