Galileo's premonition: "to understand motion is to understand nature" is seen
most clearly with the attempt to understand the kinematics and dynamics of
planetary motion. The attempts to understand the motions of the planets had been
undertaken since at least the time of the ancient Greeks, yet, by the end of the
16th century there was still no satisfactory theory for the motions of the
planets. Johannes Kepler, using data taken by Tycho Brahe, published, over a
period of 20 years of research, three "laws" of planetary motion. Though they
provide some descriptive and prescriptive power, they cannot be thought of as
kinematics since they are purely empirical. The laws are: