While it may sound more like legend than fact, Newton maintained that it was true. In 1665 Sir Isaac Newton (1642–1727), who was newly graduated from Cambridge University, escaped bubonic-plagued London and was visiting the family farm. There he saw an apple fall to the ground, and he began considering the force that was responsible for the action. He theorized that the apple had fallen because all matter attracts other matter, that the rate of the apple’s fall was directly proportional to the attractive force that Earth exerted upon it, and that the force that pulled the apple was also responsible for keeping the moon in orbit around Earth.

But he then set aside these theories and turned his attention to experimenting with light. In the 1680s Newton revisited the matter of the apple, taking into consideration Galileo’s (1564–1642) studies of motion (1602–09), from which the Italian scientist had concluded that falling objects accelerate at a constant rate. In 1687 Newton, with the considerable support of his friend Edmund Halley (of Halley’s comet fame), published Principia mathematica (Mathematical Principles of Natural Philosophy), which outlined the laws of gravity and planetary motion. Newton arranged Galileo’s findings into three basic laws of motion: 1) a body (any object or matter in the universe) that is at rest tends to remain at rest, and a body in motion tends to remain in motion—moving in the same direction, unless acted upon by an outside force (this is the law of inertia); 2) the force to move a body is equal to its mass times acceleration (F = MA, where F is force, M is mass, and A is acceleration); and 3) for every action, there is an equal and opposite reaction. These three laws allowed Newton to calculate the gravitational force between Earth and the moon.