Text: By the third law of Kepler, the squares of the periodic times, of any two planets, are to each other as the cubes of their mean distances. This shows that governing the planetary motions is a cause bearing proportionately upon all the planets. It can not be said of this cause that it is time, because time itself is an effect, the measure of a cause. The periodic times are directly as the forces producing them, of which they are the measures: wherefore, it is nearer the truth to say that the squares of the forces productive of the entire orbits of two planets are to each other as the cubes of their mean distances. It is evident that no exact proportion being found short of taking the entire orbits, and an exact proportion being found by taking the entire orbits, shows a departure from the rule as to the production proportionately of different orbits in detail, i. e. one has a greater or less eccentricity than another at a greater or less distance from the sun, without any disturbance of the rule as generally applicable. Skinner, Ralston; An Essay Upon Force and its Effect Upon Matter

See Also: KEPLER'S THIRD LAW; GRAVITY; TIME

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