Fixed stars, part II

The Middle Ages were a hotbed of astronomical controversy in Europe.

Apart from rare spectacles like Gliese 710 (which we discribed last week), most of the stars in the sky are remarkably unchanging.  That is, they rise and set and all that, but among themselves the patterns are fixed.  Orion rises as Scorpius sets (more or less, from mid-northern latitudes), and the distances and angles you measure between them stay the same over a lifetime and longer.

This was used by Ptolemy in the second century AD as an argument against the idea that the Earth moved around the Sun.  Obviously, if the Sun is the center of the universe and not the Earth, than at some times we must be closer to Orion than others.  At those times it must appear larger.  But it doesn’t, so the Earth must be at the center.  (This argument was not original with Ptolemy, but we don’t know for sure where it came from.  His Almagest was such a successful compendium of astronomy that almost no previous material has survived.  The ancients had already invented history and astronomy, but they hadn’t yet gotten around to the history of astronomy.)  Someone did point out that the stars might be very, very far away (the real answer), but that seems like a lame excuse, and absurd: why have all this empty space between Saturn and the stars?  Well, we like to think our reasoning is more rigorous nowadays, and we don’t reject things just because they seem strange.  Maybe that’s true.

Ptolemy’s work is mostly concerned with the motions of the planets.  He included a catalog of the fixed stars as the background against which the planets move.  Among them he noted a number of alignments, three or more stars in a straight line, so that observers coming afterward could more easily tell if the stars moved among themselves.  He didn’t think they did, but was open to the possibility.  In this he disagreed with Aristotle (the main authority at the time for science), whose physics did not admit of any change in the universe beyond the Moon.  His system was also in tension with Aristotelian physics in its details.  Aristotle figured that each planet must move in an exact, unchanging circle at an even speed.  Ptolemy could only get his system to match the motions of actual planets by having them move on a circle whose center is displaced from the center of the Earth, moving at an even speed as observed by a point displaced on the other side, adding an epicycle on top: violating the spirit if not the letter of Aristotle.

This tension continued in the centuries after Ptolemy.  The philosophers insisted on uniform motions in circles as the only possible eternal motion.  Astronomers and astrologers, who were concerned with where the planets actually appeared, went with what worked.  This was called “saving the phenomena.”  The conflict was not as bloody as others during that time; indeed, it only concerned the very few who could understand it; but it went unresolved for the whole Middle Ages.

The Scientific Revolution was not much help at first.  Copernicus put the Sun at the center of the universe, and pointed out all the ways in which this actually made more sense, but it didn’t produce better positions for the planets.  Kepler bent the circles into ellipses and made the motions uneven in a precise way, doing a wonderful job of saving the phenomena.  But that meant he was in even more direct conflict with Aristotelian physics, which was still the only game in town.

It took Newton’s synthesis to finally make sense of everything, putting the physics and the astronomy together.  We like to think that, since then, we haven’t been divided into the philosophers who know what should happen, and the astronomers “saving the phenomena.”