Another Stark Problem with Stark's Astronomy

Stark (Bearing False Witness) seems to think that Copernicus had to introduce “loops” (epicycles) into the planets' circular orbits to get the orbital period correct: “it would not do for the earth to circle the sun in only three hundred days” (151). This is silly: one can always change the diameter or speed of a circular orbit in one's model and thus get the orbital period correct. The real problem with positing circular orbits instead of the actual elliptical ones has to do with the relationship of different parts of a planet's orbit, as can be seen with a visual representation:


In the portions of a planet's orbit where the ellipse if flatter than a circle, the planet will appear to move too fast for it to have a circular orbit. And in the portions of its elliptical orbit where the ellipse is more curved than a circle, the planet will appear to move too slowly. So the actual problem with Copernicus's system (and Ptolemy's) is not that circular orbits show planets having years of too short (or long) a duration – that problem could be trivially corrected. Instead, the problem is that if we mistakenly assume circular orbits, we are left with having to introduce “loops” (epicycles) to explain why some portions of a planet’s orbit proceed faster than other portions.

2 comments:

  1. And in particular temporary retrograde motion as Earth passes Mars for example. No way to get that with earth centered circular motion.

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    1. Yes, that explains Ptlomey needing epicycles. But Stark is trying to explain why Copernicus needed them.

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