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

MOT'IONS OF T'I I, rAN]',TARIXY SYSTIENM, 21
return to his station at the sutl, he loses all perception of this
elliptical fornm, and of his eccentric position. — But lhe perceives
thie coaseqeanees of these fatts, —..nl alternato increase and ldcrease, of size and of velocity in tlhe planets. 1'le eartll slowly
becomels Ilarger, and moves more, swiftly, till it reachels a certaim position, and then di minishes both ill size and velocity,
andl attainls a mlinimum at a point:80~ friom the maximulll.
AMercury passes tlhrough still gteater changes of the samle kind.
35i6. A clear understanding of the motions of MAtercury, alnd
of tte relations of its olrbit to the pllane of the ecliptic, will render it easy to understand the samne particulars ill regard to each
of tlhe otller pllanets. Standing on the stunll, we should see eacltl
of tle planets pursuing a similar course to that of Mtrcury, all:moving; fi'om west to cast, difleringl from each other chiefly in
t\wo respects, namiely, in tlcir velocities, alnd in thle distances
to whlich they recede fi'om the ecliptic, or their inclinations.
Wce hlave supposed the observer to select the planle of the carthl's
orbit as his standard of reference, and to see how eachl of the
other orbits is related to it; but suchll a selection of the ecliptic
is entirely arbitrary; the spectator oni the sun, who views thto
mnotions of tlhe planets as they3' actually exist in nlature, would
make nlo distinction between the dilferent orbits, but melrely
inquire how they are mutually related to each other. T.'akilg,
llowever, tile ecliptic as thie plate to wlich all thle othersl are
referred, we do nollt, as inlthe case of tie other planets, inquire
Jhow its pllanle is iaeicined, nor wthat are its wodles, since it hlas
neither inclination nor node.
357. Suchl, in gcneral, are the real motions of the planets,
and such thle alpearances whichl tlle planetary system would
cxhlibit to a spectator at the center of nAtion. But, ill order
to relpresent correctly the positions of the planetart orbits, at,
alty given time, t1/o'ee tillngs lust 1)e regardled tflie.zclination 7
of the orllbit to thl ecliptic; the position of the line qf tAe.Nbes, andll tile position of tile lie of tAe 1Ap.sides. In our
common dliag7amlis, the orbits are incorrectly rep)resented, b)eilng all il tile same plane, as inll the following diagram, wherel
AEI3S (Fig. 68) represents tile orbit of iNercutlry (as lyingo in tho
sante plane with the ecliptic.'1ro exhiblit its position justly,