May 15, 1618

I just finished reading a book about Johannes Kepler and Tycho Brahe, Tycho and Kepler by Kitty Ferguson. According to the book it was on this date, May 15, in 1618that Kepler discovered the 3rd law of planetary motion.

The 3rd law is the harmonic law which states that “there is an exact relationship between the squares of the planets’ periodic times and the cubes of the radii of their orbits.” Kepler was so excited by the discovery that he wanted to give way to a “sacred frenzy”, as he put it:

I feel carried away and possessed by an unutterable rapture over the divine spectacle of heavenly harmony... I write a book for the present time, or for posterity. It is all the same to me. It may wait a hundred years for its readers, as God has also waited six thousand years for an onlooker.

The dedication prayer of his book, Harmony of the Worlds reads:

I should pray, devout and supplicating, to the Father of lights: O Thou Who dost by the light of nature promote in us the desire for the light of grace, that by its means Thou mayest transport us into the light of glory, I give thanks to Thee, O Lord Creator, Who hast delighted me with Thy makings and in the works of Thy hands have I exulted. Behold! now, I have completed the work of my profession, having employed as much power of mind as Thou didst give to me; to the men who are going to read those demonstrations I have made manifest the glory of Thy works, as much of its infinity as the narrows of my intellect could apprehend. My mind has been given over to philosophizing most correctly: if there is anything unworthy of Thy designs brought forth by me—a worm born and nourished in a wallowing place of sins—breathe into me also that which Thou dost wish men to know, that I may make the correction: If I have been allured into rashness by the wonderful beauty of Thy works, or if I have loved my own glory among men, while I am advancing in the work destined for Thy glory, be gentle and merciful and pardon me; and finally deign graciously to effect that these demonstrations give way to Thy glory and the salvation of souls and nowhere be an obstacle to that.