Changes in the Heavens

Changes in the Heavens

Copernicus died in 1543, on the very day that the first copies of his book left the printing press. His opponents soon had much more to contend with. Towards the end of 1572 a new star appeared in the constellation of Cassiopeia. The Danish astronomer Tycho Brahe and other tried to detect its movement relative to the so-called fixed stars, but without success. It clearly belonged to the region of the celestial sphere, a region which, according to Aristotle, was inherently divine, eternal, and unchanging. Aristotle, and the theologians who accepted his world system, were therefore mistaken. As if to drive the point home, a large, bright comet appeared in northern skies in 1577. Tycho Brahe found that it was moving at a distance far beyond the moon and in a very definite path. It changed in appearance night after night, yet moved in celestial regions. How could those region be eternal and unchanging?

Tycho Brahe observing with his large mural quadrant

The new star of 1571 was really an old star which had literally exploded. Its estimated position is now occupied by a weak source of radio emission and a few small, ragged patches of nebulosity. One of these “super new stars”, or supernovae, flared up in 1054, and was mentioned by Chinese analysts of the time. Its remains form an object known as the Crab Nebula, an enormous, expanding, and chaotic mass of gas which emits light, cosmic rays, and radio waves. Yet it is so far away that it looks no more than a small misty patch even in telescopes of moderate size. Its light takes 4,500 years to reach us. The explosion, therefore, occurred not in 1572 but some 3,000 years before the Christian era.

After observing the new star, Tycho Brahe devoted the rest of his life to an accurate determination of the positions and motions of the heavenly bodies. At his observatory at Uraniborg on the island of Hveen, he assembled a princely collection of large measuring instruments of his own design. For 25 years he diligently measured the positions of the stars and used these as reference points for tracing the paths of the five planets. For the first time in many centuries the heavens were studied systematically and in detail, although still within the limits of naked-eye observation.