<p>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.”</p>
<p>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.</p>
<p>I have translated the <em>Almagest</em> to French, if it’s a language you understand. My website has some of Ptolemy’s diagrams animated or interactive. <a href="https://ecliptiqc.ca/Almageste.php" rel="nofollow noreferrer">https://ecliptiqc.ca/Almageste.php</a></p>

CC BY-SA 4.0
2023-06-15T23:14:11.807Z
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.)
Mars
CC BY-SA 4.0
2023-06-16T04:53:14.043Z
Super! I’m seriously thinking about adding more animations, so keep an eye out!
Pierre Paquette
CC BY-SA 4.0
2023-06-15T03:54:08.533Z
2023-06-15T03:54:08.533Z
Pierre Paquette
<p>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.)</p>

<p>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.</p>

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

<p>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 <a href="https://history.stackexchange.com/questions/15329/is-there-a-general-term-for-epicycles-deferents-and-eccentrics-in-ptolemaic-as/15333#15333">answer</a> 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.</p>

CC BY-SA 3.0
2014-12-22T17:34:26.770Z
No, path curve is called [*"epicycloid"*](https://en.wikipedia.org/wiki/Epicycloid). General term is likely to be [*"a bad science"*](http://en.wikipedia.org/wiki/Deferent_and_epicycle#Slang_for_bad_science) along with phlogiston.
Free Consulting
CC BY-SA 3.0
2015-05-21T18:49:52.490Z
@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.
LocalFluff
CC BY-SA 3.0
2015-05-21T19:08:06.563Z
@LocalFluff, not just unfair.  Uninformed.
Mars
CC BY-SA 3.0
2015-05-21T19:28:20.960Z
@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.
LocalFluff
CC BY-SA 3.0
2015-05-21T19:31:31.170Z
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*](http://www.amazon.com/Worldviews-Introduction-History-Philosophy-Science/dp/1405195630).
Mars
CC BY-SA 3.0
2015-08-19T20:13:49.183Z
"Trajectory" is a generic term that should work well in this case.
Florin Andrei
<orbit><solar-system><history>
Is there a general term for epicycles, deferents, and eccentrics in Ptolemaic astronomy?
Mars
# Is there a general term for epicycles, deferents, and eccentrics in Ptolemaic astronomy?

(Asked by: username_0 on 2014-09-23)

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](https://history.stackexchange.com/questions/15329/is-there-a-general-term-for-epicycles-deferents-and-eccentrics-in-ptolemaic-as/15333#15333) 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.

**Tags:** orbit, solar-system, history
> **Comments:**
> **username_1**: No, path curve is called [*"epicycloid"*](https://en.wikipedia.org/wiki/Epicycloid). General term is likely to be [*"a bad science"*](http://en.wikipedia.org/wiki/Deferent_and_epicycle#Slang_for_bad_science) along with phlogiston.  
> **username_2**: @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.  
> **username_0**: @LocalFluff, not just unfair.  Uninformed.  
> **username_2**: @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.  
> **username_0**: 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*](http://www.amazon.com/Worldviews-Introduction-History-Philosophy-Science/dp/1405195630).  
> **username_3**: "Trajectory" is a generic term that should work well in this case.  

---

## 1 answers

### Answer by username_4 on 2023-06-15. Score: 1

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>

> **Comments:**
> **username_0**: 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.)  
> **username_4**: Super! I’m seriously thinking about adding more animations, so keep an eye out!  

---

