* fourth fatellite oC feturtu I n jt&j im C as si n i xlifcoverecJ o f the orbits-of them all, except the fifth in order front
the fifth"? in t .67.^1 he- *<fifeovered the third; and in the. planet^, coincide very nearly with^the «plane o f the
1684, he difcovered £he firjj: ajid fe c p n d .,i )R T J ert ripg o f the planet. D r . H aljley found that .‘the orbit
schel has difcovered a fixth and feventh fatellite, o f the fourth (at that time difcovered) was elliptical,
which lie wjthin the orbits pj£ the other five. T he planes T h e periodic timhs are as: followa :
H f 1 Second'1 ƒ ’ ‘l ’fiird v f Fquf®r ' . Fiftjv ’ | ! Sikth- l L "'Sevéntli' • *
aah. 37'. zj". | id. 8b. 53'. 9,,/. f rd. * i h . f 'jtf.i7h.+i'.**'' f »s'. . j 154.4 ah. 41'. Ï’ c X ^ i . 7'', S
157. Their diftahccs from faturn, in terms of minutes and Ipconds bf a degree, are as follows t
Firft . j Second 1... , I h iA 'S . T j r Sixth . j. Seventh I
- I- - » 8 1 4-3 " .5 1 56" I 1'. " 18"' . 1 3 ■ Ö i 1 ! '-8.'.;r4i'',y |
11 I I I
: 13.8» The pêriodic, times and diftances obferve the
fame law as 'thole.ofJupiter f fee art. 1-34;
On the Satellites ofifaëGeórgian.
" 139. In 1:787* D r. H erschel, the difcoverer of the
gtorgjan, difcovered two fatellites belonging to i t ; and
he determined the fynodic revolution of one; of them- to
be 84* 17b. :i> mk"3> and. of the jother?'3 3d. • 1 ih* 5 ^
; -aSo> thedm|ance of the former from the planet
in minutes and feconds of a degree, was found: to be
33'',09, and of, the jatter 44^23; And fince thefe difcd-
veries were made, the Doéior has difcovered four more
fatellites ; and foandrthat the motions of than all are
retrograde. Their orbits are nearly perpendicular to
the plane ofrthe ecliptic.
On the Ring of Saturn.
140. G alileo was the fiift perfon who obferved any
thing extraordinary in Saturn... That planet.appeared to
him like a large globe between two fmall ones.,,, In
1610 he announced this difcovery ; and continued his
-obfervations till i6 i2 9 when .he was furprifed to find
only the middle globe. But ^terwards he again difcovered
the globes on each fide, which,’■ hr procefs of
time, appeared to change their form. Upon this,
H uygens ffct about improving the art of grinding object
glafles ; and .made telefcopes which magnified two ror
three times more than any .which had been before made,
with which he difcovered thefing of faturn ; and having
obferved' it for fome time, hepnbfrflred the difcovery in
3656. The ring is broad and flat, at a djftancè from
the'planet* and edge-ways towards it. In r6jfo Cas-
ai ni obferved a dark line upon the-Hug, dividrng it, as
~ it. werè; into two' ririgsy the inner of which appeared
1 brighter than the outer. He alfb> obferved a dark belt
upon the planet, parallel to the .major axis, of thejing.;
for though thé ring rs circular, y'qt, been feen crbliqadyy
it appears aft ellipfe. D r. H erschel obfervesv that
the black mark on the ring* » not, in the middle of its
breadth. The ring is no lef% folid than the planet, and
it'5s, generally brighter"than the planet. H e takesmo-
tice-of the ,extreme thinnefs; o f the ring,, ks he’ Taw
. fatellifcè' on the edge, hanging over on éaoh'fidfe: ^0 '4!
141* The vnng^ip invifible *wheiiy its planex pafFe& -
“Through fchè earthj *■ che^funftof between them. Iu thé
firft cafe, the -fun fliines onl^r uöri rits ^ed’^ë,1-^kich-is ^töó
thin tp refléót flight' enough to'rhnde'rut^vifibl’e' i ^in the
fecond cafe, the edge only being expofed is-ini-
^vifible* for the fame reafon-; in the third Gafê, the dark
fide is towards us^^Dr. H er.sc h^ri^ fuipêétsHthat1 thé
ring is dividedinto two rings;*- f9r the^ollb^ltglreSfons r
i f t - The black “divifionê ön the twbflidgg, are
the fame fituatioUS.^ • adly^The* dirifioii the. ring*
and the open -fpace. between therin g ,andjthe bo4y,< appear
equally dark, and off^iföfame.Colaur as tHë*heavens
about the planet. H enc&9 h^'conclud^ has
two concehtric rings, fituated in one?blane, the dimep?
fions of which are m the following proportions:
Infidc diameter o f the fmafier ring- * ^
©Utfidfc diameter' | - •'
Infide diameter-of-the largerringr'^ - ^ 7740
Outfide diameter - - - 8300
Breadth oftbe inner ring- - ^ v ' \
Breadth o f the outer ring’ •• - - , 2^o>
Breadthofthejfpacebetweenthe rings - - , 1 15
■ From tb.e mean o fa gjeat many meafures o f theoutT
fide diameter oftlre largeT'Trng, DR. H er suttee makes
it 46",,67 7 at the mean di&nee- of fa turn ; and hence, he.
finds the diameter of the ring to -be 2,04883 miles j and
the diftance of -the two rings 28 35 miles.
... On I;climes, o f fh e Sun and Moon:.
142. An eclipfe of.the'wioo/ns caufedby its entering int«^
the earth’ s fliaadw, and confequently it tmift happen'at
the full, moon, or when {he is in oppofition to the tun, as
the fhadow■ o f the earth muft lie oppofite to the'fun.
An eclipfe of the fun is caufed by .he interpofition of
1 the
ttie modn between ttie earth Snd funs A,
muft happen when the moon is in conjuntSioh with the
fun, .or at the new maqn.
143. Jf the plane;the moon’s orbit ■ coincided with
the plane of the eeliptic, there would be an‘ eclipfe! at
ereryjooaju,n£i:.op and oppulitlpn ; but pile plane of"the
moon’ajorbil being inclined to tfre plane of the ecliptic,
there can be no-cblijiie-at- cosjanotiuii or oppofiti.n , uu-
lefs at that time the moon be at,',or near the node. f '
tions, E the-earth,'and let the plant o f the paper repre-
fe h lw S p ian e in which theearth mote» nr&fd the fun,
or the ehliplic5; aad|](t Maud r<^»refent the mood’s or»
bit, inclined _-K> ecliptfc, & d cutting i t in two
.points M , m, in tHe line SEP", then MEm is the line o f
the nodes?, lying in conjiinflion and' oppofition, the fun
heiftgl at'S"; and we niuft conceive half the orbit Mem
to lie above the paper, and the othef hiilf mdM to He
beloftf i t ; deferibe alfo t t t circle MaMb on the paper j
thd^dMe twdielrcles Mcmd, Maitfos will be inclined to
each others like two hoops put one into the other, and
inclined o'tVCTq 'fhebther* ^Now’ tf ’ vhe moon be at M
in cdiijmttion with the fun S, the three bodies are then
in the fame plane, and in the fame ftraight line, and
therefore the moon is intfirpofed between tlfd fnn and
earth, and^capfes.an eelipfe’o f ,the fun. B u t if the fun
.p$abl $r,4£d4tlfc moon in fonjunflion at M', fhe is then
out- o f the plane o f the eeliptic, the part M ' lying abate
the plane of the paper, or the ecliptic,' and therefore the
moon.is not in the lineijjcthtitigiS' and E £ and may
be fo nodte at A fv that it may be fo muah
.elevated .abovje the plane of tl\e eelipt’ie;.as not to inter,
pofe between S' and which eafethete. can be no
eclipfe o f the fun. Whether therefore there will he an
eclipfe, or not, at conjunction, depends upon how far
the mpon,,at Jl/'pis diflant from the node at M, a t the
time, .of G0njunfti<?9-d i Ifrfhe rnpon be at the nodee>i at
the time o f oppofition,. the three bodies aretheB la 'the
fame ftraight line,- and the mooh muft pafs through th*
eentey ofrthe .earth’ s jhadbw, and be>;totally .eelipfed.
But if at the . time o f oppofition,to the fun at S , the
moon be at m', m maybe ft> far belo w .the lhadow Mv o f
tile earth, tha tthe moon may not pafs through i t , in
whi&^.aaf^here, will be, no eclipfe. -Whether, there-
fore there will be a lunar .eclipfe at the moon’ s oppofition,
or nat^- depends Upon, how far the moon at m is
diftant from the ndde at m„ a t thatrtitpel- 1 Bat, if the
plane ilfifW- o fth e moon’, orbit c'Oineidtd with the
plane o f the ecliptio, er: the plane, o f the paper,, there
would manifeftly be a central interpofition evSty eon-
jundtion and? pppofifi'ont..and coflfa|peBtly an ee%fe»
I t is. alfo' evident, that . the place o f the earth feeu from
the fun ts the.-faTneas, the, place o f the earth’ s lhadow,
they both, lying ib the fame hue from the fun.
144. The