For Johannes Kepler, music was crucial to the emergence of a new astronomy. Kepler ’ s
Harmonices mundi libri V ( Harmony of the World , 1619) culminates in his so-called third
law of planetary motion: the square of the orbital period of any planet is proportional to
the cube of its mean distance from the sun.^1 This surprising connection emerged from
Kepler ’ s search for harmonic relations between planetary data and became a touchstone
for Newtonian celestial mechanics. As important as this result is, Kepler presents it without
fanfare, certainly not as the “ law ” it was later called. For him, it was only a part of his
much larger project of seeking harmonious relations between planetary motions. Kepler
embarked on this enterprise moved both by his Neoplatonic concern with cosmic arche-
types and also by his keen interest in practical music and contemporary compositions.^2
His strong feeling for what he called the “ song of the Earth ” illuminated and comple-
mented his cosmic concerns.
On the theoretical side, though he identified himself strongly with Pythagoras, Kepler
reinterpreted musical consonance in terms of the ratios of the sides of polygons, instead
of the pure numerology inherited from Pythagorean tradition. Indeed, Harmonice mundi
concludes with a polemic against the English Rosicrucian Robert Fludd, from whom
Kepler distinguishes himself “ in the way in which a practitioner does from a theorist. ”^3
According to Kepler, Fludd “ has advice on the composition of figured melody, an art which
I do not profess ” and “ also digresses to various musical instruments, to which I had not
even given thought. ”^4
On the practical side, though the passages in Harmonice mundi in which Kepler turns
to actual musical compositions are few and brief, they give insight into his sensibility. In
those moments, he characteristically gives examples from compositions by Orlando di
Lasso. Kepler does not say much or go deeply into these examples, yet they are still inter-
esting because they figure in an astronomical work. Indeed, in the long tradition linking
music and astronomy, Kepler is remarkable for citing specific musical examples, not just
theoretical generalizations. These examples illuminate the context of his whole project.
Like many of the other figures considered in this book, Kepler was involved in the
practice of music as well as its theory. From early childhood, Kepler was steeped in the
barré
(Barré)
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