Tycho & Kepler: The Unlikely Partnership that Forever Changed Our Understanding of the Heavens by Kitty Ferguson. Walker & co,. $28, 402 pages.

The Chapel Hill News

August 29, 2004

A skybound mind

By Phillip Manning

Tycho Brahe drank too much wine that night. Soon, he was squirming in his chair. He had to go, had to go badly, but 17th-century etiquette forbade him from leaving the banquet table before his host. So Tycho, a proud aristocrat, waited and suffered. Nine agonizing days later, he died, probably from a burst bladder. He was 54 years old, and his last words to Johannes Kepler, his brilliant assistant, were, “Let me not seem to have lived in vain.”

Kepler had arrived on Tycho’s doorstep a year earlier. They were a mismatched pair from the beginning. Tycho was rich and imposing, a barrel-chested, elegantly dressed man with a silver nose in place of the original one, which he lost in a sword fight. Kepler was frail, deeply religious, and a commoner who ranked far below Tycho in that era’s rigid class system. What the two men did have in common was a passion for astronomy, but even there, they approached the subject in vastly different ways. Kitty Ferguson tells the story of these two men’s fruitful, but dysfunctional, collaboration in her superb, exhaustively researched book “Tycho and Kepler” (Walker, $28).

Tycho Brahe was the greatest observational astronomer in the era before telescopes. He supervised the construction of huge (and hugely expensive) quadrants and sextants that gave him a more accurate reading of the positions of stars and planets than anyone before him. Kepler, on the other hand, had poor eyesight and rarely spent nights watching the heavens. He was a mathematical genius, a theoretician who wanted to make sense of the cosmos. To do this, he needed the best observational data available. And because Tycho had that data, Kepler went to work for him.

The two men did not hit it off. Kepler was an avowed believer in the sun-centered solar system proposed by Nicolaus Copernicus a half century earlier. Most astronomers, including Tycho, used Copernicus’s astronomical tables, but few believed that the Earth and the planets actually revolved around the sun. They stuck with Ptolemy’s Earth-centered system and held that the Copernican theory was a useful mathematical model, but not one that described how the solar system really worked. Common sense told them the Earth was was not moving.

Tycho with years of data at his disposal proposed a hybrid system, part Ptolemy, part Copernicus. In Tycho’s theory all the planets except Earth circled the sun. The Earth remained immobile with sun and moon revolving around it. Unfortunately, Tycho’s approach was as inaccurate in predicting the planets’ positions as its predecessors, and he hired Kepler to rework his data. The biggest problem with all three systems — Ptolemeic, Copernican, and Tychonic — was predicting the orbit of Mars. No theory accurately accounted for its wandering ways. After years of keeping his data on Mars secret, Tycho finally turned his observations over to Kepler. The data represented, Tycho said, “An astronomical treasure accumulated by the expenditure of sweat and money over so many [y]ears.”

Kepler was not free to investigate Mars as he chose. Tycho was paying him to prove Tycho was right. And so the frustrated Kepler worked, computing circular Tychonic orbits that never fit the data. But after Tycho’s death, he begin to explore his own theories. He dove into Tycho’s data, applying all of his mathematical skills and imagination. The answers came grudgingly, one by one. First, with a lovely bit of geometry, he proved that the Earth was just another planet orbiting the sun, which consigned Tycho’s system to the junk pile. Then, he derived a formula that predicted how the planets sped up when they were near the sun and slowed down when they were farther away, an insight known today as Kepler’s second law. Finally, in a stroke of genius, he determined that the orbits of the planets were not circular, as stipulated by Ptolemy and Copernicus, but elliptical. Tycho’s “astronomical treasure” now fit a single theory. For the first time in history, mankind could predict the paths of the planets. Thanks to Kepler, Tycho did not live in vain.

Kepler’s reputation soared, but he could not escape the religious strife around him. A devout Lutheran, Kepler was caught up the bloody confusion of the Thirty Years War. Catholics and Protestants fought bitterly throughout eastern Europe. Kepler had done much of his work in Prague, the seat of the Holy Roman Empire, but when he visited that formerly tolerant city in 1627, he found the head of an old Protestant friend impaled on a pike. Soon afterward, Kepler was fired from his job as imperial mathematician, then had it offered it back if he would convert to Catholicism. Kepler refused. He died three years later, nearly destitute. His grave site has been lost but, the epitaph he wrote for himself survives:
“I measured the heavens, Now the Earth’s shadows I measure,
Skybound my mind. Earthbound, my body rests.”
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