e moon which is turned towards the sun is
brilliantly illuminated, and, according as we see more or less of that
brilliant half, we say that the moon is more or less full, the several
"phases" being visible in the succession shown by the numbers in Fig.
25. A beginner sometimes finds considerable difficulty in understanding
how the light on the full moon at night can have been derived from the
sun. "Is not," he will say, "the earth in the way? and must it not
intercept the sunlight from every object on the other side of the earth
to the sun?" A study of Fig. 24 will explain the difficulty. The plane
in which the moon revolves does not coincide with the plane in which the
earth revolves around the sun. The line in which the plane of the
earth's motion is intersected by that of the moon divides the moon's
path into two semicircles. We must imagine the moon's path to be tilted
a little, so that the upper semicircle is somewhat above the plane of
the paper, and the other semicircle below. It thus follows that when the
moon is in the position marked full, under the circumstances shown in
the figure, the moon will be just above the line joining the earth and
the sun; the sunlight will thus pass over the earth to the moon, and the
moon will be illuminated. At new moon, the moon will be under the line
joining the earth and the sun.

As the relative positions of the earth and the sun are changing, it
happens twice in each revolution that the sun comes into the position of
the line of intersection of the two planes. If this occurs at the time
of full moon, the earth lies directly between the moon and the sun; the
moon is thus plunged into the shadow of the earth, the light from the
sun is intercepted, and we say that the moon is eclipsed. The moon
sometimes only partially enters the earth's shadow, in which case the
eclipse is a partial one. When, on the other hand, the sun is situated
on the line of intersection at the time of new moon, the moon lies
directly between the earth and the sun, and the dark body of the moon
will then cut off the sunlight from the earth, producing a solar
eclipse. Usually only a part of the sun is thus obscured, forming the
well-known partial eclipse; if, however, the moon pass centrally over
the sun, then we must have one or other of two very remarkable kinds of
eclipse. Sometimes the moon entirely blots out the sun, and thus is
produced the sublime spectacle of a total eclipse, which tells us so
muc