orbital mechanics

Enunciated for the first time the following year after some concerted fact-checking and re-taking measurements since the outcome seemed to elegant to be true, Johannes Kepler discovered the last of his three laws of planetary motion on this day in 1618, capturing the relationship between the distance of a astronomical body from its host star and the time it takes to complete a trip around it: that is, the value of the cube of the semi-major axis divided by the square of a planet’s orbital period is a constant—for our solar system. The publication was also delayed due to rather laborious attempts to reconcile his formula with the theory of the music of the spheres (see here and also above), thus making this third discovery known as the harmonic law.