Whereupon I computed what would be the Orb described by the Planets. … I found now that whatsoever was the law of the forces which kept the Planets in their Orbs, the areas described by a Radius drawn from them to the Sun would be proportional to the time in which they were described.
The world is full of things that exhibit rotary motion. They range in size from galaxies to electrons orbiting around atoms, and they include such familiar objects as orbiting planets, amusement park rides, flywheels, and bathtub vortices. What underlying principle explains the persistence of such motions? Can we describe them all in a unified way?
In Chapter 7 we studied two partial answers ...