The fact that of all the planets Venus most resembles this good little
earth on which our present lot is cast gives us a strong feeling of
kinship with her, and a more lively interest in all her affairs than we
might otherwise have. She and the earth are so nearly of one size that
they are often referred to as twin sisters. There is a difference of
less than three hundred miles in their diameters, the earth’s diameter
measuring 7,917 miles, and that of Venus 7,629 miles. The surface of
the planet is about ninety-three per cent. as extensive as that of
the earth; its mass is a little more than eighty per cent., and its
volume about ninety per cent. as great as the earth’s. Differing so
little in these particulars, it follows that it must differ very little
in density and gravity. The earth is the densest of all the planets,
and Venus is only one-tenth less dense than the earth. Its force of
gravity is not quite nine-tenths that of the earth. A removal from
the earth to Venus would make just a comfortable reduction in one’s
weight. A person weighing one hundred and seventy-five pounds here
would weigh on Venus one hundred and fifty-four. If through strength
of appetite and weakness of will one should take on two hundred pounds
of too, too solid flesh here, transportation to Venus would bring about
an instantaneous reduction to a solid one hundred and seventy-six
pounds--as much of a reduction as would be compatible with health.
Venus must have begun her career in much the same way that the earth
began its career. The nebula that formed her nucleus was probably
nearly the same size (contained about the same amount of matter) as
that with which the earth began its existence. The two bodies have
succeeded in capturing about the same amount of loose material, and
their gravity is such that they can hold within their bounds particles
traveling at about the same rate of speed. No molecule of gas coming
within the range of Venus’s attraction and traveling more slowly than
six and thirty-seven hundredths miles per second can escape from Venus,
and the earth can hold only such as move, when coming within its own
attraction, with a less speed than six and ninety-five one-hundredths
miles per second.
The earth has a moon, and Venus has none; but that may be because,
like Mercury, Venus is too near the sun to be permitted to retain such
a luxury. It is likely that if, in her earlier history, she had within
the limit of her gravitative attraction the nucleus of a satellite, it
would have been taken away from her by the stronger attraction of the
sun. The same thing would have happened to us if we had been a little
nearer the sun. And yet in 1645 a moon belonging to Venus was supposed
to have been discovered, and it was thought to have been seen three
times within the rest of that century, and four times within the first
half of the following century. The last supposed view of it was in
1791; it has never been seen since. There is little doubt that it was
an illusion of some kind. Perhaps, though, Venus has not the same need
of a moon that we have.