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

']'Rf1TURBATIONS OF TIlE, ]'TANJ'~,S. 239
planct was once put il 1lnotionl wit01 sucll a projectile force as
to malke it describe anl ellipse, it would forever continue to
describe tihe same fig'ure without the least variation tlhe radius
vector always passing over equal spaces in equ:l tinmes;; b)ut
now ilntrod(luce, third body so near as to exert onl it a decided
attraction, antd its Imotions no longer retain tleir siptl'icity,'
but become comlnlicated( by the conflictinig influences of' thl
two attracting bodies.'T'le s1un, hlowever, in consCquell1C of
its nmass, whfich is eiglit hundred times as great as tlhat of all
tihe plalnets, anld, of course%, vastly greater than that of anly o011
of thllem, exerts a iorcc so much superior to tlhat of any or all
the otlher distturbing bodies, that thle elliptical figure of the
orbits is tatiz.rly'maintained, andl a near approximation to the
place of a planlet is obtained by neglecting l ll th}ose minor'
forces, and sitlply contemplatinlg it as revolving in an ellipti-.
cal orblit, Still it is essential, int order to find the xc(tac place
of a lanet at at ally given tie, that all these irrceglarities,
minlito as tlhcy niay. be, be carefully sumnled ulp, and their
resultant applicld to tiel elliptical lmotions. To investigcate
tlhese perturbationls, to estimate their precise amount, alld to
reg-ister t leltn ill tables,'fior thle luse of thle practical astlronlloler
h}Nav collstitted a large plart of t he labois of modern astronomy. Tlhe knowledge gailled by astronm0lltcl:rs of the planetary
motiouins, consi ciiei:ng thlc very numerolllo s irreculalitics, 1oth
periodical a.(nd secular, to wlhicll thley are stliject, is truly woncerfll.'.the motion of Jupiter, for instancec, is so perfectly
calculated, thalt astro'nomers have comnl)uted teii years beforehand 1tlt timec at whict it will lpasi ttc metrid ian of (liilercnt
places, and w\e find tlhe prediction correct witltin half a, sCccntld(
of titne.,:' T.he Imore obvious irregrularities ltave bcCnl dletfeted
by observlation; thle more minute, by,llovi11g out tile coisecquelnccs o f' universal gravitation. Even those at first revealcl
to the inst.-rumlets of tle lastronomer 1have ) ee contfirmed and
cstimated with greater accuracy by thle atmc -far-reachinlg
lp'inil)le; and;many of the irregularities have been firlst
brouC)gltt to light by thlis ttheory, whvicl tladl eforel eluded of)servationt; althougl, when once )ointetc out as a result. of time
principle of gravitation, careful inlstrumenlltaal mcasuriements
C' Aity.