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According to Ptolemy's (c. 150 CE) account of the motions of planets, planets moved in circular paths ("epicycles") around center points that in turn moved around the center of the earth along a path called a "deferent", or around point near the center of the earth, in a path called an "eccentric". (I think I have this right.)

Is there a general term for these three kinds of paths? "Orbit" doesn't seem correct, since none of the three are orbits in the modern sense.

I'd also be interested in knowing of a general term of the centers of these circles.

EDIT: I originally posted this on History.SE, because I think of it as a question for historians of science, and the History of Science SE site is in Area 51. Someone at History.SE suggested it would be better to ask this question on English.SE. I did, but I left the original question up at History.SE. The question got downvoted at English.SE, and was migrated here. There is now an answer at History.SE that I've accepted. Perhaps this question should be deleted from Astronomy.SE? Or I should not have posted it in two places? I'm reluctant to delete a question for which someone has posted an answer.

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  • $\begingroup$ No, path curve is called "epicycloid". General term is likely to be "a bad science" along with phlogiston. $\endgroup$
    – Free Consulting
    Commented Dec 22, 2014 at 17:34
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    $\begingroup$ @FreeConsulting That is unfair! Ptolemy did marvelous work according to his mission, which was to predict the apparent movements of the planets. It wasn't bad science, it was the best science ever, for 1500 years. $\endgroup$
    – LocalFluff
    Commented May 21, 2015 at 18:49
  • $\begingroup$ @LocalFluff, not just unfair. Uninformed. $\endgroup$
    – Mars
    Commented May 21, 2015 at 19:08
  • $\begingroup$ @FreeConsulting I want to add to the defence of Ptolemy, that his calendar saved the life of millions of human beings, because of the better navigation and agricultural timing he provided. Physicists and engineers should give a bit more attention to the history of the giants on whose shoulders they stand today. $\endgroup$
    – LocalFluff
    Commented May 21, 2015 at 19:28
  • $\begingroup$ If anybody would like to read an easy, informal presentation that clarifies why Ptolemaic astronomy was not "bad science" at the time, take a look at Richard DeWitt's Worldviews. $\endgroup$
    – Mars
    Commented May 21, 2015 at 19:31

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Orbit can also mean "the usual course of one's life or range of one's activities" so I would find Ptolemic orbit quite acceptable if you hesitate to use just orbit when the path isn't elliptical.

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  • $\begingroup$ Good point. I agree that this common metaphorical use of "orbit" suggests that I could use it, metaphorically, to refer to the circles in Ptolemy's or Copernicus's system. I'd rather have a non-metaphorical term. My edit to the question references an answer that provides that. $\endgroup$
    – Mars
    Commented Sep 23, 2014 at 14:32
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Ptolemy himself only talked about these as “circles.” Words like deferent and epicycle and even equant only arose in mediaeval times, in Latin. For example, for the equant, Ptolemy talked about a “center of uniform motion.”

You are right to point out that the word “orbit” is too modern. If I recall correctly, it was coined by Kepler, so ≈1,500 years after Ptolemy.

I have translated the Almagest to French, if it’s a language you understand. My website has some of Ptolemy’s diagrams animated or interactive. https://ecliptiqc.ca/Almageste.php

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    $\begingroup$ Thanks very much Pierre! I do read French, and I'm always interested in animated diagrams of such things for teaching. (I'm not a historian, but a philosopher, and it's useful to teach students some history of science when teaching philosophy of science.) $\endgroup$
    – Mars
    Commented Jun 15, 2023 at 23:14
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    $\begingroup$ Super! I’m seriously thinking about adding more animations, so keep an eye out! $\endgroup$
    – Pierre Paquette
    Commented Jun 16, 2023 at 4:53

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