APHELION

, or Aphelium, in Astronomy, that | point in the orbit of the earth, or a planet, in which it is at the greatest distance from the sun. Which is the point A (in the fig. to the art. Anomaly) or extremity of the transverse axis, of the elliptic orbit, farthest from the focus S, where the sun is placed; and diametrically opposite to the perihelion B, or nearer extremity of the same axis. In the Ptolemaic system, or in the supposition that the sun moves about the earth, the aphelion becomes the apogee.

The times of the aphelia of the primary planets, may be known by their apparent diameter appearing the smallest, and also by their moving slowest in a given time. Calculations and methods of finding them have been given by many astronomers, as Ricciolli, Almag. Nov. lib. 7, sect. 2 and 3; Wolfius, Elem. Astron. § 659; Dr. Halley, Philos. Trans. No. 128; Sir I. Newton, Princip. lib. 3, prop. 14; Dr. Gregory, Astron. lib. 3, prop. 14; Keil, Astron. Lect.; De la Lande, Memoires de l'Acad. 1755, 1757, 1766, and in his Astron. liv. 22; also in the writings of MM. Euler, D'Alembert, Clairaut, &c, upon attraction.

The aphelia of the planets are not fixed; for their mutual actions upon one another keep those points of their orbits in a continual motion, which is greater or less in the different planets. This motion is made in consequentia, or according to the order of the signs; and Sir I. Newton shews that it is in the sesquiplicate ratio of the distance of the planet from the sun, that is, as the square root of the cube of the distance.

The quantities of this motion, as well as the place of the aphelion for a given time, are variously given by different authors. Kepler states them, for the year 1700 as in the following table.

Of the new planet, Herschel, or Georgium Sidus, the aphelion for 1790 was 11<*>23°29′42″, and its annual motion 50″ 3/8. See Connoissance des Temps, 1786 and 1787.