Fig. 60.—Kepler’s second law.
141. Thus were established for the case of Mars the two important results generally known as Kepler’s first two laws:—
1. The planet describes an ellipse, the sun being in one focus.
2. The straight line joining the planet to the sun sweeps out equal areas in any two equal intervals of time.
The full history of this investigation, with the results already stated and a number of developments and results of minor importance, together with innumerable digressions and quaint comments on the progress of the inquiry, was published in 1609 in a book of considerable length, the Commentaries on the Motions of Mars.[89]
Fig. 61.—Diagram used by Kepler to establish his laws of planetary motion. From the Commentaries on Mars.