APHELION, or Aphelium, in Astronomy, that Point of the Earth’s or a Planet’s Orbit, in which it is the farthest distant from the Sun that it can be. See ORBIT. Thus a Planet being in A, (Tab. Astronomy, fig. 1) its utmost Distance from the Sun S is said to be in its Aphelion.
See PLANET, SUN, etc. In the System or Supposition of the Sun’s moving round the Earth; the same Point is called the Apogee.
See APOGEE. The Aphelion stands opposed to the Perihelion. See PERIHELION.
The Aphelions of all the primary Planets are at rest; excepting that those Planets nearest the Sun, viz. Mercury, Venus, the Earth, and Mars, being acted upon by Jupiter and Saturn, their Aphelions move a small matter in Consequents with respect to the fixed Stars, and this in the sesquiplicate Ratio of the Distance of those Planets from the Sun.
Hence, if the Aphelion of Mars move 35 Minutes in Consequents, in respect of the fixed Stars in 100 Years; the Aphelions of the Earth, Venus, and Mercury, will move in 100 Years, 18 Min. 36 Sec. 11 Min. 27 Sec. and 4 Min. 29 Sec.
The Method of finding the place of the Aphelion, is by observing several of the great Digressions of the Planet from the Sun; till by two or three repeated Observations it be found to remain at a stand—in the Philosophical Transactions, No. 128. we have a Geometrical Method of finding the Aphelia of the Planets, by Dr. Halley.
Kepler places the Aphelium of Saturn for the Year 1700, in 28°. 3’. 44”. of Sagittarius: De la Hire, in 29°. 14’. 41". —The Aphelium of Jupiter in 8°.10'.44 of Libra: De la Hire in 10°. 17’. 14”.—The Aphelium of Mars in 0° 51’. 29” of Virgo; De la Hire in 0°. 35’. 25".—The Aphelium of the Earth in 8°. 25'. 30”. of Cancer— The Aphelium of Venus in 3°. 24’. 27”. of Aquarius; De la Hire in 6°. 56'. 10”.—And the Aphelium of Mercury in 15°. 44’. 29”. of Sagittarius; De la Hire in 13°. 3'. 14".
The annual Motion, according to Kepler, of the Aphelion of Saturn is 1'. 11". of Jupiter, 27". of Mars, 1’. 7". of the Earth—of Venus, 1'.26", and of Mercury 1'. 39”.
According to De la Hire, that of Saturn is 1'. 22". of Jupiter 1'. 34". of Mars 1’. 7". of the Earth—of Venus 1'. 26", and of Mercury 1'. 39".
The Word comes from the Greek ἀπό, from, and ἥλιος, Sun.
