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

20o8 TlE i'P,LAN ETS.
maxinnmu breadth to the minimum, and the reverse. These
chlanges are visible in a telescope of moderate powers, except
tlhat til ringg is too thlin to be seen at A. and E.
3145.:l her ae tlare wre ays in which thle ring may fail to be
visible during tlte p1)riod Ahen the line of its nodes is crlossing
the earth's orbit. lst.. It may present its edge exactly to the
ealrth, rlhe-n in common telescopes, it subtends too small lan
angle to be seen. 2(1. It may present its edge exactly to thie
sta1n, so t hat neitlher side of the ring is Clnllghltned. 3d. Its
plane may be directed bet'ween. the earth and sul, when thle
dark side is toward us.'The two first causes mlay be considcred as only momentary; for tile plane of thie ring p)asscs; th
breadth of tile sun\l in less thllan two days, Sand of the cart'h ill
about 20 minutes. RBlut tlhe third cause may continue to render the ring invisible for severac l months.
I1n Fig. 67 let S be the sutln, 0 the place of Saturn, when the
Fig. 67.
Y'I!o(.................~................../..
i~: ".-... xI.x.~. -~~~ —-,........... -......................................................'........
nodes of its ring are in the linle CS, passingr thirough the sun.
Lct El1:0-1I[ bte the earth's orbit; then P'IB, O1), par a llel to SO,
will incellde 1:1), the are of Satulrtn's orbit in whlich the ring
will presett its edge to the earlth's olbit. If tle orbitas are
supptosed to )be in the plane of the paper, wf e must conceive, the
smniall ellipses at B, 0, 1), reprcsenting' the ring, to be inclilned
ab)ou1t 28~, fhaving their sction withl the orbit, inl,31 CS, and
1)0, resl)ectively. While Saturn passes over the are 1!), thl,
plane1 of its ring Nwill paS OnCe t'rough the sun, and may repeaitedly pass through thle earth, on account of thte revolution
of tile latter. Th'le time of describing:DB:l) diflrs only about
six days friom a ycar; for, sillco Saturn is 9.051. timlcs as far