, thq fame magnitude in, the-horizon.' -jdly, l i t oamial
lie in the,intermediateqahjeéls, they-.remaioiifgthafam« ;
5 alfo, when thefe ..ôqjeéls.. ,aix ëxeludedJfrom figlit, it
" makes no qltèfatiqn, ■ 4fthly, I t cannot- be the vifible
magnitude, becaufe that .is'1 leaf! in the horizon. The
‘ caufe Thereforemult lie jn the vifible iajïpearance, which
proceeds from the greater paucity o f rays coming to the
■ eye, producing^iihb$b, Ma. R owni n g fuppofes that
the .moon appears ^the&iHgm'-jUs in .the.hqrÿson, becaufe
the portion ofT'the ilcy which.we fee, appears, not
an entire hemifphere^but, only a portion o f one ;j and
hence we judge the moon ter be further from us„ in 5the
horizon, and theréfqfélarger. D r. S mith, in his Optics,
gives the; fame feafqn,— TJJie fame circumilances
take place in the fun. A lfo , if >ve take two ftars near
each other in the hbrizohj.and two other hars near the
zenith a t’the;fame, angular diftance,.the two-former will
! appear'at a much greater diftance from each other,, than
the two latter. On this accpunt,"people.-are; in general,
'much deceived in ellimating the altitudes,.of.the heavenly
bodies above the horizon, judging them to be much 1
greater than they a?£. T he lower part o f a rainbow alfo
appears much wider than the upper part j and this may
'be* confidered as an argument that the phæuomenon cannot
depend entirely iippn the greater degree o f faintnefs '
o f the objeft when in the hpfizpnj becaufe ih c lower
part oPfhebowfrequentfy a p p e ^ brighter than the-upper
p’ârf, at the Tame time, that it, appears broader. A lfo ,
faintnefs can have no effedt upon the angular diftance,Of
the liars ; and as the difference o f the apparent diftance
o f the two" ItarsT^whofe angular diftance is the fame in
the horizon and the zenith, feems.tobe fully fufficient to
account for the apparent variation o f the moon’ s diameter
in thefe fituations,, it may: be doubtful whether the
faintnefs o f the objeét enters into" any p a r t ixfthe caufe.
‘ ' T2Ó.’ The mean diftance o f the moon from the earth is
%Heut-^39 thoufand milesi;' and hér femidiamcterris nearl
y Of-the radius o f th'e earth, or about 1081 miles.
A n d as the magnitudes o f fpherical bodies are as the
cubes o f their radii, the magnitude o f the moon : (mag-
'nitude ó f the earth : : 3 1 1 1 3-:: 1 : 49 nearly. _
,, jJÊÆ m Rotation\ of.the- Sun and,Planets. \ .
■" 12.1'. T h e times o f rotation'Of thé fun and planets are
determined b y ; thè fpots' which are pbfervéd' upon their
•filrfaces; either b y finding the arc which- is deferibed
in a given-timeJby'a fpot, or b y obferving how long it
is in palling oyer the whole difei ?
“ . On the Rotation of.the Sun.
- I2 2 ." lt is doubtful by whom the fpotsBon thé fun
werefirff. difCoyered: "S cheiner obferved them in May,
’ .16 1 1 , andpublilhed an account Of them in td 'iz . G alileo,
in a publication in:-1613, fays, that being at
Rome, in A p r il 16 I I , lie lhén fhowed the fpots on the
■ fun to feveral people,' arid' that he.had fpokeii o f them
fome.'months before, to his friends at -Florence. He ima-
-gined«them*ta :adher.oto; the" fan,->* K eïder fays, they
vyefe otdènsed,b^ajfoftlefs®as"i j> Fîüibri cniisfWho'pulf-
I liM C *)l3w l i l j | f ( f f ' " - U m t e f j t q W i A '
of -Ha r Rtjqf/,) not ; yeôpublifhedyit .le faid'that fpot»
-u^!E)n,-the'.ihxn were' dhferved in* December, « '-Frtitn
pofetvÿîg:the motion of'the. fpots,-the 'time -of-the fun’s
rofetibh ts determined to be 2yd. 14b;; 81. tt* I
123,- BefideS'th'e dark fpots upon thefftnf thefc-arc
;alfh parts i>f the*-fun,’ • called facula,-yluètUf'-%h.c. .which:
,asd brighter than she général lut’face ; thefe abound pioft
m the neighbourhood 1of‘ -fpots,1 or where’ fpOts drive
lately been. Molt pfitherfpots appear wltifin'd®*1 ofpie
Taft’s equatpr-ifOn April« id»'{17(7*9, D r. HiitShtiSL
fpw a fpot whofe diameter was I-'lrff', which as equal in
length to" mare than .31 thoufand milesqfthis Was vifiblfc'
.toqihe naked-eye. .... i>,
: if. On'hhe’-lR&tâtiohi'pffh%.^ïiàeWP--f
124.. TheiÿeV^^ds^ife.ÆWKtaidhlaàreiithsit^àflfb«-
nomers bave ,not-been,able t» determine, «whethep-ifcfeali
125. Saturn wasifafpcctctf by'Ë-Àssiv r at»dJFATcf, in
1683, to have-a irweïlutisn about itsiaras ; Ton theyloqe
day faw-«â lpjghtdireeikv w^ich idifappeared; f ié n&t'f,
-when another came ‘into- iew .near it»;ffi&.. ’Thfefe
ft leaks are-called ie/ts. - In 1719,uwhen -t Ifef iangt Sifap-
peàredr I3|^s;tEl:fa^' its! fhadaw upon the planet«’ and
.albeit on tusclpfide,. parallel to thedlradoW: « Hr. H fr-
, s op Erjjjfapad that fher arrangement «ofjlhfe' belts Slwayh
followed that>,qf.,tiiearing}; And.duringilhik1 'qbftrià!-
tionson Jpne iig^ÆOgarithzi, tt78c^ehetfs)w?the faint
in thfee|4|f^entlhtuafKlas-; : from^i^hfoh-he^Ht,:
fipli|ded thapfetum refalvedtabout-àn; axiis rW®:h‘ is perin
fupport.af,its rotation, id, thht'tbe plan'etrfe'an' oïJfete
fpherord,' ha’ln^''the>, diameter îin«;tKe..idirtï®|oh* iof^hb'
, ring, to they diameter perpendicdlar«kct it,; àâîfîlfô^ïS,
, according ,to, the “Da&Ot.o 1 Thestrutbl of idiis’ èG'njédtùre
‘Jfeaftenraj^s -we^Sedy thaÿi^dstêiâhided'« thât Ôtüni
revolves about its axis in job. 16!. ' ;
■ . 12’6. fjnjnler is obferved tathave belts,and îtlfo Ip'dts,
by whVhithe'time of ÿecrotsJiod Mabeendéferthïrftib
Fronya fpqtghich QassiürcibfervfedJtilidiSÿsIfèfoûnd
the .Frodi 6t;hâ«ffihfh
, i^-Oétohpr î 691, hêfound the time« ghl 15
pjjiqr y)pts hq determined,the time to bt « and',
in general, he found that the« nearer« thevfbpts were to
the equator,, the1 quicker they revolved 9 from, whence
it is probable that’ the fpots are not upon the body of
jüpiterj:but, in its atmofpMdr«.. Hr. H erschel'alfo
found the time of rqtatioipt Q.vary, from different fpots. ;
and that the time of revolution of thé famé fpot dimi-
niftied ; and obferves, that fuch a circumftance-is. agreeable
to, .the theory of equinoftiifl wihdsj as it may be
fome time before the fpot can acquire the' velocity o'fèhë
wind. D r. Boulin .made the ipplarllid ftheyequalorial
diameter as; 12: 13.. D r. Bradley- .made them as
12,J.:,-1,3,51 Sir XsAac Newton .made 'them as gf-s
1. io*»by
eof .’hy4heo^r.i * il- af«g^eraB^ip®L
raHel«to«hfis eqdïtbr,« and ar'c.fübjééUto great variations,
.Jffsis' prfehablfettl^trKheijE etsiflsdnt-theittiriqfphere. .
",«;. attsk
fyhïchöo^e ’/tki/ianftraw a fpqtdm t® Si J i.’b§6',
finedïfpotéj-and- the-latter ^determinca thq-d-imesof^rota-
tion to be 24I1. 40’. Maraldi made it 24I1. 39’ ; and
.dffddvered- .i vai\ In Yrht a,art near the fou.ttieni -pole ;
hu. tno' brigbtnüfyifdlübl'f'E , uiu^Jetcl.Uige, hoptetihing
uobtn-pWfe. D r.
11 ■ r i hT. 1 ny a f f i the tjn>e_qt,i flSl'U to be 24I1. 39'.
j21",A7. He alfo concludes, that mars has a confider-
nïft?''atrfiö{jdrerë. -j
I i28."&AirréEO“firlt difcavered the'phlïfes^of" «udv.vi,
% m [ lit i;^5oi3;.®A"r I n at thUlina. when W .,us ua, ,
dibbotoinifed; difcovf'redqa; bright Tpot»npp'fi/it> at >its
^li light,edge, aad,tjyabfj .ing it nuit'O'U.bcyfuupd 1-hc
time of, rotation, to b©j^3±ii»|y5«jt: M. Shroeter has
endeavoured tqf ftiow tliat venus has an atmofphere,
frarn bbfqiyjng illuminated lirab,,,when hqmedi4
«Jteeeds aifemieirldi:*as.in jheiQtfe pf,the moon
cafpsffoiriétSmes- ran ,i5?.»-n<Jljiny,o»lhe- dark hemifphere.
He.'HiakeSrthe time qfrot^tjon 2,jh.21' ;.uan,d,,coniludes
from Hat obfervations, that there- are very high moun-
. tains upon the fur face.
. ^1213 , .qibe,pK'afest:of- mercury« are, eafily diftisguiftied,
hdt-?POs*§lotq havq-yet hqeg^difcovpte-ih'.by. which ,can.
Ije vafcqqtained .Whether it has any ro^atfon. ■
* ! On the R o tatio niif the Satellites.
ÏJRièftfih l^tellitKjóffat-urn waVqbïnteBJjy,M.
ein pait ot its oib^t, to appea. h f ai d Tel, till it Ue-
rarae inyifihle'; and in thguWeftprn part, to, iocrqafe
again. ,Thefe phsenpmena qan^ha^dly, be„ftcc,qpn.te,d for,
butiby &p.poling. fomc- partsfqf-.t-hefurface to be- inca-
pable o f rculedling light,uind thcrefore wlicn fuch parts
are' turned1 towards the earth, they appear to grow lefs,
or to' .difappear. - A n d as the lhme circumftances always
returned' -again when the fateftite returned, to the
fame part o f its orbit, it affords a ftrorig argument that
the time of the rotation, about its axis, is equal to" the
time o f its revolution about its primary, a ■ circnmftan'ce
fimilar tod the cafe o f the moon. D r. H erschel has
difeoyere'd that all the fatellites oi'jufiter have a rotation
about their, axes, o f the fame .duration as their re-
fpeiftive. periodic times’ about their primary. r
ftrt the Satellites of Jupiter-.
131 . On January 8,;i,6.to, G a l il e o "difeovered the
four fatellites o f jupiter, and. called them MedkeanJlars,
in. honour, o f the family o f the- Medici, his .patrons. This
was,a difeovery ,o f great: ituportjanSje, as it furnilhed a
ready method o f finding the longitude o f places upon
the earth’ s furface, by means ,of their eclipfes. T h e
eclipfes- led M . R oemer. to the difeovery, o f the pro-
greffive motion o f lig h t; and hence D r. B radley was
, enabled to folve an apparent motion o f the fixed ftars,
which- eduld not otherwife have been accounted fiir.
a ^ * !3ffi^{fatellftes,df jupiter in going' front' wbft to
eaflkirft dchpfed jby.the-lhadow o f jupiter, and R4 they
go fromi-eaft fojWeft,ithdy.,ai'e«obferved to pafs'oVer its
dife. Hence,’ ...they revolve about jupiter. The" three
.firft * fatellites: are always ecUpfe'diwhen they are in op-
pqihjpnj to th e fun, and« the! left^slhiofi their eclipfes ia
found to vary; but fometimes the fourth fatellite 'paftes
through oppofition without being- cclipfed. : Hdnceit
4ppe®, .that the planes o f their;otbits dS not eajflcide
With; thq planq^Bf(jpp^terls jorbft; for m" that cafe# they
would always pafs through the center o f his fhadow, and
be always! equally ■ eclipfed' at every oppofition. 1 The
pexio die- times, are as folio ws :
’ Firft5 : : IE J -. Third | Fourth..' ;
Ü ü & l i - . 4 2". 1 Yd." 3h. 42V 33"1 |-i'Sd.k io h .1,32’ " 8"
The* diftandesf of'the futcibtes’fi'o'nl’jiijiiter, mtèrrn?; ofqhqTenildiametei; of jupiter, are as follows :
Firit -f Hi ■ ■ ' Second .. 1 ü |. Fourth iSe?:
S i« - 9 >f9+ 1 ’ .T f .H * 1
' 134. T he periodic times, and diftanoes o f thefe&tel-i o f j.upiter without .entenngq-ihto'its fcadow.;. qnd.thif
Fites oblerye the fame law as thofe o f the primaries re- h called an'occultation.
fgefting .the, fun s that as,,the .fquares.of. the peiiod’ic 1
tunes hwV’ ffit-mmv b r^ ^ ^ o h td' each ’OtHef,1 as th e .j};0-' x ir/i, -iO-K theiSa-toHifiS o fiS a tU m . ■
cubes, o f their,refpeftive -diftanges... >; . . . . ,
. 135. A fateliitc is fometimes hidden behind the body 13ti- In the ycar.-ltib^, 1 lu vc; 1:ns’ difeovered the
• Thtifirft fstellite it.thst neareft to the flanet, and the others in their order from j t ,
■ w t v a f a : fourth