New Astronomy and The Harmony of the World

Summary

This record joins two Latin books by Johannes Kepler that together contain the three laws of planetary motion and the religious vision that drove his astronomy. The library does not store a text: the English translations of both books (William Donahue's New Astronomy, 1992; E. J. Aiton, A. M. Duncan and J. V. Field's Harmony of the World, 1997; and Charles Glenn Wallis's earlier version of Harmony Book V) are under copyright, and no public-domain English translation is available, so the record gives a summary and guide only. Astronomia Nova (New Astronomy), issued in 1609, tells the story of Kepler's long campaign against the orbit of Mars, which he called a war. Working from Tycho Brahe's unprecedentedly accurate observations, he first built a circular model that matched them within eight minutes of arc, and then rejected it because Tycho's data could not be wrong by that much. He showed that the Earth's orbit also requires correction, and he looked for a physical cause of planetary motion in a force issuing from the turning sun, drawing on William Gilbert's work on magnetism. Along the way he found that a line from the sun to a planet sweeps out equal areas in equal times, and after many failed attempts he concluded that the orbit of Mars is an ellipse with the sun at one focus. These are now called Kepler's first two laws. The book's introduction argues that the motion of the Earth does not contradict Scripture, which speaks of nature in the language of ordinary appearance. Harmonices Mundi (The Harmony of the World), published at Linz in 1619, is a vast study in five books of the geometric and musical ratios Kepler believed God had built into creation. Book I treats regular polygons and which can be constructed; Book II shows how regular figures fit together in tilings and solids; Book III derives the musical consonances from the division of a string; Book IV applies harmonic ratios to astrological aspects and to the soul's response to them; and Book V finds harmonies in the planets' speeds at their nearest and farthest points from the sun, giving each planet a melodic range. In Book V Kepler announces what is now his third law, that the squares of the planets' periods are proportional to the cubes of their mean distances, which he says he found in 1618. The book closes with a hymn of praise to the Creator.

Top Lessons

• Eight minutes of arc

  Kepler had a model of Mars that most astronomers would have accepted, but it disagreed with Tycho's observations by about a quarter of the moon's apparent width. Because he trusted the observations, he discarded years of work. The episode shows how respect for accurate evidence can overturn a satisfying theory.

• Asking why, not only how

  Earlier astronomers aimed mainly to predict where planets would appear. Kepler insisted on finding a physical cause for their motions, and his search for a force from the sun led him to the area law. His causes were later replaced by Newton's gravitation, but his question shaped the future of physics.

• Science as praise

  For Kepler, finding the geometric order of the heavens was a way of reading the Creator's thoughts and giving thanks. The Harmony of the World ends in prayer. In his work, faith supplied both the motive for inquiry and the expectation of finding order.

• Truth mixed with error

  The same book that announces the third law also assigns melodies to planets and reforms astrology. Kepler's discoveries arose together with ideas later abandoned. Reading him teaches patience with the way real inquiry advances.

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Medical Lens

These two books are astronomy, geometry and cosmology and contain no medical material of note. Kepler's contribution to the history of medicine lies elsewhere: in his optical work of 1604 he gave the first correct account of the inverted image formed on the retina and explained how spectacles correct vision. Book IV of the Harmony discusses the soul's perception of harmonic proportion in astrological terms, which belongs to the history of natural philosophy rather than medicine.