Bunch-Hellemans, Immutability of the heavens

The Greek concept of the universe was detailed by Aristotle. This Greek cosmology included the idea that something had to hold up the stars and planets, something that could not be seen. Since the only hard substance known that was also transparent was crystal (transparent calcite), Greek logic insisted that spheres of crystal hold the stars, Sun, Moon, and planets in place.

Greek tradition also included the idea that the heavens are very different from Earth. On Earth, everything is changing all the time. In the heavens, nothing ever changes, they believed. Comets, however, appear in the heavens, last a few days or weeks, and disappear. This constitutes change. Therefore, Aristotle argued, comets are earthly stuff, not heavenly stuff, and must be some kind of spontaneous fire in the upper atmosphere.

Then the comet of 1577 showed that Aristotle could not be right about the heavens. Tycho Brahe, Michael Mästlin, and other astronomers were able to plot the comet’s path. Tycho Brahe also used parallax in an effort to determine the distance of the comet of 1577 from Earth. Parallax refers to the apparent angular difference in position of an object when seen from two locations. The amount of this difference can be used to calculate the distance to the object. These studies refuted much of the Greek view of the heavens.

For one thing, the comet was farther away than the Moon, so it had to be heavenly and not earthly. Secondly, the path of the comet was not part of a circle, so heavenly objects need not all move in circles. Finally, the path of the comet of 1577 was such that it must travel through the crystalline spheres. Hence, something other than crystalline spheres must be holding the Sun, Moon, planets, and stars in space.