The same
invisible force that made the apple fall to the ground must, he is said
to have reasoned, control the moon, sun, and stars. The earth is pulled
toward the sun, as the apple to the earth, but it is also pulled toward
the stars, each of which is a sun so far away that it looks to us very
small. The result is that the earth neither falls to the sun nor to any
one star, but moves around the sun in a regular path.
This suggestive principle by which every body in the universe is pulled
towards every other body, Newton called the law of universal
gravitation. Newton's law [Footnote: It was really only a shrewd guess,
but it appears to work so well that we often call it a "law."] was
expressed in a simple mathematical formula [Footnote: "The force
increases directly in proportion to the product of the masses, and
inversely in proportion to the square of the distance."] by means of
which physics and astronomy were developed as mathematical sciences.
When a modern astronomer foretells an eclipse of the sun or discusses
the course of a comet, or when a physicist informs us that he has
weighed the earth, he is depending directly or indirectly upon Newton's
discovery.
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