You ask me how the planets are weighed? I reply, on the same
principle by which a butcher weighs a ham in a spring-balance.
When he picks the ham up, he feels a pull of the ham towards the
earth. When he hangs it on the hook, this pull is transferred from
his hand to the spring of the balance. The stronger the pull, the
farther the spring is pulled down. What he reads on the scale is
the strength of the pull. You know that this pull is simply the
attraction of the earth on the ham. But, by a universal law of
force, the ham attracts the earth exactly as much as the earth
does the ham. So what the butcher really does is to find how much
or how strongly the ham attracts the earth, and he calls that pull
the weight of the ham. On the same principle, the astronomer finds
the weight of a body by finding how strong is its attractive pull
on some other body. If the butcher, with his spring-balance and a
ham, could fly to all the planets, one after the other, weigh the
ham on each, and come back to report the results to an astronomer,
the latter could immediately compute the weight of each planet of
known diameter, as compared with that of the earth. 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