An Etymological Dictionary of Astronomy and Astrophysics

1. In → Ptolemaic system, a circular → orbit of a body around a point that itself orbits circularly another point. Such a system was formulated to explain some → planetary orbits in terms of → circular motions in a → geocentric cosmology.
   

2a) Math.: A circle that rolls, externally or internally on another circle, generating an → epicycloid or → hypocycloid.

2b) In → galactic dynamics models describing the → spiral arms, a → perturbation of simple circular orbits. → epicyclic theory.

Etymology (EN): → epi-; → cycle.

Etymology (PE): 1) Falak-e tadvir, from Ar. falak al-tadwir, from falak “sphere” + tadwir “causing to turn in a circle.”

2. → epi-; → cycle.