into a globe would equal the size of the
moon. The superficial extent of the moon is equal to about one
thirteenth part of the surface of the earth. The hemisphere our
neighbour turns towards us exhibits an area equal to about one
twenty-seventh part of the area of the earth. This, to speak
approximately, is about double the actual extent of the continent of
Europe. The average materials of the earth are, however, much heavier
than those contained in the moon. It would take more than eighty globes,
each as ponderous as the moon, to weigh down the earth.

Amid the changes which the moon presents to us, one obvious fact stands
prominently forth. Whether our satellite be new or full, at first
quarter or at last, whether it be high in the heavens or low near the
horizon, whether it be in process of eclipse by the sun, or whether the
sun himself is being eclipsed by the moon, the apparent size of the
latter is nearly constant. We can express the matter numerically. A
globe one foot in diameter, at a distance of 111 feet from the observer,
would under ordinary circumstances be just sufficient to hide the disc
of the moon; occasionally, however, the globe would have to be brought
in to a distance of only 103 feet, or occasionally it might have to be
moved out to so much as 118 feet, if the moon is to be exactly hidden.
It is unusual for the moon to approach either of its extreme limits of
position, so that the distance from the eye at which the globe must be
situated so as to exactly cover the moon is usually more than 105 feet,
and less than 117 feet. These fluctuations in the apparent size of our
satellite are contained within such narrow limits that in the first
glance at the subject they may be overlooked. It will be easily seen
that the apparent size of the moon must be connected with its real
distance from the earth. Suppose, for the sake of illustration, that the
moon were to recede into space, its size would seem to dwindle, and long
ere it had reached the distance of even the very nearest of the other
celestial bodies it would have shrunk into insignificance. On the other
hand, if the moon were to come nearer to the earth, its apparent size
would gradually increase until, when close to our globe, it would seem
like a mighty continent stretching over the sky. We find that the
apparent size of the moon is nearly constant, and hence we infer that
the average distance of the same body is also nearly constant. The
average