epicycle
noun
epi·cy·cle ˈe-pə-ˌsī-kəl 
1
in Ptolemaic astronomy : a circle in which a planet moves and which has a center that is itself carried around at the same time on the circumference of a larger circle
2
: a process going on within a larger one
epicyclic
ˌe-pə-ˈsī-klik
adjective -ˈsi-klik
Example Sentences
Recent Examples on the Web In order to reconcile circular orbits with actual planetary behavior, Copernicus continued the tradition, developed by Ptolemy, of arguing that planets spin on wheels, known as epicycles. 
National Geographic, 9 Apr. 2019
National Geographic, 9 Apr. 2019 Word History
Etymology
Middle English epicicle, from Late Latin epicyclus, from Greek epikyklos, from epi- + kyklos circle — more at wheel
First Known Use
14th century, in the meaning defined at sense 1
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