The aphelion of a circumsolar planet or periodic comet is the point of its orbit farthest from the Sun (in Figure A25 it is point A, ADPF being the orbit and S the Sun). It is in contrast with the perihelion (q.v.) or point closest to the Sun (P in Figure A25). These two terms were first introduced by Johannes Kepler in his Mysterium Cosmographicum of 1596. He formed them from the Greek roots αφ = απo, from, and περι, near + ηλιos, Sun, thus constructing them in analogy with the terms apogee and perigee used by Ptolemy to denote the points on an epicycle or eccentric circle farthest from and closest to the Earth's center. When Copernicus set the Earth in motion about the Sun, he employed the Ptolemaic terms, contrary to their original meanings, to denote the points on a planetary orbit respectively farthest from and nearest to the mean Sun, or center of the Earth's circular orbit.
Aphelion and perihelion in an elliptical orbit. ADPF = orbit, S = Sun at focus, C = center, A =...
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Wilson, C. (1997). Aphelion . In: Encyclopedia of Planetary Science. Encyclopedia of Earth Science. Springer, Dordrecht. https://doi.org/10.1007/1-4020-4520-4_14
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