The Collaborative International Dictionary of English v.0.48:

Aphelion \A*phel"ion\ (?; 277), n.; pl. Aphelia. [Gr. ? + ?
   sun.] (Astron.)
   That point of a planet's or comet's orbit which is most
   distant from the sun, the opposite point being the
   perihelion.
   [1913 Webster]