the reason why the moon continued to orbit the earth to all eternity.”
“Smart. But not so smart really.”
“Why not, Sophie?”
“Well, if the moon was drawn to the earth with the same force that causes the
apple to fall, one day the moon would come crashing to earth instead of going round
and round it for ever.”
“Which brings us to Newton’s law on planetary orbits. In the case of how the
earth attracts the moon, you are fifty percent right but fifty percent wrong. Why
doesn’t the moon fall to earth? Because it really is true that the earth’s gravitational
force attracting the moon is tremendous. Just think of the force required to lift sea
level a meter or two at high tide.”
“I don’t think I understand.”
“Remember Galileo’s inclined plane. What happened when I rolled the marble
across it?”
“Are there two different forces working on the moon?”
“Exactly. Once upon a time when the solar system began, the moon was hurled
outward—outward from the earth, that is—with tremendous force. This force will
remain in effect forever because it moves in a vacuum without resistance...”
“But it is also attracted to the earth because of earth’s gravitational force, isn’t
it?”
“Exactly. Both forces are constant, and both work simultaneously. Therefore the
moon will continue to orbit the earth.”
“Is it really as simple as that?”
“As simple as that, and this very same simplicity was Newton’s whole point. He
demonstrated that a few natural laws apply to the whole universe. In calculating the
planetary orbits he had merely applied two natural laws which Galileo had already
proposed. One was the law of inertia, which Newton expressed thus: ‘A body remains
in its state of rest or rectilinear motion until it is compelled to change that state by a
force impressed on it.’ The other law had been demonstrated by Galileo on an
inclined plane: When two forces work on a body simultaneously, the body will move
on an elliptical path.”
“And that’s how Newton could explain why all the planets go round the sun.”
“Yes. All the planets travel in elliptical orbits round the sun as the result of two
unequal movements: first, the rectilinear movement they had when the solar system
was formed, and second, the movement toward the sun due to gravitation.”
“Very clever.”
“Very. Newton demonstrated that the same laws of moving bodies apply
everywhere in the entire universe. He thus did away with the medieval belief that
there is one set of laws for heaven and another here on earth. The heliocentric world
view had found its final confirmation and its final explanation.”
Alberto got up and put the inclined plane away again. He picked up the marble
and placed it on the table between them.
Sophie thought it was amazing how much they had gotten out of a bit of
slanting wood and a marble. As she looked at the green marble, which was still
smudged with ink, she couldn’t help thinking of the earth’s globe. She said, “And
people just had to accept that they were living on a random planet somewhere in
space?”
“Yes—the new world view was in many ways a great burden. The situation was
comparable to what happened later on when Darwin proved that mankind had devel-
oped from animals. In both cases mankind lost some of its special status in creation.