epicycle

noun

1. A small circle whose centre is on the circumference of a larger circle; in Ptolemaic astronomy it was seen as the basis of revolution of the "seven planets", given a fixed central Earth.

   “Is it not [Philosophie], that […] teacheth miserie, famine and sicknesse to laugh? Not by reason of some imaginarie Epicicles, but by naturall and palpable reasons.”

2. Any circle whose circumference rolls around that of another circle, thus creating a hypocycloid or epicycloid.
3. A ring of atoms joining parts of an already cyclic compound
4. (figuratively)An ad hoc complication added to a model to make it fit the known data

   “If two chronicles seemed contradictory, instead of trying to choose between them, a rationalization (epicycle) was devised to cover both.”

   “Rather than solve the theoretical problem of how to produce a method of political ethics, the contractarian device introduces an unnecessary theoretical epicycle into what is otherwise a coherent account of social justice in particular and political morality in general.”