ther occurrence of the same class which is known as a
_transit_. This takes place when an apparently small body passes across
the face of an apparently large one, the phenomenon being in fact the
exact reverse of an occultation. As there is no appreciable body nearer
to us than the moon, we can never see anything in transit across her
disc. But since the planets Venus and Mercury are both nearer to us than
the sun, they will occasionally be seen to pass across his face, and
thus we get the well-known phenomena called Transits of Venus and
Transits of Mercury.

As the satellites of Jupiter are continually revolving around him, they
will often pass behind or across his disc. Such occultations and
transits of satellites can be well observed in the telescope.

There is, however, a way in which the light of a celestial body may be
obscured without the necessity of its being hidden from us by one
nearer. It will no doubt be granted that any opaque object casts a
shadow when a strong light falls directly upon it. Thus the earth, under
the powerful light which is directed upon it from the sun, casts an
extensive shadow, though we are not aware of the existence of this
shadow until it falls upon something. The shadow which the earth casts
is indeed not noticeable to us until some celestial body passes into it.
As the sun is very large, and the earth in comparison very small, the
shadow thrown by the earth is comparatively short, and reaches out in
space for only about a million miles. There is no visible object except
the moon, which circulates within that distance from our globe, and
therefore she is the only body which can pass into this shadow. Whenever
such a thing happens, her surface at once becomes dark, for the reason
that she never emits any light of her own, but merely reflects that of
the sun. As the moon is continually revolving around the earth, one
would be inclined to imagine that once in every month, namely at what is
called _full moon_, when she is on the other side of the earth with
respect to the sun, she ought to pass through the shadow in question.
But this does not occur every time, because the moon's orbit is not
quite _upon the same plane_ with the earth's. It thus happens that time
after time the moon passes clear of the earth's shadow, sometimes above
it, and sometimes below it. It is indeed only at intervals of about six
months that the moon can be thus obscured. This darkening of her light
is known a