noun
epicycle
Technical term in astronomy and history of science, referring to the small circle whose center moves along the circumference of a larger circle in the Ptolemaic system.
プトレマイオスは惑星の逆行を説明するために周転円を導入した。
Ptolemy introduced epicycles to explain the retrograde motion of planets.
周転円 is a concept within the geocentric model (天動説).
Compound of 周 (circumference), 転 (revolve), and 円 (circle). A calque from Western astronomical terminology, likely coined in the Meiji period.