In 1665 C.E., Issac Newton graduated from university and retired to his little hometown to avoid the Plague, which was sweeping London. According to a probably apocryphal tale, Newton supposedly hit upon his inverse square law of gravity when observing an apple falling from a tree outside his study window. The inverse square law declares that the force of gravity is squared when one halves one's distance from an object. With this realization, and the calculus he invented to describe it, the orbits of the planets (and, indeed, any celestial object) could be accurately modeled. Although his law was refined by the 20th century works of Einstein, Newton's laws still describe the force of gravity quite well, except for the most massive and the very tiniest (quantum) of objects.