German astronomer Johannes Kepler (1571-1630) derived his mathematical laws of planetary motion from astronomical data meticulously collected by Tycho Brahe, who, as he was dying, beseeched young Kepler to use the data to discover the laws of motion of the planets. Based upon Kepler's laws, Sir Isaac Newton later developed his law of gravity. This is a chapter from Sir Robert Stawell Ball's Great Astronomers (2nd ed. 1907). Ball traces Kepler's life...