Sir Isaac Newton was contemplating the universe in his garden one day in 1665 when he noticed an apple fall from a tree. Newton wondered to himself, "Why should the apple always fall to the ground? Why does it not go sideways or upwards but directly toward the Earth's center?" After much contemplation Newton concluded that objects are pulled to the Earth through a force that he called gravity, which means heaviness in Latin. On July 5th, 1687 Newton formally expressed his law of universal gravitation in a series of books called The Principia.