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

258 coWNIms.
agres, it sall undergo new perturbat again in
some sm'aller orblit as before.~
ORBITS AND MOTIONS OF COM.
397. The pAleancts as wseen (it the exception of
tlhe asteroids, whlichl seem to be an intermediate lass of bodies
between planets and comets), move in orbits which are nearly
circular, and all very near to the plane of the eliti, and all
move in the same direction from est to east. ut the orbits
of comets are far more eccentric than those of the planets
they are inclined to the ecliptic at varios angles be soetimes even nearly perlp)en(licular to it and the motions of
cormets are sometimes retrograde.
t398. TThe elements of a comet's orbit as usually obtained,
are live: (1), longittude of the to t
ecliptic; (3), per'iheliont distance; (), onit of'el
(5), time qf/peri/leion ]assa/e.
1i ceoml)aring these with thl lements of a pantary orit
(Art.. 36i),'we p~erceiv'e two to be omitted- -the l&wie time,
and the eccentricity; while perTihelion distance is substitutedl
for mean distance. Thle reason for this is, that in) these orblits,
no reliance(, c,-anr be lplaced on the (letermnination of the Size and.
form of orbit, or the time of describing it, from observations
which are limited to a, shiort arc near the perihielion. Theo
comlplete problem is not only extremely diflicult, as. Newton
lproiioviicedl it, but rather wholly impracticable. It is during
only a very,small portion of their course that they are visible
from the, earvth, and the observations made in thvat -short period
can not afterward lbe verified on more convenient occasions;'whereass in thie case, of the l)laiiets, whiose orbits, are nearly cirenular, and whose imovements mnay lbe followed unninter-upted ly
Athr~oughout a Complete revolution, no ssuch impediments to the,
(let eriination of their orbits occur. There is also some, unavo ida leo uncertainty in observations mnade uipon bodlies whose'
ontlines are so ill-defined. It is, not unfrequently the ease too,
that comets mnove in a dIirection op)posite to the ordler of theli
*Millue.