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

ORBITS AN,) MrIOTIONS OF COMF,S.;s
s in the zodiac, and someties nearly l)erplendiclar to the
Ie of the ecliptic; so that thleir apparent course through
the heavels is rendered extremely coml)licatcd, onl account of
the ontrary mnotion of tihe earthl. Since it is possible ther
should e allny number of' elliptic orbits, whose perihelion distances c quatl, it is obvious that, in the case of very ccentric
orbits th slightest change in the position of thle curve near the
vertx, wheire alon the tc omet can be observed, must occasi0
a very sensible diflerence in the length of the orbit (as wil
o Ivious fromn ig. 79); and therefore, though a smlall rror
iprodul' 11 perce~ptible d iscrepancy betweenO the olbserved and
the clalculated course, while the Comet remains vi'silble from the
earth, its effect, when difiusedr. 79extent of the
or-bit, may acquire ta most material or even a fiatal importance.
On accouint, of these Circumstances, it is found execeedingty
diffiult o laydown eve in the rudest'man ner, the pa th
difficult to lay dowil, evelf I 1 (:
which at comet actually pur1sues, -when gone for.years beyond
the limit of dur11 visiionl; andl least of all to (Icterinine wVith ac1........
eraythe length of the ma~jor axis of the ellipe soas. to deCrive f'oan1 it, by Kepler's third law, the time of its, revolutionl,
and thus to lie able to lpredlic its next perihlelion passage. An
err-or of only a, few s-econds, ma.y causle at difleren'e of manly
hitudred,yeaxs. In ti manner, though Bessel determined the
revolut ion of the Comet of 1-7(9 to be 2/iOyeritws on
_..~~'/ ~.........
J,?.'p