same object. Galileo discovered that the same thing applied, for instance, to a
cannonball. It is propelled into the air, it continues its path over the earth, but will
eventually be drawn toward the earth. So it will have described a trajectory
corresponding to the marble’s path across the inclined plane. And this was actually a
new discovery at the time of Galileo. Aristotle thought that a projectile hurled
obliquely into the air would first describe a gentle curve and then fall vertically to the
earth. This was not so, but nobody could know Aristotle was wrong before it had been
demonstrated.”
“Does all this really matter?”
“Does it matter? You bet it matters! This has cosmic significance, my child. Of
all the scientific discoveries in the history of mankind, this is positively the most
important.”
“I’m sure you are going to tell me why.”
“Then along came the English physicist Isaac Newton, who lived from 1642 to
1727. He was the one who provided the final description of the solar system and the
planetary orbits. Not only could he describe how the planets moved round the sun, he
could also explain why they did so. He was able to do so partly by referring to what
we call Galileo’s dynamics.”
“Are the planets marbles on an inclined plane then?”
“Something like that, yes. But wait a bit, Sophie.”
“Do I have a choice?”
“Kepler had already pointed out that there had to be a force that caused the
heavenly bodies to attract each other. There had to be, for example, a solar force
which held the planets fast in their orbits. Such a force would moreover explain why
the planets moved more slowly in their orbit the further away from the sun they
traveled. Kepler also believed that the ebb and flow of the tides— the rise and fall in
sea level—must be the result of a lunar force.”
“And that’s true.”
“Yes, it’s true. But it was a theory Galileo rejected. He mocked Kepler, who he
said had given his approval to the idea that the moon rules the water. That was be-
cause Galileo rejected the idea that the forces of gravitation could work over great
distances, and also between the heavenly bodies.”
“He was wrong there.”
“Yes. On that particular point he was wrong. And that was funny, really,
because he was very preoccupied with the earth’s gravity and falling bodies. He had
even indicated how increased force can control the movement of a body.”
“But you were talking about Newton.”
“Yes, along came Newton. He formulated what we call the Law of Universal
Gravitation. This law states that every object attracts every other object with a force
that increases in proportion to the size of the objects and decreases in proportion to
the distance between the objects.”
“I think I understand. For example, there is greater attraction between two
elephants than there is between two mice. And there is greater attraction between two
elephants in the same zoo than there is between an Indian elephant in India and an
African elephant in Africa.”
“Then you have understood it. And now comes the central point. Newton
proved that this attraction—or gravitation—is universal, which means it is operative
everywhere, also in space between heavenly bodies. He is said to have gotten this idea
while he was sitting under an apple tree. When he saw an apple fall from the tree he
had to ask himself if the moon was drawn to earth with the same force, and if this was