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

o101sf\ toT'oNs or cotO'rs. 231
ascensions ad declinations are accurately known) afiford the
mean-s of calculatingY these elements.
he appances of the same comet at diffebrent peroiods of its
etun a so various (Art. 392), that we can neveir )ronounce
a given coet to be tsame with one tlhat has appeared before,
frm any pecuiaities in its ysical aspect.'The identity of
a coet it one alady on record, is determined by the
d iy of the1 elelnts. When a new comet appears, we fir;st
deterine its lements and then turning to a. Catalogue of
comets wose elements lhave previously been1 found and placed
on record, we se whether thenews elmlements agree with allny
t of those in the catalogue. If they (do, we infer that the
present coet is ientilwth thalt ol record; alnd thel interval
ween the two appearnces of the body will indicate its peIf, for example, wr fi lnd respecting a co. et now
visible in sky, that its pathl makes the same angle with tlhe
ecliptic as that of a ceain comet in our catalogue, that it
cosses teclipti in the same dlegree of longitude, that it
comes to Its peilion ln the same place, tlhat its p)erilelion
disanc i th saeand its course, the same in regard to the
order, of the ssigus thien wve infer that theo twvo bodies are one
andl the same; and liv the numer of years that have elapsed
S'ihee its former, appearance, indlicates the period of its revohittion around theo sun.'But if these lpartieulars differ wholly
from any s.,et of rccordled elements, wev infer that thie present, is;
a. comet whbichi hias never visited our sphiere before, ors at least
one, whose elements hiave not been determined and recor(Ied.
lIt was by this me-ans that H1-alley first established thec idlentity
of the Conmet whbich bears his name,10 withi one that had appeared
at several preceding a ges of the, world, of whichl so many particular-s were left on record, as to enable him to calculate the
elements at eachi periodl. Th'lese were as in thec followving table.
Timeo of P.P Inclintdion of L W'1-11f53 of LongR1tuio of i'oLrlhelion1
Pe Mille. the orbit. the NodQ Pecr~ihdlot). JDistanco. Curo
1460 I 1iO 60 4 80 30O' 3010 00 0.68 lRetrog)inde.
16-3 1 17 60 49 25.301 39 0.67 IRetrograde.
16007 1 7 02 60 2 1 302 106 0.68 lReti'ogimlde.
10682 17 42 6 0 4 8 301 306 0.58 Rectrogr'ade.