Introduction Kepler's New Astronomy Three Models Kepler's Departure Finding "Oppositions" The "Mean" Sun Twelve Observations Ptolemy's "Equant" A "Vicarious Hypothesis" Earth's Motion An "Immaterial Species" Area-Time PrincipleAn Ellipse Conclusion On Proportion Recommended Books
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One day in 1594, while Kepler taught a secondary school class in Graz, Austria, about the conjunction of Mars and Jupiter, it occurred to him that he could inscribe an equilateral triangle in the distance between the two planets.
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The insight struck him like a lightning bolt, and launched one of the most fruitful, if idiosyncratic investigations in the history of science. Kepler decided, that day in 1594, that a nested ordering of the five regular three-dimensional figures (called the Platonic Solids) must determine the distances between the planetary orbits. |
At age 25 he published his idea in a short book entitled Mysterium Cosmographicum (The Sacred Mystery of the Cosmos).
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One astronomer who learned of Kepler's hypothesis, the Danish nobleman Tycho Brahe, was impressed enough by Kepler's technical work to extend an offer of employment. From 1597 on, Kepler writes, Brahe “did not cease ... to invite me to come to him."
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As other prospects for employment in Kepler's adopted Catholic city in a climate of rising religious tensions (Kepler was protestant), at the start of the Thirty Years War, in 1600, Kepler took up Tycho's invitation and traveled overland 800 miles north to Tycho's island observatory in Uraniborg, Denmark.
Kepler writes:
“It is true that a divine voice, which enjoins humans to study astronomy, is expressed in the world itself, not in words or syllables, but in things themselves and in the conformity of the human intellect and senses with the sequence of celestial bodies and of their dispositions. Nevertheless, there is also a kind of fate, by whose invisible agency various individuals are driven to take up various arts, which makes them certain that, just as they are a part of the work of creation, the likewise also partake to a certain extent in diving providence." (Chapter 7 of The New Astronomy, Donahue translation, p. 183)
He saw his encounter with Tycho as the work of divine providence.
When Kepler arrived, Tycho's assistant was puzzling over discrepancies in the theory of Mars.
Kepler says he thought it happened by "divine arrangement” that he arrived when Tycho's assistant "was intent upon Mars, whose motions provide the only possible access to the hidden secrets of astronomy, without which we would remain forever ignorant of those secrets.” (Donahue translation, p. 185)
Mars's extraordinarily irregular orbit facilitated Kepler's key insights. Had he started his work on a planet with a more regular orbit, his work might have been less fruitful.
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