Underrated: Tycho Brahe

This great Danish astronomer made a heliocentric theory credible

Mark Ronan

In a large and imposing church in Prague’s Old Town Square lies the grave of Tycho Brahe, a memorial plaque in high relief close by. The body of this Danish astronomer — his first name is a Latinised form of Tyge-was exhumed in 2012 to investigate whether his death in 1601 was the result of natural causes or poisoning. The negative results in no way invalidate the colourful nature of this astronomer, whose metal prosthetic nose replaced the one he lost in a duel with another Danish student. His strong personality, allied with noble lineage, charisma and determination, helped generate funds and space for a great observatory on the Danish island of Ven which became the centre for astronomical study and discovery in northern Europe. He built a printing shop to create and bind manuscripts to his own design, brought in Italian and Dutch artists to decorate the observatory, and craftsmen from Augsburg to build very accurate instruments made to his own specifications.

Years earlier, however, in 1563 when he was only 16 he had made a significant discovery. An expected conjunction of Jupiter and Saturn, when they reach the same stage along the sun’s path, occurred at the “wrong” time. His careful observations showed the current tables of astronomical positions based on Ptolemy’s Almagest to be grossly inaccurate, and he was determined to create his own.

Eight or so years later, after studies at Leipzig and travels in Germany, where he acquired all manner of mathematical and astronomical instruments, he was back in Denmark and constructed a small observatory on one of the family estates he had inherited. From there on November 11, 1572 he was astonished to find a star that became as bright as Venus almost directly overhead in the constellation of Cassiopeia. Tycho’s methods showed it to be a very distant object — now known as Tycho’s supernova — not some atmospheric or sub-lunar phenomenon as comets were supposed to be. His detailed work achieved international recognition, and in 1576 Frederick II of Denmark granted him the island of Ven, along with funds to found the observatory there that he called Uraniborg, after Urania, the muse of astronomy.

In theoretical astronomy several attempts had been made over the ages to view the planets as revolving around the sun, most recently and notably by Copernicus, who published his definitive work on this heliocentric theory shortly before his death in 1543. Tycho was very keen on the idea in his early days, and developed the important technique of parallax to determine distances and hence learn more. The method involves measuring the angle at which an object is visible from two different positions, yielding a triangle for which you know the angles and length of the base. The distance of the object is then the height of the triangle.

Tycho had used this technique on the supernova of 1572, showing it to be very far distant, and in 1577 he applied the same methods to a comet, which he showed must be farther away than Venus. This confounded Aristotle’s view of immutable celestial spheres on which the planets moved, with the stars on a distant sphere beyond everything else. He also used the method to test whether the earth revolved around the sun. If it did, a parallax shift should appear when viewing the stars from different points of the earth’s orbit, but Tycho could find none. Therefore either the earth was stationary or the universe extraordinarily large. Lacking further evidence he felt compelled to accept a stationary earth, but here was an experimental scientist for whom evidence was primary, and it was his extraordinarily accurate observations that paved the way for Johannes Kepler’s theory of planetary orbits.

Kepler became Tycho’s mathematical assistant in Prague where he moved in 1599 at the invitation of the Holy Roman Emperor, Rudolf II. This was after Frederick II of Denmark had died and his heir was not so amenable to Tycho’s requests. When Tycho died, rather suddenly at the age of 54, the use of the telescope for celestial observations was still a few years in the future, but his superb observations made the link from the old Ptolemaic scheme to the new heliocentric view of the solar system. They enabled Kepler to explain planetary orbits, as ellipses with the sun at one focus, plus two additional rules on the orbital speeds and periods.

Like Tycho’s observations, Kepler’s laws were spot on, and can now be derived by any competent mathematics student using Newton’s law of gravity and differential calculus. Kepler of course paved the way for Newton, but it was Tycho Brahe’s superb observational work that fully propelled things forward from the complicated and detailed geocentric universe of Ptolemy to the modern solar system that we now know and accept. Tycho did what all modern scientists should do: first nail down the evidence, then develop the theories. That is what leads to progress. If the theory comes first and observations are massaged to fit in or small theoretical tweaks are made, the result is stagnation, as it was when Ptolemy’s Almagest held sway.

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