telefcd'pe'/tfiey arfc ’found to have all thefe phsenomenai-
we ftyriilude fKkt theymuft:bhjnffrio,r planets/' . Now
flie an gle ' chS'is /he-'Igreateft^diftaiid'e '"at wlifch-t-here,_
planets appear fiiomjthe fun\ 'or the greatf^^l'©n^ation ;
arid; as this angle is found fto:b e ; greater for -venus than.it
i f f or mercury, we know thaVrhercurj ianear'ertolthe f in
thah&riz^y. |§S v‘
' 84, 'When the ^lriri'et # a t 'd ,f it -appears in the heavens
amongft the fixed ft'ars atTP'$* 'when it is at 3, it appeafs
when' it is "-at c ‘it k jffe iffik t 'R i | ^ e n it is at
(f, 'ife'aljpe^^af A£ ^ ;:wheriTitris »at e, it - appear^ a t -M-f
tvh’eh it is at/^it^appears at fS.tj and' whefi itiJetJofrisitx*
a9 it appears at 'P / at vvhich pMce'alfoHhe dun ^appears;
Xtj is marijtfeft*’ therefore; that-an inferior planet appears
to _ move. Backwards rafick fotwdrds • irit^tM^heaVeas,1
from S to i? , and ’from R to S ; ; and therefore there
ihuft’ he1'; t wo points (where * the planet appears, ftatfon-
a r y ; for i f a planet-fieft Appear5 to move one way and
then back again in a c-ontrary dire^on^hf? motipp^n^ift.
®gj| ceafe in one direftion- before it takes~p$18<^jn a >con,-
trary direction. W e have here fuppofe.d the1, earth tp
be at reft at X9 but all the fame phenomena will take
place i f we fuppofe the earth to be in motion ;, for an
inferior planet moves fafter about the fun .-than the
earth does, and therefore 'when_ it- comes in.to~inferior
conjunftion at a, it will immediately leave the earth/be-/
hind it, and have the fame relative fitklafipns'.in refpedh
to the earth, and fun, as we have defcrised. abovp^If the
- earth were at reft, the two ftatioriary ppintslwotild be at
R and S, when the .planet was on each fid e a trigs- greateft
elongation from the fun ( appearing at- Vj) ; the
earth is .in, motion, thefe will not be the . ftationary
^ppin|sv T h e -true ftationary points (which call PVand S )
are determined, by; fin ding when a line joining theearth
and planet continues parallel to itfelffor a very fmall time$
85. T he earth and "all the placets revolvK aboutthe
fun .in. the d^reftion” X T ; that djreftion is therefore
direct, and the contrary dir^dtlon T X is retrogradeflfee:'
a r t . H e n c e , am>mfexiof planet appears to move,
dir eft, from ..the ftationary-point R , beforerit comCSito“.
the fuperior conjunction, till l't^qomes to the ftcftibnrilry*
point S after ; anp k appears to move retrograde, from,'
the ftationary point ^before it comes to thdrirrforioj con-;
jun ftion ,fill it,dome8 to,the ftationary point R a f t e r ; '
therefore wlplft an inferior planet iff, paffing. through its<
inferior /pnjupftiorh, i t . is retrograde,; and, whilft' it is
paffing tlirougnlts fuperior conjtiii&ion, its mdtidnris
difeft, ** A s the arc c e j Is greater than the arc fac% the
planet is ldriger direft than it is retrograde.': I t appeafs
aifo from hence,' that the Iwo'iiiferior planets will cpm
the fun, receding to a certain diftance on
each fiae, and then' refurning fojagatn to him. A s thd
hrbfts o f rihe£piarietS are ribtp-chcle&j biit' ellipfeS,the
great eft elongations of vetius and mercury are riot always
tlie fanie ;; the greateft: eionjdtions 'fof 'vrdrius are from
44°* 5 7 . \ $ f e -48'^ and o f mercury\ from '/ 7 ° ,'36'' to
28*°. 20'. A s mercury recedes b.ut'tp1 a fmall .diftance
front the fun, it lVnot often that it can be feen, ps i t
jnuftbe in the moffc favourable fituation for that' purpofe;
and ’the atmosphere tjruftèlfo ",betyeiy vplear at^the
fame.time: , f
1 Hf6,rrWhen ^enus. is at the diftange , o f 4 4 > from.the
fun, betweenitsahfta*ibtó<3©bjuhd1ü.öh.and its gfeètèft|elongation,
mfth'ei^^v.es rthe^greaVepi antityc-of.li'gh/ to.-the..
earth.; .and at that timepïfirp^ fo; greatfas '&
caufe' ,a^{fiadpW.' .vAnd:;if |f|^hafe-rii^a
greateft north latitude,-hei brightnefs is fo gi eat that.
théfl^ied? at an^y' "^irrfeld^’pie./d^y
; when file is ab^^t^rhori'zon ; for, when hen^rth/la*-
titude isuftie, g.reét€Ö^' lfe rifes'^?^h^|ïalp'owe' the; hori-
zon', farid |her ra^^fouiing^’fihn^^lpefs of- the atmof-
phere, -fhe is moie.eafily fecn. This.bapp,ens once-in
' about 8 y e a r s , ; a r i d , t h e v^rV>6 T d|fe ing veiy nèarly-
- to the fame rparls o f their orbits Sfter'fhljtrintervakof
. 87. ??2or«i«o^ftar from inferior to^foperior
eon|3nftion^lia'nd an evening infe^
rior conjunftion. ^ T n é akis^c-
eording to the order of,,the letters mnvhv ; when the
• fpeftatör is at «, if is then night .to him.; and as,, b y
^the»earth’ s rotation, he is' carried towards «u, it is mani-
f^^fhait^^eTpal't ace of' t h # S'veirus. will ;e©mêr
into view before the fun iS1 does ; hence, i f venus be any
: where in thaf^’p'arf- o f her orbit, fhe w ill. appeaf§iri5thp
mornin g . before fun-rife, and therefore fhe-is th'en '•a
??20f72i«§--ftar. A s the fpeftator paffes th o u g h vw'fd0xé\
as day^-and 'at m 'the fun will1 f e t ; but the part téjfa o f
thb^orbit o f venuslwill; ftill beaboyg-#is^Jh©/izon, and'
I therefore- i f Venus be in that W^1 vilible.
after fun-fet, and'will then be an-^n^zA^- fM’!'' 1 (*
? 88. ,The orbits o f pinèMêüd jnercury are inclined to-;
the orbit o f the earth, and c”ut it sat' iwbi'^pfep^lipoints,"
called- the nodeir 'fo fehat i f the--
l^ trm 4 to' Me ;M 'the \ la n e o f /Ke^jgper,J impfb rbps '’
-.of venus /.aria ‘ mercury ' will', lie, one 'half above
fthe ‘^pape-r/ „and-the other .halfbelow. I t is Spbrithis
pecöönt that yenus' ^and mercury, when they come-inta
Ithèir inféflórdpnjunftion^ut^fe;domo#always ’appear to
' or make a
the-nodes happen 10> lie in,^cd^^n6libri
then, when thfe plaMB|p0mes infe^conjunftion atx,2, it is.
fin a line joining”t“he earth and fun, and.it will -aJofeaPtbi''
-pals over, the- difejMiili^fon.,- Ifke fa mallj-rouliWpkpk
fpót. , But if th e nodes he a tv a "certain diftance from.'
QÖ^^nftioi^and.oppofitïqn^wh'é|fthèb l% et comes|m|6^'
COTjhnftidnyit rmayJb¥ fd! !f i r above' dr -below ‘the-1 fine,
joining- the earth and' fun, as ' nbsf^b^ p‘® óver the^fun.
TMe1 tranftts'of venus- do. 'not happen -fo often-asSfho’fè
o f irièrcury. T he la ft tlah'fit
and the next will He in 1'8J74-. The la ft Iran fit o f mercury
'happened' in and fthc/next ’will be in 'i8b’è/'
89. When D r. 'H aA leV was at St. IfehtiUs whithet-
Ke wèrit for the- pnrp'ofe óf/inakiug 'avdat’èliogu© èï"'fhë
foüthefn- ftarSi he obferved^ a tranfit o fa^ rcu ry Öve^ the
ftiri’ s drfc, arid this fugg&ftefd tb him ^ïriêtHbèdffbkdlng
the fun’ s parallax from' fuch'obïerfritions, from the dif-
ference ó f ‘thé times o f tranfit óver the futi,- -at difPerént
plaeès upon<the earths furface. B u t-thé diference' o f
1 8 «
l m~ -T R O - D U C T I O No icxi
the rtimes - being Icfs^ for m e r c u r y f o i ; werjus,:
the i?®dtukon& will be 'more* accura^e^for yenm than .
, 'f-Tne
mine the parallax o f the fun from the tranfit o f wcniis 1
and as it was not probable that he himfelf Ihould live -to
icchferve^the two next tranfits, which h’app^q^i'iii:';i 761 '
yeiyrekjjbeM^'-recdtlimeride^lM'a^^Mon
|^Mem ^o, ’the | 'then .
jM ^ m m e | g y e ^ England a
to thjefmb)ft^©|)@r parts o f the earth, to^ofefferiviefbpth
tthefe' trajifits-; -from’-iwhieh -#t|iat
’j:Ke-hArizonta;lvparaUa%-oDtn^ tfun at his mean /diiMnce,
is S J " ; hence,, b y article 14, fin ■> 8 j'; : ra'd’./ / rad.
o f earth f mean d iftjficj^^the,furi from me e a ^ l lm pwf
*fM Mi V 23S*i^ ftrJjerefofe1 tjad m b a^ a ifB eS
'io’f th e fun'frbm_ the earth is ve%ial"to‘>3575 femidi'apl'-
ters o f ■the cai th ; and,, as we have determined (fee ait. .
; f l § | a'dfu§ foff the ■ea rtM i f f the m^an
diftance o f fthe' eaffh ^th^ •;
9347-48
^qntad^parallax, we ref^r the reader' to the Treafife o^
| A ftx©r#nyjShicli we have UefoR^\ne|i tlonedk
^ffipw ly^v ingamcrfSea .
[^Jeriot'praaefsf we proceed to defcribe thofe which at-
,t;end the fuperior0 f /
fixed ftara; draw VKZ&IQj,- C&P,
F bR , mbP ; then when the planet is at /, it is in oppo-
Jjdon ~to ,the .fu 1 j , ■ a^d’aï K> it is in conjunction» • Now the
^ajrtfr 'mores' fafter than a'fuperior planet; whilft1 the
cartM the/eforé moves from C to Edzviqlïrèïd E i o F^ .
let the planet^defQrijDe the fmaller arcs a l, I I . Then it
is iriahifeft, that when Ih^eS'fefiïds at C, the planet at a
k g lA ï l 18 M | 8 | : yvhel/ the Veai/h Ms at E f .
'earth- is at
appears • at R ; wiiijft therefore the
earth moyesVfrdn) C to if,, ,the, planet appears to move
from P to to its real' -motipri ^hence^a fu -
.4periQrvpranet is {retfogrjide whilft it paffes tnrbirg% oppo•
Jition. - ^upppjfe^now that/(when the:^earth-is at K the
Blanqt.'isjat; lè f the
f|sa#h ^pyë'from-iAT'to ® whilft thé plan et’ moves from I
it-will-appear in the heavens to have moved
©li accordirijg to< its real motion. Hence,
direct p^jie^phrough^conjunc--
A s therefore, a fuperior 'platièf appears '/b^ipove,
l ^ a r A y w h e r e the motion*
Lëhlai.^,s.frc)m dne.tp tihe.-;öther.
1 .11 y z j When'the planet is ïmoptómipri/at /j or in con-
p|unBiM'''a^ ^T, the eartlii M‘eing at^^, ‘ ii ls^manifeft
thats/ta^^pne ’face of towfrds
the fun,is alfo towards the.earthy, and-; therefore the;
KpJapMji^arl^fo^^ IojbeidBut,; i f | I S | | W .pofi..
t^.e.. pr^ét;x!then the fö^ftator fok/thé; earth
‘ at w il'ha ved' Mfriéloll/m^dark part o f the planet
will .
I not ' f u l l orbê th e e a r th , but* will appear like
r tlie iridon,-a:- lit^e'bdfore'or;'after' its-- full: | B u t.if the
I pjanef|1^at\ a very-' g r e a t w h ^ n
’compared with the earth’ s diftance^ithere willb&fo'littde •
r 'i^ th e dark'part'.tuined towards .the earth, that itiw ijl,,
J asïto fenfe,1 appear full orbed. ^ Now this is the cafe, with
all the fuperior planets,' except otaa-j-, which between.
I |^njyn^ion. and| .appofition, is-, ;-obferyed‘'to “appear njbt
g rea^ -
diftancès. | ;
93'. ï t is found by obfervationj that the placed o f the
aphelia oFth.e orbits.of the plarièts,; :ari^l the places o f '
[/thein nodes,, have' ,a motiö'n,.--and that therincljbattoiis o fI
1 theirforbits; to tne-' edligtic^ arh -iubjöft.to. ,a - variation,
f|l^e|e- jriVcpmftances arifefrórh. the mutual att^aftipn^of-
th e-planets^ 5 ;
y V 94* I t appears, from what'we havealffeady;pb^rved5,
th.zi'mer6ury; venus, and mars are opaque bödiés^-as they I
d© not- alwaySvftdue with '011 *faces, that part towards 1
the earth wbichfis^CTot -towards the fun, being dark..
jfupiter\ pd! •.faturn 'caft ihadows, fand eplipfe their -fatel- -
Mtës', .aiydfthefoT1^ J^ey mull be!opaque bb'dies. ; TJie-r
georgian has uevex'beèn-feen to eclipfe. itsTateffiteasjas/hje _
‘ fatelli^s hdve - not,’ fince the difeovery o f-the/planltj,
Liheern i^i^^pafeión to^bérecMpied By/hepïfencfVaDtït j it -
being a body revolving-;abqut/he fun, like the-'otljér -
planets, arid haymgdlfó/atelMtes revolving about it, wee
giaypöncltïde b y analogy, that i t is an opaquè hódy.
95. K epeÈk made three very importarit,dUcoveries- -.
' ;■ J4-, . , * •; - ' re^eftieg^