Astronomia Nova - The "third Law"

The "third Law"

Kepler discovered his "third law" a decade after the publication of the Astronomia nova as a result of his investigations in the 1619 Harmonices Mundi (Harmonies of the world). He found that the ratio of the length of the semi-major axis of each planet's orbit (cubed), to the time of its orbital period (squared), is the same for all planets.