Man's physical universe

xanabras

104 THE UNIVERSE A VAST SYSTEM OF PARTS

forty or fifty miles in diameter. The mountain chains and the crater

walls rise three, four, and even five miles. The cause of these lunar

craters is uncertain. Perhaps they were formed by huge meteors.

Fig. 25. Lunar crater Theophilus.

(Photograph from the Yerkes Observatory,

reprinted by permission of

the Chicago University Press.)

Small craters have been formed in the earth's crust at least twice in

fairly recent times, and possibly some craters of previous times have

been obliterated by the erosive agents which are lacking on the moon.

The unequal distribution of the craters and their concentration near

the mountain ranges lead many

astronomers to attribute their formation

to ancient volcanic activity.

The density of the moon is only

60 per cent of the density of the

earth and its mass only 1/82 as

great. Its gravitational attraction

is

1/6 that of the earth.

We are sure that the moon has

no atmosphere because there is no

refraction of light as stars appear

from behind it. The masses of atmospheric

particles and their velocities

are known, and calculations

based upon Newton's laws show

that the moon could not hold such

particles to its surface. The absence

of erosion and spectroscopic evidence

also indicate that these calculations

are correct.

Inasmuch as the moon has no

atmosphere, the side facing the sun must be much hotter than the

Sahara desert, while the other side must be much colder than our

Perhaps man may contrive to make a journey

coldest polar regions.

to the moon in the future ; but its temperature extremes, complete lack

of life, and oppressive silence would lead one to terminate his visit as

soon as possible. With no atmosphere, man's voice and ears would

be useless because sound would be impossible, and he would have to

carry along his own supply of oxygen for life itself.

The light from the moon is, of course, reflected sunlight. The different

phases of the moon (that is, the periodic change of shape from

crescent to full and back again every twenty-nine days) depend upon

the relative positions of the earth, moon, and sun, as shown in the

diagram.

It has already been noted that the planets closer to the sun

than the earth pass through similar phases for the same reason.

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