
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
Keplers 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 Keplers 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
wasnt correct, Isaac was positive that his theory was right.



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