3

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.

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

|improve this question
  • 1
    You would likely get a better answer for this on the English Language and Usage SE. I'm not sure why "Orbit" doesn't seem correct though, it's a much older word than heliocentric astronomy. It is basically "move in a circle" in Latin. – Comintern Sep 23 '14 at 4:19
  • Thanks @Comintern. That seems like a well-justified answer. I'd be willing to accept it, but maybe I'll try the English language SE as well. (I wanted to ask on the History of Science and Mathematics SE, but it's been languishing in Area 51.) – Mars Sep 23 '14 at 4:26
  • 2
    The technical term for this family of mathematical curves is "epitrochoid". Epitrochoids are just one kind of a much more general family known as roulettes. See the roulette wiki for a brief overview of associated terminology for describing their basic properties. – David H Sep 23 '14 at 4:42
  • Thanks @DavidH. I'm interested in a term for the circles in these roles, not the curve traced by the center of the outer circle. – Mars Sep 23 '14 at 5:37
  • Thank you for mentioning the History of Science SE. I had no idea it existed. – Joe Sep 24 '14 at 5:09
3

The general term in Ptolemaic astronomy is kyklos (cycle), which encompasses the epikyklos, the ekkentros kyklos etc.

|improve this answer

Not the answer you're looking for? Browse other questions tagged or ask your own question.