CYCLE, in astronomy, a period of time at the end of which some aspect or relation of the heavenly bodies recurs. (Gr. riJicAos, a circle.) (See CALENDAR and ECLIPSE.) In physics, the term is applied to a series of operations which, performed upon a system, brings it back to its original state; "Carnot's Cycle" is an example (see THERMODYNAMICS). The word is used loosely of any long period of time. From the phrase o Eirucos Ki Aos, the epic cycle (see below), comes the application of the term "cycle" to a series of prose or poetical romances which have for a centre one subject, whether a person, as in the Alexander, Arthurian or Charlemagne cycles, or an object, such as the ring of the Nibelungenlied. In music "Song-cycle" (Ger. Liederkreis) is similarly used of a series of songs written round one subject or set to poems by the same author. Beethoven's An die ferne Geliebte (Op. 98), published in 1816, is the earliest instance.
For the epic cycle, or cyclic poems, of Greece see HOMERIC