aphelia

Aphelia is the plural form of aphelion, the term used in astronomy to denote the point in an object's orbit around a focus at which the distance to the focus is greatest, most commonly the Sun in a heliocentric orbit. The concept applies to planets, comets, and artificial satellites, and is a standard element of orbital mechanics.

In an elliptical orbit with semi-major axis a and eccentricity e, the aphelion distance is r_a = a(1+e) and the perihelion distance is r_p = a(1−e). The line joining perihelion and aphelion is known as the line of apsides. In real solar-system dynamics, perturbations can cause slow changes in the orientation of this line over time.

Usage and examples: aphelia describes the moments when an object is farthest from the Sun, and it is used in orbital calculations, mission planning, and historical data analysis. For example, Earth reaches aphelion around July 3–6 each year, at about 1.0167 astronomical units (AU) from the Sun. Halley’s Comet has an aphelion near 35 AU, reflecting its highly elongated orbit.

Notes: while aphelia is a recognized plural, some texts also use aphelions as a plural form. The related term perihelion denotes the closest approach to the Sun. See also aphelion, perihelion, and orbital eccentricity.