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

)IET'EIIMINATIO(N OF T) 1IIF ILANEI*Y I.\t OrBlt, 223
axes of their orbits. \V'e llay now learn more part icularly1
the process by w\\lich the illustrious Ktepler was collducted to
tfle (isicovery of these grand laws of thlc planetary systeml.
Ftrom tile appl"'rent motions of the h]eavenly bodies, as seenl
pr'ojected on' tlhe face of the slk(y, t]he ancient astronolmers i1nfierred tlat theiir orbits were ntecessarily circular, a(nd tile mol
tions actllall: uniform. Still, Itipparclus and IPtolemy were
not ignorant of the fact that the sun moves faster tlhrough(l the
winter thita tllroulgh the sumncler signs, perfo)rmiillg the half
of his revolution around the earth nearly eigilt (lays sooner
firom thte autumnal to the vernal, thllan from the vernal to the
aut.umllal equinox. This led thenm to infer thlat thl earth is
not ill thle centelr of' the circle, butt nearer to one side of the
circle thlan to the otilher, by wlich mealns thle sunt woul\ d alppear
to move mlore rapidly in tlhat part of its orbit tllan ill the op)posite 1)art, just as a steamboat appears, to a spectator on thte
shore, to mlove faster when nearer than l when more remote
fi'otm1 thtc slhore, althougll ]her actual speed is tie sanic in both
eases. On a sililar supposition Tycho l'ralle made a grecat
number of very accuraate obserc!atiotas onl the planetary motiDls,
wllichl served Kepler as standards of comparisonl fo)r results,
tiicllh lie leducedt fi'omt c(alclIlatitots, foundedt oln thle applicatiol of geometrical reasoning to various 1lypotlheses nwhich li
successively assumlled as to tlheo figlure of thle planetary orbits;
firzst suplosingl tlhe orbit to be of at certain figure, then deterin-'min2g fi'onl tle geometrical properties of the curivt e -what motions the body would appear to us to have whlen moving in
such a. path, and finallly testing his conclusions by comparing
t.1hen with the fiacts, as determined by T'yelyho, from obscrva,tion.
363.:Kepler fir st applied himself to investigate tilhe figure
of thle orlbit of Mlars, thle motions of whvich p)lanot appeared
more irregular than thllose of any other planlet except M1[ercury,
whlich, being seldom seen, had beell very little studied. ]Like
Ptolemly and l ycho, hc lie fst supl)losed t:he oirbit to be circular,
and the plalnet to move uniformly about a point, at a certain
distance fronl thte suil. le made scvvetty suppositiolls before
he obtained one tlat agcreed wit.h obslervttion, tile calculation
of \llicll was sextremely long and te(ious, occupying him lmore