Kepler's three laws provide a convenient and accurate way of
describing the orbits of planets. They do not, however, give any
physical reason why planetary motions obey these laws. Newton's three
laws of motion, coupled with his law of gravitation, provided the
reason. When Newton published his Principia in 1687, he
layed the foundations of celestial mechanics. In this, the final part
of the celestial sphere course, we will summarize Newton's laws and
then look at how they can be used to derive Kepler's laws and predict
the motion of celestial bodies.