centers around the sun.”
“And that world picture was the right one?”
“Not entirely. His main point—that the earth moves round the sun—is of course
correct. But he claimed that the sun was the center of the universe. Today we know
that the sun is only one of an infinite number of stars, and that all the stars around us
make up only one of many billions of galaxies. Copernicus also believed that the earth
and the other planets moved in circular orbits around the sun.”
“Don’t they?”
“No. He had nothing on which to base his belief in the circular orbits other than
the ancient idea that heavenly bodies were round and moved in circles simply because
they were ‘heavenly.’ Since the time of Plato the sphere and the circle had been
considered the most perfect geometrical figures. But in the early 1600s, the German
astronomer Johannes Kepler presented the results of comprehensive observations
which showed that the planets move in elliptical—or oval—orbits with the sun at one
focus. He also pointed out that the speed of a planet is greatest when it is closest to the
sun, and that the farther a planet’s orbit is from the sun the slower it moves. Not until
Kepler’s time was it actually stated that the earth was a planet just like other planets.
Kepler also emphasized that the same physical laws apply everywhere throughout the
universe.”
“How could he know that?”
“Because he had investigated the movements of the planets with his own senses
instead of blindly trusting ancient superstitions. Galileo Galilei, who was roughly
contemporary with Kepler, also used a telescope to observe the heavenly bodies. He
studied the moon’s craters and said that the moon had mountains and valleys similar
to those on earth. Moreover, he discovered that the planet Jupiter had four moons. So
the earth was not alone in having a moon. But the greatest significance of Galileo was
that he first formulated the so-called Law of Inertia.”
“And that is?”
“Galileo formulated it thus: A body remains in the state which it is in, at rest or
in motion, as long as no external force compels it to change its state.”
“If you say so.”
“But this was a significant observation. Since antiquity, one of the central
arguments against the earth moving round its own axis was that the earth would then
move so quickly that a stone hurled straight into the air would fall yards away from
the spot it was hurled from.”
“So why doesn’t it?”
“If you’re sitting in a train and you drop an apple, it doesn’t fall backward
because the train is moving. It falls straight down. That is because of the law of
inertia. The apple retains exactly the same speed it had before you dropped it.”
“I think I understand.”
“Now in Galileo’s time there were no trains. But if you roll a ball along the
ground—and suddenly let go...”
“... it goes on rolling ...”
“... because it retains its speed after you let go.”
“But it will stop eventually, if the room is long enough.”
“That’s because other forces slow it down. First, the floor, especially if it is a
rough wooden floor. Then the force of gravity will sooner or later bring it to a halt.
But wait, I’ll show you something.”
Alberto Knox got up and went over to the old desk. He took something out of
one of the drawers. When he returned to his place he put it on the coffee table. It was