Of all the patriarchs of science, Johannes Kepler is the least known. We often talk of Isaac Newton and his law of universal gravity (and laws of motion, and the calculus, and laws of optics), of Galileo's impetuosity and his telescopic discoveries (and law of free fall and pendular motion), and of Copernicus, the man who put the sun in the center of the cosmos. But Kepler? Sounds familiar; but what was it again?
We need to do better. Kepler is, hands down, one of the most fascinating characters in the history of science. Of course, most of the readers of 13.7 know this already; they know Kepler discovered the three laws of planetary motion, the first mathematical laws of astronomy: that planets orbit the sun in elliptical orbits; that the imaginary line connecting sun to planet sweeps equal areas in equal times; and that the square of the planet's orbital period equals the cube of its average distance to the sun. (These laws apply to any planet orbiting a star.)
I know, sounds kind of boring. But, as with much in life, relevance depends on context. Kepler was the link between the old and the new, a visionary who lived to show that the order we see in the cosmos was the handiwork of a divine mind, well-versed in geometry. To Kepler, faithful to what the Pythagoreans preached two millennia before him, only math could unveil the mystery of creation. The relationship between man and cosmos obeyed the same resonances as the planetary orbits, an expression of the harmony of the spheres.
If his spirituality may seem innocent to us today, we must recall that his dreams of cosmic harmony inspired his work throughout his life. They were the fire that breathed life into his breakthrough scientific discoveries. Kepler found the ellipse not because he looked for it but because it was the only curve that fit the data collected over decades of meticulous observation by the Dane Tycho Brahe. In this, Kepler showed his modernity: if a theory is in conflict with data, change the theory. This was not as obvious in 1609 as it may (or should) be now. The circle, after millennia of prominence in the skies, gave way to the imperfect ellipse.
Nature, not mind, had the last word.
Even if his search for a cosmic harmony, his mysterium cosmographicum, was more a holy grail than science, it represented one of the noblest aspirations of the human spirit, to transcend its mortal chains in search of eternal knowledge.
Today, we can identify similar trends in the search for unification in physics, also inspired by dreams of a universal harmony, albeit one based on the vibrations of fundamental strings as opposed to planetary orbits. From Kepler, we learn that we must dream. But we also learn that such dreams are only useful if, when we wake up, they help us make better sense of the observed world.