In Aristotelian cosmology the region above the Moon was unchanging: stars neither appear nor die, they only turn. Change, comets included, was reckoned an atmospheric matter. When a new star as bright as Venus appeared in Cassiopeia in November 1572, that framework was tested directly.

The contribution of the young Danish nobleman Tycho Brahe was not seeing the object but measuring it. If the new star lay in the atmosphere or below the Moon, it would shift against neighbouring stars as the observer's position changed through the night — it would show parallax. Tycho measured for months with large instruments of his own making and found no shift. The conclusion was unavoidable: the object lay among the fixed stars, and the heavens changed. We now know the event was a supernova, SN 1572.

Tycho's real legacy exceeds this one observation. Uraniborg, the observatory he built on an island granted by the Danish king, was the best-equipped of the pre-telescopic age and produced twenty years of systematic, high-accuracy measurement — planetary positions good to better than an arcminute. His own cosmological model was a compromise (the planets circling the Sun, the Sun circling the Earth) and did not hold; but his data passed after his death to his assistant Johannes Kepler. Kepler's laws of elliptical orbits came precisely from those measurements refusing to fit a circle. Modern astronomy's habit of resting on data before theory was established at Uraniborg.