Harmonices Mundi (The Harmony of the World)

Harmonices Mundi is Johannes Kepler's own 1619 treatise setting out his theory that the geometry of planetary orbits reflects a musical, harmonic order, the primary source in which he stated his famous third law of planetary motion alongside genuinely mystical and numerological reasoning about cosmic harmony. As a primary source it is the strongest possible evidence for what Kepler himself actually argued, including the parts of his reasoning later science kept and the parts, rooted in his own religious and Pythagorean convictions, that it did not. Its limit is that it is a seventeenth century Latin text reflecting the mixed scientific and mystical thought of its era, so a modern astronomical claim drawn from it should be distinguished from Kepler's own explicitly religious and numerological framing around it.