Newton had studied Kepler's three laws, Galileo's falling experiment and Descartes' vortex theory when he was in college. 1665, in order to avoid the plague, he returned to his hometown and began to explore the problem of gravity, trying to explain the motion of planets by relying on the mechanical principles on the earth. 1672, Newton was elected as a member of the Royal Society, and later became the president of the Royal Society until his death.