Tycho Brahe (1546-1601) was a Danish nobleman who devoted his life to observing the heavens. In 1575 he convinced the Danish king, Frederick II, to give him the small island of Hven off the coast of Copenhagen. On this island, Tycho built an estate that he called Uraniborg (from Urania, the muse of astronomy). The estate contained two observatories, an extensive library of scientific works, a printing press, and an alchemical laboratory. Tycho and his family lived in Uraniborg, and so did Tycho’s assistants, students and visiting colleagues. Uraniborg became a kind astronomical research center and scholars from all over Europe either visited or corresponded with Tycho. Tycho had a falling out with the next Danish king, Christian IV, who took the island away from him in 1597. Tycho eventually reestablished his observatory in Prague in 1599, where he was appointed Imperial Mathematician by the Holy Roman Emperor Rudolph II.
In his lifetime Tycho was known across Europe for the accuracy of his data, and today he is known as the greatest naked eye astronomer ever (“naked eye” means without a telescope). Tycho was able to make extraordinarily accurate observations because he designed (and had the wealth to have built) very large measuring instruments. In 1598, he published a book with illustrations of these instruments, Astronomiae Instauratae Mechanica (Instruments for the Restored Astronomy). Many of them are so large they require several people to operate, which is why Tycho had large numbers of assistants. For example, the following illustration shows Tycho’s mural quadrant, an instrument used to determine the height above the horizon of celestial objects. The celestial object can be seen through the hole in the wall on the left. One person observes the rising of the celestial object and the others reports the exact time of its rising. A third person records the data.
In 1572, something totally unexpected happened: a new star appeared in the constellation of Cassiopeia in early November. In the picture on the left, the new star is the one labeled “NOVA” on Cassiopeia’s chair. It was visible for a couple years and then disappeared. It was actually what we today would call a supernova, the last explosion of a dying star. Supernovae fade from view after several weeks or months. The supernova of 1572 remained visible until 1574. It was one of only eight historical supernovae visible to the naked eye. For Europeans in 1572, this unprecedented object caused serious problems for the accepted Aristotelian view of the heavens. The celestial realm was supposed to be made of one perfect unchanging substance, the ether or quintessence. New objects should not appear and disappear in the celestial realm! Tycho established his reputation as an astronomer with his observations of this new star.
In 1577, Tycho made another set of observations that challenged Aristotelian cosmology. He observed a comet. Now comets were unusual and unpredictable objects, but they were certainly not unheard of. Observations of comets long predated the Greeks. However, because comets were irregular and unpredictable, they were believed to be terrestrial phenomena. From Aristotle on, the standard explanation of comets was that they were meteorological phenomena, like storms. They occurred at the edge of the terrestrial realm in the sphere of fire. They did not occur in the perfect, unchanging, absolutely orderly and regular heavens. However, Tycho’s astronomical instruments enabled him to make more precise measurements of the position of the comet of 1577 than anyone had ever made before. And his observational data proved that the comet was at least six times farther from the earth than the moon. That, of course, put it solidly in the celestial realm, and created another big problem for the Aristotelian view of the cosmos.
Tycho’s findings led him to reject the traditional Aristotelian/Ptolemaic geocentric cosmos. Although he gave serious consideration to the Copernican model, in the end he rejected this as well. He was bothered, in part, by the enormous size and almost complete emptiness of the heliocentric cosmos. In Ptolemy’s model of the cosmos, each planet’s orb was as large as it needed to be to incorporate the planet’s epicycle, but no larger. One of the beautiful aspects of the Ptolemaic system was that all the orbs fit together perfectly, with no overlap and no empty space between orbs. So the orb of Mercury, for example, just touched the orb of the moon on its inner surface and the orb of Venus on its outer surface. This was not true of the Copernican cosmos. Rather, the orbs of the planets had large gaps between them. You either had to assume that the orbs were much thicker than they needed to be to accommodate the necessary epicycles, or you had to accept large empty regions in between the orbs. Further, there was an absolutely enormous gap between the sphere of Saturn and the sphere of the fixed stars. In the Copernican cosmos, the sphere of the fixed stars on the outer edge of the cosmos is stationary, because the daily rising and setting of the stars and other celestial bodies is accounted for by the daily rotation of the earth about its own axis. Because the earth is moving around the sun, these fixed stars should appear to change position relative to one another, a phenomenon known as stellar parallax. The fact that they are not observed to change position relative to one another can only be accounted for if the sphere of the fixed stars is extremely far away from the earth. Tycho calculated that the sphere of the fixed stars would have to be at least seven hundred times farther away from the earth than the sphere of Saturn in order to avoid stellar parallax. But this would mean that well over ninety-nine percent of the cosmos was empty space. All this empty space violated Aristotle’s physics, which explicitly denied the possibility of a void. But it also seemed wildly improbable to Tycho, and indeed to most thoughtful Christians, that God would have created a cosmos that was almost entirely nothingness.
Instead, Tycho proposed his own system as a rival to both the Aristotelian/Ptolemaic and the Copernican systems. In the Tychonic system, the earth is stationary at the center of the cosmos. The sphere of the fixed stars (and all the celestial bodies) rotated around the earth once every twenty-four hours, thus accounting for the daily rising and setting of the sun, moon and stars. The moon and sun rotate around the earth in periods of one month and one year respectively. The five planets all rotate around the sun. Here is an animated version of the Tychonic system.
When Tycho first devised this system, he was still thinking of celestial bodies being embedded in solid orbs. However, he found that in his geo-heliocentric system, the orb of Mars intersected the orb of the sun. This led Tycho to take a radical and monumental step: he rejected Aristotelian physics with its solid spheres and revived an old rival to Aristotelian physics, that of the Stoics. Stoic natural philosophers, including the Roman writers Lucretius (ca. 99 BCE – ca. 55 BCE) and Seneca (ca. 4 BCE – 65 CE), had asserted that the heavens were fluid rather than solid and that planets moved through the heavens like fish through water or birds through air. Tycho’s system is often treated rather disparagingly as a sort of transitional system between the Ptolemaic and the Copernican. However, Tycho’s abandonment of solid celestial spheres was in its own way no less radical than Copernicus’ abandonment of geocentrism, and of nearly equal importance to the further development of astronomy. It led to sustained debate in the late sixteenth and seventeenth centuries about the reality of the celestial spheres, a debate that was not satisfactorily resolved until the work of Isaac Newton.
Postscript: In the seventeenth century, Jesuit missionaries brought copies of Tycho Brahe’s books, including his instrument book, to China. The Chinese were keenly interested in astronomical observation and they constructed instruments based on Tycho’s models. Although the instruments once housed in Hven have not survived – indeed nothing survives of Uraniborg except some of the foundations – the Chinese versions of these instruments are still on display in Beijing. Note that they are decorated with Chinese dragons!
References and further reading:
John Robert Christianson, On Tycho’s Island: Tycho Brahe, Science, and Culture in the Sixteenth Century (Cambridge, 2000)
Tycho Brahe from Galileo Project
Exhibit on Tycho’s Astronomiae instauratae mechanica at Royal Library of Denmark
Before Newton 
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