The Sleepwalkers: A History of Man's Changing Vision of the Universe

by Arthur Koestler

the longer it takes to complete a revolution, but this is only roughly true: an exact mathematical ratio was lacking. Saturn, for instance, is twice as far out in space as Jupiter, and should therefore take twice as long to complete a circuit, that is twenty-four years; but Saturn in fact takes thirty. The same is true of the other planets. As we travel from the sun outward into space, the motion of the planets along their orbits gets slower and slower. (To get the point quite clear: they not only have a longer way to travel to complete a circuit, but they also travel at a slower rate along it. If they travelled at the same rate, Saturn, with a circuit twice as long as Jupiter’s would take twice as long to complete it; but it takes two and a half times as long.) Nobody before Kepler had asked the question why this should be so, as nobody before him had asked why there are just six planets.