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

[OTIoNS OF'ile 1ILANPAt'i RY SYSTE'I. 919
quil're into tile nature and causcs of the (q)iataret I otiots. T1l
apparent motions of the planets are exceedingly unlike the real
motionIs a fatct whlich is owing to two causes: first, we view
tltheml out of thle cltter of their orbits; isecondly, wve are otu'selves'in no0tio.. From the first caluse, the apparent places of
thle placts are greatly cllanged by perspective; and, f'rom the
second cause, we attribute to the planets changes of pllace
wlhichlt 11rise friom1 olf1 own motionls, of vwhllich we are unllconsciouls.
359.'l'tle situation of a leavenly body, as seen from the
center of tile sunll, is called its /elioeelrieh place; as seen filom
thle center of the earth, its geocentric place. The geocentric
llmotions of the planets nullst, accordingcl to what las just b)een
sail, be far morel i'l'Cregular anl complicated than the heliocentric, as will be evident from thle followin~lg diagramn, Nwhlich
r1epreselnts tile geocentlrie rmotiolns of ielrcullry for two entilre
revolutions, embracing a period of nearly six months. L:'et; S
(Fig. 69) represent thle sun, 1, 2, 3, &c., the orbit of Marcury,
a, b, c, &c., that of the earth, and G1?: the Concave sphere of th}e
lheavenlt. IlThe orblit of Mtercury is divided into 12 equal 1arts,
eachl of lich he d1escribes in - Cdlays; anlld a portion of tlhe
carth's orbit described by that body in tie time that Mttercury
describes the two complete rcvolutions, is divided into 21t
equal parts. Let Xus now suppose that hMercury is at tlle point
I: ill his orbit, when thle carth is at the point a; TMrcury will
thtel appear in tile ]eavens at A. In I7- days lelrcurly will
thave reached 2, while tle earth s reached, tl b, when r rcurly
will appear at 13. iBy laying a ruler ol thle point c and 3, d
andtl 1, and so on, in tlhe olrder of tile alphlabet, t le successive
apparent pltlaes of Mcleuryr in thle hcavens will be obtained.
Fllroml A to 0, thle apparent motion is direct, or in the order of
tile signs; from C to G it is retrograde; at t it is stationaryr a
wh]ilc, and then direct through the whole are CT. At 1T the
planlet is again stationary, and afterward retrograde along thle
arc' TX'I. lilenee it appears tlhat the motions of an itnferiorl
plln1Ct, as vie\wed firom the earth, are xcedingllly irregular and
complicated, althoulghl it is all tilem while pursuing its course. at
a nearly uniform rate, and in the same unlvarying direction
atround tlte s1un. It moves forward when near the superior