Tycho

Danish astronomer Tycho Brahe was a good example of those who admired Copernicus’s achievement in tying all the motions of the planets more closely to the Sun but who were unable to accept the motion of Earth. Brahe worked out an alternative cosmology, known as the Tychonic system. In this view the Moon and the Sun revolve around Earth, but all of the other planets revolve around the moving Sun. Tycho’s system had the same explanatory advantages as Copernicus’s. It was what the Copernican system would look like if Earth was made to stay at rest.

Like many other astronomers, Tycho was fascinated by the brilliant new star that appeared in Cassiopeia in 1572. He made extensive observations to determine if it shifted its position with respect to neighbouring stars from night to night. For an astronomer or a philosopher of an Aristotelian frame of mind, it would be difficult to admit that a new star really could appear in the heavens; one would more likely consider it to be some sort of phenomenon in the upper reaches of the air and fire (the elements that, in Aristotle’s cosmology, surround Earth and the seas). If the new star displayed a parallax (i.e., if it shifted back and forth with respect to the real stars), one could be sure that it was near Earth and not a part of the cosmic sphere. Tycho’s demonstration that the new star had no measurable parallax and that it therefore really was a star in the celestial sphere did much to dismantle the old physics.

In 1577 there was a second gift from heaven—a particularly bright comet. In antiquity and the Middle Ages, comets were regarded as atmospheric phenomena. Thus, Aristotle did not treat them in On the Heavens but rather treated them in Meteorology. After all, they are transient, appear suddenly, rapidly cross from one constellation to another, and then disappear. However, Tycho was able to model the motion of the comet by putting it into an orbit around the Sun. He pointed out that the comet was therefore sometimes closer to Earth, and sometimes farther away, than Venus and Mercury were. This seemed to imply that it crashed through the celestial spheres that carried these planets, thus calling into question these vast constructions.