Newton discovered the
principle of gravity while thinking under an apple tree that was in
the garden. In the fall of 1665, he began to solve problems using
fluxions. The problems that he was interested in were about the
movement of the planets. Newton began by reading Kepler’s
three laws of motion:
1. Each planet travels in an ellipse around the sun. The sun
is at one of the focal points of the ellipse.
2. The speed of a planet traveling around the sun changes all
the time. It changes in a way that a line drawn from the center of
the sun to the center of a planet sweeps over equal areas at equal
lengths of time.
3. The time it takes a planet to orbit the sun is related to
its distance from the sun.
Using Kepler’s laws of motion, Newton was able to figure
out that the moon was both falling toward the Earth and trying to
shoot off into space at the same time. To have proof, he needed to
know the radius of the Earth. Although the given measurement
wasn’t correct, Isaac was positive that his theory was