Man's physical universe



Just as the sun has a number of satellites, so the planets have varying

numbers of satellites of their own, which are called moons.

Mercury Is the Dwarf among the Planets.

Mercury, having a mass about one-twentieth of the mass of the

earth and having a diameter only half again as great as the moon's, is

the dwarf among the planets with the possible exception of Pluto.

Mercury is so close to the sun that it is visible only in a few certain

positions in its orbit and then only shortly before sunrise or after


Mercury makes the circuit of the sun in eighty-eight days, and its

time of rotation around its own axis is the same, so that its day is the

same length as its year and it always presents the same face to the


It is too small for its gravitational attraction to hold atmospheric

particles to its surface.

This conclusion, based upon Newton's law of

gravitation, is confirmed by the observation of Aldrich, who found that

the earth with its atmosphere reflects about 44 per cent of the sunlight

w^hich reaches it, whereas the moon with no atmosphere reflects only

7 per cent. It was then found that Mercury also reflects only 7 per cent

of the sunlight which reaches it.

Because Mercury is so close to the sun, it receives about seven times

the radiation per unit area that the earth does. For this reason and

because so little radiant energy is reflected, the side of Mercury facing

the sun must be a parched desert too hot to support life,

while the

opposite side is always very dark and must be very cold inasmuch as

there is no atmosphere or bodies of water to act as an equalizing

medium. Certainly the conditions on Mercury would make life, as we

know it, impossible.

Venus Is the Brightest of the Planets.

Taking the planets in the order of their proximity to the sun, we

come now to the planet Venus. At times it appears as a bright evening

star and at other periods as a brilliant morning star. It is nearly the

same size as its nearest planetary neighbor, the earth. At times of

greatest brilliancy, Venus can be seen in full daylight.

Inasmuch as Venus reflects 59 per cent of the incident light, it is

known to have an atmosphere. The clouds which surround Venus are

so dense that the few brief glimpses we have had of its body have been

insufficient to enable man to determine the period of rotation about

its axis. Spectroscopic evidence indicates that there is little oxygen in

the atmosphere of Venus, which in turn indicates that there is little

vegetation like that of the earth on Venus, because it is the process of

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