Legend has it that Isaac Newton, an English mathematician and physicist, was led to discover the law of universal gravitation when he saw an apple fall from a tree. He showed that the same force of gravity that pulls an apple to the ground also holds the moon in orbit. In other words, orbital motion and falling are exactly the same thing.
Newton’s law of gravitation describes a force of attraction between any two bodies in the universe. This force increases in proportion to the mass of each body and decreases in proportion to the square of the distance between them. (That means, for example, that if the distance between two bodies increases by a factor of two, the force of attraction between them falls to one-quarter as much; at three times the distance, the attraction is one-ninth as much; at 10 times the distance, it’s only one-hundredth as much, etc.)
Newton also discovered the laws of motion. Any body moves in a straight line at constant speed unless an outside force acts on it. Such a force produces a change in the speed or direction, called an acceleration.
Newton was also a first-class experimenter. He demonstrated, for example, that white light is a mixture of the entire rainbow spectrum of colors. His study of light led him to invent the reflecting telescope, which is still the basis for the design of the large telescopes astronomers use today.