The Mystery of the Planets

Yesterday my class observed the motion of the planets against the starry background using Stellarium.  In part this was an exercise in quantitative astronomy.  They learned how to measure the synodic and zodiacal periods of the planets, and they will need to know how to make such measurements when they begin examining their personalized solar system later in the semester.

But the first part of the activity was qualitative, and this is where they were in for a big shock.  As they follow, say, Mars eastward across the stars they find that it suddenly stops, turns around and goes west for a bit, and then resumes its usual eastward motion.  This retrograde motion must have perplexed the ancients to no end, and my even my students with their modern educations ask me "why does it do that??!!"  It is one of the most satisfying moments of the course, especially when I get to answer their questions with "we will spend the rest of the semester answering the question you just asked."

These qualitative observations also help us to establish the first major classification of planets.  Mercury and Venus exhibit similar qualitative behavior.  They both remain relatively close to the Sun at all times.  During their retrograde motion they pass the Sun, so that at the center of their retrograde they are in conjunction (near the Sun in the sky).  In contrast, Mars, Jupiter, and Saturn have a different type of motion.  They can be far away from the Sun, even on the opposite side of the sky (in opposition).  In fact, it is when they are in opposition that they do their retrograde thing.  Curiously, they are brightest at this time as well (although the effect is only dramatic for Mars).  These two different types of motion lead to two different classifications: Mercury and Venus are "inferior planets" and Mars, Jupiter, and Saturn are "superior planets". 

I'm actually very interested in the origin of these terms.  I know that eventually ancient astronomers settled on a system in which Mercury and Venus were below the Sun, while Mars, Jupiter, and Saturn were above the Sun.  But not all ancient astronomers used this ordering.  Eventually, Copernicus would shift the meaning of inferior and superior, but I'm very curious as to how these terms first came to be used, long before Copernicus.

One thing I have found in teaching this material is that modern students jump easily to what was known as the Capellan system: with Mercury and Venus orbiting the Sun, but no such claim about the other planets.  This may be partly because the high-speed view you can get from Stellarium is very suggestive of this arrangement, but I think it is also a product of their schooling.  They have been told all their lives that planets orbit the Sun, so when they see Mercury and Venus running back and forth across the Sun in the sky it is very easy for them to connect this to what they "know."  They are just seeing what they expected to see.  But it is harder with the superior planets.  The motions  of the superior planets are tied to the Sun in a much subtler way, and even that subtle connection does not really suggest that these planets are orbiting the Sun.

For now, I want the motion of the planets to remain a mystery for my students.  We have established some empirical facts (qualitative and quantitative), and now we must come up with some rules that will help to explain these facts.  As I said above, we'll spend the rest of the course doing just that, with history as our guide.