In due time he secured access to all the unpublished observations of his great predecessor, and these were of inestimable value to him in the progress of his own studies.
Kepler was not only an ardent worker and an enthusiastic theorizer, but he was an indefatigable writer, and it pleased him to take the public fully into his confidence, not merely as to his successes, but as to his failures.

Thus his works elaborate false theories as well as correct ones, and detail the observations through which the incorrect guesses were refuted by their originator.

Some of these accounts are highly interesting, but they must not detain us here.

For our present purpose it must suffice to point out the three important theories, which, as culled from among a score or so of incorrect ones, Kepler was able to demonstrate to his own satisfaction and to that of subsequent observers.
Stated in a few words, these theories, which have come to bear the name of Kepler's Laws, are the following: 1.

That the planetary orbits are not circular, but elliptical, the sun occupying one focus of the ellipses.
2.