In applying
this principle to the heavenly bodies, we at once meet a
difficulty that looks insurmountable. You cannot get up to the
heavenly bodies to do your weighing; how then will you measure
their pull? I must begin the answer to this question by explaining
a nice point in exact science. Astronomers distinguish between the
weight of a body and its mass. The weight of objects is not the
same all over the world; a thing which weighs thirty pounds in New
York would weigh an ounce more than thirty pounds in a spring-
balance in Greenland, and nearly an ounce less at the equator.
This is because the earth is not a perfect sphere, but a little
flattened. Thus weight varies with the place. If a ham weighing
thirty pounds were taken up to the moon and weighed there, the
pull would only be five pounds, because the moon is so much
smaller and lighter than the earth. There would be another weight
of the ham for the planet Mars, and yet another on the sun, where
it would weigh some eight hundred pounds. Hence the astronomer
does not speak of the weight of a planet, because that would
depend on the place where it was weighed; but he speaks of the
mass of the planet, which means how much planet there is, no
matter where you might weigh it.
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