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

TH'E O MOVl0TgAHATO SrYEt, 309
443. The systems off the world which hiave been most coleblrate(l are three l. — -thle Ptolemaic, the chllonic, and the Coperlican. WAV shall conclude this part of our work w\ith a, concise statement and discutssion of each of thlese systems of them
A:Mchanllism of the HIeaven1s.
TIFF XPTOL.EM.AIO SYSTEM.
444. T..i, doctrines of the lPtolemnaic System were not originated by Ptolemy; but being digested by him out of imaterials
fulrnished by various halds, it lhas come down to us unller the
sanction of hIis namle.
According to thlis system, the earth is thie center of the mtiverse, and a11 the helavenly bodies daily revolve around it fiom
east to west. nl order to explatin the planetary motions, P]tolemy liad recourse to dferelnts anld epicycles,.-.........an explalnation
devised by A pollonlius, one of the greatest geometers of antiquity7.x'lie conceived th.at, ill the cirtcumference of a circle,
having thle earllt for its center, there moves thle cente of another circle, in tlhe cilcumference of whltich thle planet actually
revolves. The circle surroun ding the earth Nwas called the
dft'reint, while the smaller circle, whoso center was always in
the lperipl'ilery of the deferent., was called tile qe)iyele. Thte motion in eachi wVas supposed to be uniform. Iastly, it was conceived that tlhe motion of the center -of the epicycle in the circulnifellcce of the deferent, and of the defcrcent itself, are in
oplposite dtirections, thl flst being toward the east, and the
second toward the west.
445. 13But these views will be better understood friom a diaIS'gram~. tcreflort, let Af30 (Fig. 81) represent the d(tfSc-i'ent, Ebcitlgo the earthll a little out of the center. Let tab ret'leslnt
the ejicq/cle, having its center at v, oil tile periphery of the defcrnt. C onceive the circumiference of tlhe ldeftrent to be carried
about the earth every twlenty-four hours in thie order of the
letters; and at the same timlle, let the center u of the opicyclo
bed, hlave a, slow motion ill the opposite direction, and let a
body revolve in this circle iin the direction abed. Then it will
o Playfair, 1)isscrtatiol Second, P. 119.