Lesson 3: Astronomy after Copernicus
New stars, comets, and the Corruption of the heavens
Tycho was also involved in one of the most important intellectual changes of the late sixteenth century–the decline of the idea that the heavens were perfect and incorruptible. Since the heavens were unchanging, Aristotle and most Aristotelians thought that changes in the sky were actually atmospheric changes on earth. (Other ancient authorities thought differently.)
Comets were not celestial bodies, but "exhalations" of the earth, as it belched forth incandescent gas. Another problem for the theory of the unchanging heavens were "new stars" suddenly appearing as bright bodies in the sky. We now call them "novas," from the Latin word for "new."
A new star bright enough to be seen in the daytime appeared in the constellation of Cassiopeia in the winter of 1572. It steadily dimmed in brightness, disappearing from sight by March 1574, but it made a remarkable impression. Some astronomers claimed that it was a comet, and others argued whether it was the positioned above or below the "sphere of the moon"–in other words whether it was an atmospheric or celestial phenomenon.
Tycho published more on the star than any other astronomer. On the New Star (1573), combined a record of his observations with a demonstration of the new star’s position in or near the sphere of the fixed stars.
Although not widely distributed, this work helped make Tycho’s reputation, and he returned to the subject throughout his career. Even the leading Ptolemaic astronomer in the late sixteenth century, the Jesuit Christoph Clavius (he was the leader in the Gregorian reform that created the Gregorian calendar we use now), agreed that the new star demonstrated that the heavens were changeable. The new star was followed in 1577 by a great comet, as bright as the planet Venus. Tycho and other astronomers studied its parallalax, the degree to which its background against the stars changed when it was observed from different points, and demonstrated its position "above the moon."
The comet’s position showed the falsity of the Aristotelian theory of comets, providing another example of how the heavens were not unchanging. Of course, many questions still remained, and astronomers argued whether comets were permanent bodies or ephemeral, and what their physical nature was.
Kepler believed that comets were ephemeral objects, on straight line trajectories. He argued that comets were formed out of fatty globules in space, and that the sky was full of temporary comets, only a few of which could be seen from earth.
Rene Descartes, who we’ll be meeting a little later, argued that comets were dead suns covered over with sunspots. Without any way of examining comets directly, the arguments went on and on.
