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

260 c5111"'r
that an error of no more than 5" in observatin, ld alter
thle period either to 2,678 yeas, or to 1,9 years. o as,
t.rollnmers, ill calculating tle orbit of the reat coet of 16,
have found the length of its reater xis 426 times the earth's
distance fromn the sun, and consequently Its riod 8, year;
whlile others estimate thlte greater axis 430 times the earth's distance, which alters the period to 8,916 years. Newton and
1talley, however, judged tlhat this comt accomplished it rvolution in only 570 years.
399. IDisheartened by tle iculty of attaining to iy -
elsionll in the elircumlstances by which l elliptic orbit is characterized, alnd, moreover, takn into account the laborious calcultatiols necessary for its ilnvest- ion stronomers uslly
satistf themselves with ascertaining the elements of a comet
oni the supposition of its descrilbing a araola; and, as this is
a curve wh}o0se axis is infinite, the lrocedure is greatly simli
Iied l)y leaving entirely out of consideration the periodic revolition. It is trute that a parabola may not reresent, ith,mathematicall l stiictness. the course which a coet actually fo
lowvs; but as a lpanabola, is the intermediate, curve betwNeen thle
hyperbola and ellipse, it is found that this method, which is so
much more1- eon xvenient for Complutat~ion, also acecords suffilciently with observiations, except in cases when the elliipse is a comnp)nratively short one, ais, that of Enekec's comret., for example.
When the elements of a comet. are determined, Kelrslwo
areas enables astronomers,, to find, by computatwon, the exact
place of the, comet in its orbit, at anly given time, Onl thle supposition that its 1),ath is at paralbola; and comnparing thias lplae
with that determined by obser'a-twn, for the same ins tant., it is
seenI whethler thle orbit is truly p~arabolic, or whether its (leviationt from that lpath is suchll as to indica-te that its real path is an
ellipse; and the amount. of such (leviation wvill give some1 ideat
of the, d(y~ree of eccentricity of thle ellipse.
400. The elemen,1ts of a comret, wvith thie, exception of its periodlic ltime,, are calculated in at manner shimiar to thiose-, of the
planets. Three goodl oliservatinS, Onl the,, righdt ascension and
(lee~ilnaton of thle comret (wvhich are, usually found by ascertaining its Position with res~pect to certalin stairs, whose riio'ht