Tycho Brahe (1546–1601) was a member of the Danish high aristocracy, born at Knutstorp Castle in Scania (then Danish, now Swedish) to one of the wealthier branches of the Brahe family. He had begun observational astronomy in 1572 with a public report on a new star in the constellation Cassiopeia — the supernova of 1572 — that established him as the leading Danish astronomical observer.
By 1575 Tycho was considering moving permanently to Germany. The Holy Roman Imperial court at Augsburg, the elector of Saxony, and the city of Basel had all offered him academic positions with research funding. King Frederick II of Denmark — concerned about losing the prestige of Tycho’s residency to a foreign court — made a counteroffer.
February 1576
The royal grant of February 1576 conferred on Tycho:
— The fiefdom of the entire island of Hven in the Øresund Strait, approximately halfway between Copenhagen and Helsingør, perpetually — The collected feudal rent from the island’s approximately 200 peasant tenants — A royal pension of 500 daler per year — The income from the Chapel of the Three Holy Kings at Roskilde Cathedral — Capital funding for the construction of an astronomical observatory
The package was, in 1576, the largest single private research endowment in European history. Tycho’s annual income from the combined grants was approximately 4,000 daler — roughly 1 percent of the entire Danish Crown’s annual revenue. He was, for twenty years, the most heavily-funded scientist in Europe.
He accepted.
Uraniborg
The observatory complex at Hven was named Uraniborg (“Castle of Urania” — Urania being the muse of astronomy). Construction ran from 1576 to 1580. The building was a three-storey square Renaissance villa, approximately 17 metres on each side, with corner turrets housing the major astronomical instruments and a basement-level alchemical laboratory.
The major Uraniborg instruments included a great mural quadrant (a quarter-arc of radius 2 metres, fixed to the wall, used to measure altitudes to approximately one arc-minute precision), an azimuthal quadrant, a 5-foot armillary sphere, and a large sextant. Most of the instruments had been designed by Tycho himself, drawing on his 1560s European travels during which he had studied the equipment in working observatories at Augsburg, Basel, and Wittenberg.
By 1582 the Uraniborg instruments had outgrown the building’s stability. The wooden floors carried enough vibration from ordinary movement to compromise observational precision. Tycho built a second observatory — Stjerneborg (“Castle of the Stars”) — approximately 100 metres south of Uraniborg, with the instruments mounted in subterranean chambers below ground-level domed enclosures. The Stjerneborg arrangement reduced building-vibration noise and isolated the instruments from local wind.
The combined Uraniborg-Stjerneborg observation programme ran from 1576 to 1597 — twenty-one years. The output data were the foundational positional dataset of pre-telescopic astronomy: approximately 777 stars catalogued to one-arc-minute precision (about 10 times more accurate than any previous European star catalogue), approximately 7,000 individual planetary observations of Mars, Venus, Mercury, Jupiter, and Saturn, and complete observational coverage of the 1577 great comet that Tycho had used to demonstrate that comets are extra-atmospheric objects (overturning the Aristotelian doctrine that they were upper-atmospheric phenomena).
The Mars dataset
The Mars observations were the most consequential single data set in the history of astronomy. Mars moves more rapidly and on a more eccentric orbit than the other naked-eye planets; predictions of its position based on circular-orbit Ptolemaic or Copernican models were off by several arc-minutes within months. Tycho’s Mars observations, recorded with one-arc-minute accuracy across twenty-one years, were precise enough to resolve the discrepancy.
The Mars dataset was the foundation on which Johannes Kepler — who had joined Tycho as an assistant in Prague in 1600 — derived in 1609 the first two of his three laws of planetary motion: Mars moves in an ellipse with the Sun at one focus, and the line connecting Mars to the Sun sweeps equal areas in equal times. Kepler’s 1619 third law followed from the same Tycho-derived dataset and from observations of the other planets.
The modern Kepler-Newton model of planetary motion would not have been derivable without Tycho’s Hven data.
How it ended
King Frederick II had died in 1588. His successor Christian IV, who had been a minor at his father’s accession, came of age and assumed direct rule in 1596. Christian and Tycho had a difficult relationship. The young king’s advisers — particularly the chancellor Christoffer Valkendorf, with whom Tycho had a personal feud — pressed for a reduction of Tycho’s substantial royal funding. By spring 1597 the royal pension had been cancelled and the Roskilde income suspended.
Tycho left Hven in April 1597, taking his instruments and approximately 30,000 individual observation records. He spent two years searching European courts for a comparable patronage arrangement. In 1599 the Holy Roman Emperor Rudolf II appointed him Imperial Mathematician at Prague with a salary of 3,000 daler and a residence at Benátky Castle near the capital.
He died at Prague on 24 October 1601 — possibly from a ruptured bladder, possibly from mercury poisoning during a banquet at the Bohemian court (the modern forensic analysis is contested). He was 54.
The Hven observatory complex was demolished within a generation. The remaining Uraniborg foundations were excavated in the 20th century and are now a Swedish UNESCO World Heritage Site museum.
The data set Tycho compiled on Hven outlived the observatory by approximately four hundred years. Modern stellar-position catalogues built on early 20th-century photographic-plate measurements remained, until the 1989 Hipparcos satellite, only marginally more accurate than Tycho’s late-16th-century one-arc-minute observations.
A medieval Danish king gave an island to an astronomer in 1576 and got, in return, the data set that Kepler used in 1609 to derive the elliptical-orbit foundation of Newtonian mechanics.