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

Fit;ll ti; (ithX01 1, iCI'LTILS OrtttElt itt 8S
a better metholid of ttcasurint' the relative distatlcs of t1ec
earthi from tile sun1. "By a principle i optic, tle iappar-ent
diameter of at- olbject, at diffrcttnt (istalicus front the spcctator,
is invet x y as the distanCCe. IIneltce, tlte liles n i a, Eb, &c.
(Fig. 32), are to be drawnl proportional to tle rceiprocals of the
apparcltt diatlteters of the sun.
16(4. The point w wtere tle cartl, or an} planet, int its revolution, is near1llest the suntll is called its jw.'ri/et ion; thle point
whecre it is frthest firomlll tlte sutl, its p/,Iton. 11he place, of
the earthl's perihello n is known, since there the 11)parcent magnlit(ude of the st1un is greatest; and whlent tihe sUn's lnt nltitude is
tiast, thle earth is knolm.' to be at.its aphtclion. Ii}t sill's atp)arent diameter wlhen greuatest is 32i' 83".G6; alld whlent least.
31.' 31"; hlence the radius vector at. thec al telion: l tad v cetor
at: the pctriheliont 32.59O333::5t..5 67:: 1.034:1. t laf of the
diffterence of' tile two is equalt to thle distatc:e of thle focus of the
ellipse firom the cenlter, a quantity whicht is always takcl a s
thi nme-asnure of thle ccernticit_,? of a plantchtary orbit. i'lrom
twentllxy-four observations mlldt witht tlhe greateit carlc by D)r.'Maskelync, at the Royalt Ob1.scervatory of Gfreenwich, thle folloNwinot distaancos of tlhe earth from tle sun are determined foir
each month l in tlhe year.
tito of Obsuervatilon. l)istanccs. T'ite of Observation. D)istances.
January 12 —13, 0.984 18 July 18-19), 1.01658
Februart y 17-.-18, 0.98950 Augtust 26.-27, 1.01042
MT.arch 14 —15, 0.9962)2 September 22. —-23, 1.00283
April 28- -29, 1.0(800 October 24.-25, 0.99303
Mfay 15-16), 1.01234 November I 8 -— 20, 0.987t46
June 17 -.18, 1.01654 )ecember 17-1.8, 0.98415
165. Illavintg determined the florm of the solar orbit, we
ar1e preparcd to see what relationl exists betwelen thec sllun's
rlytn(7dtr v.wlodit/ in this orbit, andi( the length} of tle raditus
vector. It has been already noticed (IAXrt. 105), that tile sln's
progr1eCss in telt ecliptic is stest tnear thl l)eriltclihiol, and slowest near the aphellion. PFoi instance, tile stlln at perihelion ad-t
t.-.....-...................................-............... —........- - —. —-—.........t
o Motri:c exactly, the tanentt of til apparentt diamlneter is inversely as tithe di'stance; butl in sttall angles like those concerned in the i)resent inqtuiry, the anrglo
tand its tantcnllt vttary ali.