Page:Astronomy for Everybody.djvu/271

principle, the astronomer finds the weight of a body by finding how strong is its attractive pull on some other body.

In applying this principle to the heavenly bodies, you meet at once 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 more exactly the difference 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. But there would be just as much ham on the moon as on the earth. There would be another weight of the ham on 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.

At the same time we might, without any inexactness, agree that the mass of a heavenly body should be fixed by the weight it would have at some place agreed upon, say