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 right.