7.5 Comets

Comets played an important role in the revolution of astronomy and cosmology that occurred between 1500 and 1700. In the earlier Aristotelian geocentric cosmological scheme, the universe was divided into two regions with very different characteristics. The heavens, which reached from the sphere of the Moon to that of the fixed stars, were perfect and unchanging; motion there was exclusively circular. Below the Moon was the world of corruption and change. The Earth was the center of the universe, the natural place of all heavy bodies (bodies in which the element earth predominated). Around it were arranged in successive spherical shells the elements of water, air, and fire. The sphere of fire reached up to the sphere of the Moon, the first heavenly sphere. Although there are references in Aristotle that some of the imperfection of the sublunary region may have rubbed off on the Moon, basically the divide between the heavenly and sublunary regions was absolute.

If the heavens were perfect and unchanging, then no change could occur in them. Any phenomenon that involved change was, therefore, by definition a sublunary one. Whereas heavenly bodies moved around the Earth in never ending circles, repeating their patterns over and over, comets came and went. They appeared suddenly, moved across the constellations for a brief period of time, and then disappeared. There was no regularity, no pattern to their appearances and motions. They were therefore considered changing appearances and therefore by definition their location was "below" the Moon. (It is to be noted here that this is true only in western cosmology after Aristotle. In the cosmologies of other cultures, comets were defined differently.)

The Aristotelian cosmology was dominant in the Islamic world and in Christian Europe. We find no coherent record of comets in the astronomical annals of these cultures (as we do, for instance in China). Comets were, of course, observed, and they are mentioned in chronicles and other non-astronomical documents. They were considered omens, bad omens, and since there was always a major disaster (plague, war, flood, fire, etc.) that happened shortly after a comet had been seen, there was no easy way to prove this notion wrong.

The first recorded efforts to study the paths of comets across the heavens as an astronomical exercise occurred in Florence in the fifteenth century, about a century before the birth of Galileo. By the early sixteenth century, astronomers were observing and measuring the positions of all comets, and in the 1530s Peter Apian in Germany discovered that the tail of a comet always points away from the Sun. His discovery was illustrated in a tract written in German, meant for popular consumption. Among the philosophers (and cosmology was a part of philosophy) there was as yet no doubt that comets were sublunary phenomena.