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

242' TII PLANE'1S.
and will continue to undergo them in all future ages, canl never
vary beyond a certain moderate limlit, entirely "within the
bolunds of safety to the stability of thle system.'lTlhe eccentricity of tlle earth's olrbit, for examp)le, has been diminishing ever
siltce tlle creation of manl; and although, as we hYave seen, tlhe
rato of diminution is exceedingly slow, yet, in the 1)rog1ress of
cnturlices, it would totally ehlange the character of thle earth's
orbit; first reducing it to the circular form, and finally carrying its eccentricity to a fatal extreCme. In like lanlner, thl
Inclination of the earth's orbit to the equator is constantly diminiishing, and is nowt about two-fiftlls of a degree less than it
was in the days of A. ristotle; and, were this to proceed in the
same direction, the equator and ecliptic would coilncide, the
chlancge of seasons would cease, and the whole economy of nature would be subverted. -But Laplace las ldemonstrated, that
sluclh an vcent can never occur, nor can thO entire extent of
this valriation exceed three degrelcs. It is worthy of rc mark,
that thlose p)erturbatmions, such18 as cl;anges i the place of the
perihelion, affecting a change of direction in space of tlhe major
axis of tlhe orbit, or in te place of tile nodes, twhichl, Iby aceu1mulzatinlg, do not endanger the stability of thle system, proceed
oitnard through tle enltire circuit of the heavens; llile tl)rt.rbationls wvlicll, by indefinite accumulation, would bring ruini
to the system, such as variations of ccccntricity and of inclinationl, are not uprogrcssi\ve, but oscillatory, wavincg to and firo
witlhin the limits of entire safety.
38G6. These great ends would not have been secured, had
the systecn been constructed differently friom what it is. NImlerous conllitions must concur in order to p)rodulce these results: t]he mass of tlh stln nusit hlave greatly exceeded tlhat of
any o' all the planets; the eccentricities of the orbits iust have
been small; and the p)lan1ets must all have revolved around the
suti in the samen- direction, and in planes but little inclined to each
other.'":. it was also necessary that the periodic titmes of the
)ltanlts slhould, in gencral, be incommensurable; for wvere
tlheir leriods such that onlc planet would revolve a certaill
* Lapltacc, ysdt l Moc. 1Icrscihl's outlines. Grant's History of Physical
Astronomly. Pontecoulautt's Trait. Etl6mcn. (1 Phys. C6testo.