🇩🇪Johannes
Kepler

Johannes Kepler spent five years trying to make Mars fit a circle. He had inherited Tycho Brahe's planetary observations — the most precise measurements ever made — and was determined to show that Copernicus was right: the planets orbited the Sun in perfect circular paths. But Mars refused. No matter how he adjusted the size and position of the circle, no matter how many smaller circles within circles he added, he could not reconcile Brahe's data with circular motion. The discrepancy was only eight arcminutes — one seventh of the apparent diameter of the full Moon — and any other astronomer of the era would have dismissed it as observational error. Kepler, who trusted Brahe's measurements more than he trusted the ancient conviction that celestial motion must be circular, did not dismiss it. He started over. He tried an oval. Then an ellipse. The ellipse worked. In that moment of abandonment — of letting go of a two-thousand-year assumption — the science of planetary motion was born.

Johannes Kepler was born on December 27, 1571, in Weil der Stadt in the Duchy of Württemberg, the son of a mercenary soldier who abandoned the family when Kepler was five and a herbalist mother who was later tried for witchcraft — a charge Kepler spent six years working to defeat. He was a sickly child, nearly blinded by smallpox at age four, and was educated on scholarship at the Lutheran seminary school at Maulbronn and then at the University of Tübingen, where he intended to become a Lutheran minister. There he encountered the Copernican heliocentric model under the astronomer Michael Maestlin and was immediately convinced of its truth, though he understood it as a theological as well as an astronomical proposition: if God placed a light source at the center of creation, He would place it there for a reason, and the Sun was a more fitting center than the Earth.

In 1594 he was appointed mathematics teacher in Graz, and in 1596 published his first major work, Mysterium Cosmographicum, in which he attempted to explain the spacing of the planetary orbits by nesting the five Platonic solids between the spheres. The geometry was beautiful and entirely wrong, but the book brought him to Brahe's attention. Brahe, based in Prague as Imperial Mathematician to Emperor Rudolf II, invited Kepler to join him in 1600. Kepler arrived expecting to receive Brahe's data freely; Brahe, who was protective of it, assigned him the single most intractable problem in observational astronomy — Mars — and died the following year, leaving Kepler his records and his position as Imperial Mathematician.

The Astronomia Nova, published in 1609, contains the first two of Kepler's laws: that planetary orbits are ellipses with the Sun at one focus, and that a line from the Sun to the planet sweeps equal areas in equal times — meaning planets move faster when closer to the Sun and slower when farther away. The Harmonices Mundi, published in 1619, contains the third: that the square of a planet's orbital period is proportional to the cube of its average distance from the Sun. This is the harmonic law, and it applies to every planet in the solar system with extraordinary precision. Newton, working sixty years later, derived his law of universal gravitation directly from Kepler's third law. The inverse-square relationship of gravitational force with distance is mathematically equivalent to the harmonic law; the two are different expressions of the same underlying physical reality.

Kepler also made major contributions to optics — explaining how the eye forms an image and improving the design of the refracting telescope — and to mathematics, developing early forms of integral calculus to calculate the areas swept by planetary orbits. He died on November 15, 1630, in Regensburg, while traveling to collect an overdue salary payment. His grave was destroyed in the Thirty Years' War. But the epitaph he wrote for himself survives: "I measured the skies, now the shadows I measure. Skybound was the mind, earthbound the body rests."