An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

308 SYSTESX OF TIlE WOIltX,.
itnudoxus. According to this system, the heavenly bodies are
set like gems iln hlollow solid orbs, composed of crystal so pellucid that no anterior olb) obstructs in the least the view of any
of tile orbs that lie behind it. The sunl and the planets hlave
each its separate orb; but the fixed stars are all set in the same
g[randl orb; and beyond thlis is another still, the Ptitmum.h1.bile, which revolves daily from east to west, and carries along
wit.h it a1 the other orbs. A)bove the whole spreads tlhe
Gtrand J.l13.pyreat,, or third heavens, the abode of:perpetual
serenity.*X
To account for the planetary motions, it was supposed that
each of the planetary orbs, as well as that of the suln, has a
motion of its own eastward, while it partakes of the common
diurnal motion of the starry sphere. Aristotle taugllt that these
motions are cffected by a tutalary genils of ceach planet, rcsiding in it, and directing its motions, as the mind of man directs
his 1m1otions.
442. On coming downe to the time of ilipparhulllls, who
flourislhed about 150 years before tle Ollristian era, we meet
witl astronomers1 who acquired far more accurate knowlvedgo
of the celestial motions. Ifipparelus w\as ill possession of instrumiolts for measuring angles, and knew hlow to resolve
spherical triangles. IXe ascertained thle length of tile year
within 6m. of the truthl. ille discovered tile eccentricity of the
solar orbit (altlhougll e supposed tlle sun actually to move
llifOlrmly in a circle, but tihe earth to be p)laced out of thle
celter), and tle positions of the sun's apogee and )perige. 1:o
formed verty accurate estimates of tire obliquity of thle ecliptic
and of thie precession of the equinoxes. lie computed the exact
period of the synodic revolution of the moon, and the inclillation of thle lunar orbit; discovered the motion of her node and
of lher line of apsides; anld mlRad the first attemlpts to ascertainl
the horizontal parallaxes of tlhe sunll and lnoon.
Such was the state of astronolical knowledge whenl Ptoelemy
wrote the Alllmagest, in which lhe ]as transmitted to us ani oen
cyclopxtdia of the astronomy of tlhe ancients.
o IXong's Ast., ti., p. 010; R obinson's Mech. Phl. ii., p. 83; Gregory's Ast.
p. 132; Playftir'r Dissertationst, p. 118.