APOGEE

, Apogæum, in Astronomy, that point in the orbit of the sun, moon, &c, which is sarthest distant from the earth. It is at the extremity of the line of the apsides; and the point opposite to it is called the perigee, where the distance from the earth is the least.

The ancient astronomers, considering the earth as the centre of the system, chiefly regarded the apogee and perigee: but the moderns, placing the sun in the centre, change these terms for the aphelion and perihelion. —The apogee of the sun, is the same thing as the aphelion of the earth; and the perigee of the sun is the same as the perihelion of the earth.

The manner of finding the apogee of the sun or moon, is shewn by Ricciolus, Almag. Nov. lib. 3, cap. 24; by Wolfius in Elem. Astr. § 618; by Cassini, De la Hire, and many others: see also Memoires de l'Academie, the Philos. Trans. vol. 5, 47, &c.

The quantity of motion in the apogee may be found by comparing two observations of it made at a great distance of time; converting the difference into minutes, and dividing them by the number of years elapsed between the two observations; the quotient gives the annual motion of the apogee. Thus, srom an observation made by Hipparchus in the year before Christ 140, by which the sun's apogee was found 5° 30′ of and another made by Ricciolus, in the year of Christ 1646, by which it was found 7° 26′ of ; the annual motion of the apogee is found to be 1′ 2″ And the annual motion of the moon's apogee is about 1<*> 10° 39′ 52″.

But the moon's apogee moves unequably. When she is in the syzygy with the sun, it moves forwards; but in the quadratures, backwards; and these progressions and regressions are not equable, but it goes forward slower when the moon is in the quadratures, or perhaps goes retrograde; and when the moon is in the syzygy, it goes forward the fastest of all.—See also Newton's Theory of the Moon for more upon this subject.