Johanne Kepler's "Harmonice mundi" was planned in 1599 as a sequel to the "Mysterium cosmographicum." In 1618 Kepler discovered the third law of planetary motion relating to the periodic times of the planets to their mean distances from the sun - a crowning achievement that enabled him to bring the "Harmonice mundi" to completeion. The authors have presented and interpreted Kepler's Latin text to readers of English, by putting it into "the kind of clear but earnest language which we suppose Kepler would have used if he had been writing today."