# A last bit of erudition for the night

I only gave Kepler’s first 2 laws of planetary motion this morning. The reason for this is those were the only two listed in the Astronomia Nova, which is the book I was concerned with in my paper. The third one follows. One must strive for completion in these things. And I knew how disappointed that you all were that I’d only put down 2 laws…

Kepler’s third law (1618): The square of the sidereal period of an orbiting planet is directly proportional to the cube of the orbit’s semimajor axis.