ess than a degree). This is known as the
_parallax_ of the Moon.

Here is a more or less alarming word; yet it is one that we can not
dispense with in discussing the distance of the stars. This astronomical
term will soon become familiar in the course of the present lesson,
where it will frequently recur, and always in connection with the
measurement of celestial distances. "Do not let us fear," wrote Lalande
in his _Astronomie des Dames_, "do not let us fear to use the term
parallax, despite its scientific aspect; it is convenient, and this term
explains a very simple and very familiar effect."

"If one is at the play," he continues, "behind a woman whose hat is too
large, and prevents one from seeing the stage [written a hundred years
ago!], one leans to the left or right, one rises or stoops: all this is
a parallax, a diversity of aspect, in virtue of which the hat appears to
correspond with another part of the theater from that in which are the
actors." "It is thus," he adds, "that there may be an eclipse of the Sun
in Africa and none for us, and that we see the Sun perfectly, because we
are high enough to prevent the Moon's hiding it from us."

See how simple it is. This parallax of 57 minutes proves that the Earth
is removed from the Moon at a distance of about 60 times its
half-diameter (precisely, 60.27). From this to the distance of the Moon
in kilometers is only a step, because it suffices to multiply the
half-diameter of the Earth, which is 6,371 kilometers (3,950 miles) by
this number. The distance of our satellite, accordingly, is 6,371
kilometers, multiplied by 60.27--that is, 384,000 kilometers (238,000
miles). The parallax of the Moon not only tells us definitely the
distance of our planet, but also permits us to calculate its real volume
by the measure of its apparent volume. As the diameter of the Moon seen
from the Earth subtends an angle of 31', while that of the Earth seen
from the Moon is 114', the real diameter of the orb of night must be to
that of the terrestrial globe in the relation of 273 to 1,000. That is a
little more than a quarter, or 3,480 kilometers (2,157 miles), the
diameter of our planet being 12,742 kilometers (7,900 miles).

This distance, calculated thus by geometry, is positively determined
with greater precision than that employed in the ordinary measurements
of terrestrial distances, such as the length of a road, or of a railway.
This statement may seem to be a romance to ma