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Let's go to Mars

Tycho Brahe builds Uraniborg and measures Mars by eye

Tycho fixed Mars to about an arcminute with sighting instruments alone, cross-checking with duplicate observers and correcting for refraction — and the logbooks went to Kepler.

A hand-coloured engraving on an open book page: a tall red-brick Renaissance house with blue onion domes and corner turrets, cut away below the ground line to show its vaulted cellars.
The main house at Uraniborg in elevation, the cellars cut open beneath it — Joan Blaeu's engraving of 1663, after the woodcut in Brahe's own Astronomiae Instauratae Mechanica. · Image: Joan Blaeu, Atlas Maior, 1663, after Tycho Brahe

On 23 May 1576, Frederick II of Denmark granted a 29-year-old nobleman an island. Hven lies in the Øresund between Zealand and Scania, covers about seven and a half square kilometres, and came with an annual stipend. Tycho Brahe spent it on Uraniborg: part observatory, part alchemical laboratory, part Renaissance palace, with a formal garden and a printing press. Wind shook the instruments on the upper floors, so he dug a second observatory, Stjerneborg, into the ground beside it, with the instruments in pits.

Over the next twenty years Hven produced the last and best body of naked-eye astronomy. Tycho's quadrants and armillary spheres were large on purpose: the longer the arc, the finer the divisions that can be engraved and read. His best instruments held stellar and planetary positions to roughly an arcminute, a sixtieth of a degree. He also ran duplicate instruments and separate observers on the same object, and corrected for atmospheric refraction, which almost nobody else did.

Mars took particular attention. Its orbit is the most eccentric of the planets a pre-telescopic observer could follow well, and its predicted position was wrong in every existing table. Tycho logged it opposition after opposition, building a record of Martian positions that existed nowhere else.

The Tychonic system had the five planets circling the Sun while the Sun and Moon circled a stationary Earth, which bought the geometry of Copernicus without conceding that the ground moves. He was wrong in some details about the comet of 1577, but showed that it moved through the region where the crystalline spheres were supposed to be, so there were no spheres.

Tycho fell out with Denmark's new king, left Hven in 1597, and died in Prague in 1601. After a scramble with his heirs, the Mars logbooks passed to his assistant, Johannes Kepler, a short-sighted, quarrelsome German mathematician who had been given the planet as a work assignment.