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

230 TrE rL~ANS.
observations suflicient.ly Ilmnerous..erefore, t e can not so
readily ascertain by) siplle observationl, tlhe motions of the
ilnferior planets seen from the sun, as we cfan those of thte su37 4. Ifenco, to obtain accurately tfhe periodic time of a sutp)erior planet, we find the interval elapsed letween two opp)1)si.
tions seplarated by a long interval, when the plainet was nearly
in the saml e psart of tlhe zodiac. From the periodic tinec, as
determined apl)roximniately by other amethods, it may be found
\whlen the planet las the same heliocentlric longitude as at thel
first observation. Thus the time of a complete numbeIr of rcvolutions will be knownl, and thelnce the tinec of one revolution.'iThe greatel the interval of time b)etween the two oppositions,
tho more acculately the periodic time \will be obtained, bccause t;lle rrors of observation vill be divided 1)ctween a g.reat
nluttl)er of periods; thlrelfore i)v mlsinlg very accurate observalions, mucth precision may 1)e attained.,'or cxample, the
planlet Saturn was observed in tlhe year,2238. e., Mafrclh 2,
(according to our reckoning of time), to 1)c near a certain star
called y Virginis, and it vwas at the same timce nearly in opposition to the sun. Tlie samle planclt was again observed in opposition to thle sun, and having nearly the same longitude, il
Feb., 17114..The oexact difercce betwccn these dates was
1943y.:118d 21m. tit5.'it is knowll friom other sources, that
the tinme of a revolution is 29t.- years nearly, and hence it Nwas
found that in thle albove period there were 66 revolutions of
Saturnl and dividing tho intervalt by this number, wte obtain
9U.44-1: years, tvwhich is,nearly tlhe periodic time of Saturn accordling to thle most accurato d(eterminatioln.
375.'Thirdl y, to dotermine the disltance frtol Ie sun, a.nd
mq'ofr Caxes qf the planet ay orbits.
The ldistance of the carth fromn tlhe sun bein known thoe?neattl distance of any planet (its periodic time being known)
Ayl b1) folund by IKepler's law, tthat the sqtiarls of thle periodic
times are as the cubes of tho distances. The mlethod of Itfindilng thle distance of an iqfeirior0 planlt from thL sun bly obsYrvao lth'ikley, 1. 167.