I N T R O D U *C T I O N.
im
H i
iifolfihle, even before it Is wKoIly < fonmerfccf 'in the
ffiadnw ; th i iin® ^ therefore; feafe i t fovifible
wiil depend upon the jjuantkyiof light whfch the telef-
cope rfcceiyes, (and its magnpyfog poweri T he inftant
therefore o f its appearance will b e t t e r , the better the
telefcope, is j and'ffiebfoonierfo wffl'appear a t Its eauer-
fion. k Nbw the iinmcrfion is the inftant the fatelhte is
g o t into the fhadow, and the emerfion is the Inftant before
it begins to emerge from the fhadow ; i f therefore ■
two telefcopesfhow the disappearance or appearance o f
the.fatdlite at the fame diftande o f time from the im-
merfion'or 'emerfion, [the d i& e n c e p£ the times Iwillibe
the fame, as the difference o f the true times df imnier- ■
lion of emerfion,' and therefore will fhow the difference
o f longitudes accurately, t B ut if the obfei-ved 'time at
one place- and the computed time at another be com- “
, pared, we imift. allow for the difference o f the. apparent t
and true times o f immerfioh and emerfion, ; fat order to
g et the true time where the obfervation was made, to
compare with ithe true time from computation'.at the
other place. This difference may.be found, by.:bbfery-
ing an eclipfe at- any-place whofe longitude is known, ‘
and comparing it with the time b y computatiom. Ob>
fervers, therefore, . (hould fettle the difference b y the
mean o f a great number o f obfervations thus compared
with the computations, b y which means the longitude
will be.more;accnrately afcerUSned. all, however,
thexfiffeieni ftates o ffhtfair^and.qf ,tfie eye,-wai eaufe
fome uncertainty ; b u t the latter-may jnja.great raea.-
fure be oKviated, i£:the bbferver remove himfcif from, all
warmth and light, for a little time before he obferves.
' i o f in d th e fio n g it t td e hy th e M o o t f s D if ia n c e
| • f r om t h e S ite , o r a f ix e d Sta r .. ■
...... j 1__n i
this method, arethefe,: _
• i . From the oblerved altitudes o f the moon and the
fon, o f a ftar, and their obferveddifiance, find their true
diftance..
.2. From GicNautioal almanac find the apparent time
at Greenwich when the moon was at that, difiance.
3. From the altitude of. the lun or ftar, find the ag-;.
parent time at the place d f obfervation. £
] 4. T lie difference' o f . the times, thus found, gives the
difference o f the longitudes,/ or th e . longitude from
. Greenwich.
W e will here fully explain each o f tbefe.
L e t Z ’h e du»' of i t a place;offobfervatfop, M
the apparent place o f the moon, m its true place, 5 the
apparent place of-the W f> H the;(tt;Ue.pl^ce; tjien" .
as .the parallax lof the nioon-edeprejies it more.dhah re- .
fiction:raifes it, .the.appareut. p la ceA f is hefoyyfoegfruu
place m ; tvjfc the ftar is jelcvatpji by reffo^tiop, and
np.pajaBa^.ftojlqig^tbj^and .th e r f^ ia rap^e, e j^ i|^ | Ijy *
r e f r a $ Q p & m & thet;efore the '.fru?.
place j'_%befowit|ieagjgjifepi^pla^i^ ,
rent altitudes being 'foand byi.ohfer-'^tion, ;we knpvjjthg:;
apparent zenith diiiajices Z 'M ifiS j ancl knowing their .
apptofoE^iftP!3^ •
triangle Z S M i hence, we can .find the .angjej^. t jNow
-find fiofo—tlie .ta lfo s th e . parallax landbefr^ion: d f the
moon, and- thetp dlffirenjae ,'is Mm ; 4° ; fo e. fome for tlie
fun,-and we g,e.t 5j , o r '.if lt fo fo fta r , gjei^p^pib|i gives
St. F r o m fijhtraft iV/m^qnd we g et.if« ., raml^tor.
Z S add St , and we get Zs ; hppqe^jp the trfapgle Zsm
we know Zs,-Zm, and .the angltjpZ, to im d . tm the true
difiance o f the foepn from the foft.<>.r,ft8r. j ;
Example. Suppofe on June 29*;i7gg, the fuu’ sjqppa-., .
rent zenith .diftance Z S w a s obferved to (b e « | ^ . ,^ . ,
24/^the, .mpon’ s ,apparent,,tten^,.:J|K^ee;,^ M to be
4? ° . S3 -50 ’»and their apparent difiance M S i t o ^ p
j02°a zglcr2 l" . Then -the(tpieydi-ffopce sjn bemg-c.dpi,-
putedncqpfding fiajthp above ItEethod^tas.fonnditq^be..
103?. 18". .. -
202. T lie true ■ difiaheeJjof .the moon from the,..fun.
being found* the next .tfong.is to fold from tfonce! ; the
tijne at Greenwich, hipw in the Hautical Mlniatytc .thetrne
diftance p£ foe -tadon foorp'-foe fjfo, of' certain fotefo
ftars, fuch'^s lie
for every three hoprV, Tins fSVf diftanoe' jlfoeiieforq,
befog hhpwhj.fodhfotp.the^^ufo^f if/mnndr, ;anfo foke.
out two difiance^, one. greater jjnd the {other lefs than,
the known true diftance as,':found abo,vej and the 'diffec-
ence D -of tfiefe diftances fhows.how much the "muon.
• approaches.
fopprfoiches tofor-foeede's from the fuii> or ftar, in- three
ftou^Sfand take th e differ'ence'dlshlsween, t.he''}nodn!s
■ sijdj tfo^dif-
1^^fefo¥KdL 'wSithmlfare^ D r d f : 3
hour fheaimh'lhe moonfn a'ecet h^j'to' 9 l.c itlm g
f f^ K 'w;nioh1Sg36if fo
the time at the beginning o f the interval, gives the apparent
time at Greenwich, corrcfponding to the true diftance
o f the moon, as deduced from obfervation.
E x a m p l e . Taking the^moon’ s true diftance 103“. 3’.
■ 118" on Tune 29, 179 J. .as deduced in the laft example,
to fipd the* apparent time a t Greenwich.
True diftance o f 1 1 from O i tB C i lM r lw
.,Ti‘uedfoa,n ceconjun.e .29, ,at,3ha h*f Nautical Almanac
T in e dilfonce
103°. ' 3f ' 18"
:i.P3-.: : 4 *:-; j ?
t&t , ' 26. t 4*
P = '* ■< k ! i v ‘> - ■ I . 38.
d — - - - - - - o. -1. 40
•m. Hence, 10.. 38’. Tt6%'h o°. 1'. 4 0 " : 3 h : oh. 3'; 3", which added to 3 hours gives 3b. 3'. '3" the apparent time
GrMlo’te^i
. .203.. T h e next thing .to be done, is to find the time I
^fecthd plaric offhhforvatidn^kn&wfo^ithe^Cui^isid'ecHna'-
tion, jheSSiitiide of the. place, ami. the fun’ s .altitude. .1
itJ.;.|E,XA'M,eL‘E.-. .The-iivnJsc .dbeiinfoicih was. 24 ’. 14’. 4",
and , J r ’ 190. 3'. 36", and the la!i-l
„tud? lyas vL°.
Apparent' time at Greenwich, jun'c 29
■ Appal ent’Jlime at place -.of obfervatidn^I-J J
, ' Longitude Qf place qfiobfervatiofif^fof tiufe
- i-Which'fgSSS.^t^ ilfo ilfe li? 68»!^ s j^ f.9
Ndw the ifefiaflioh was 2'. 44",. and the parallax 8* l
Jinnee, foe true altitude .was I9 ° if x' •, and by aitii^e
,6 f , the apparent time.is found to be June 28, «8h.
f . l f * , I f epee, f.
- - ’ . ' - - ;3h. S% 3”
i - 8- 57; 34-
■ longitude ,o£ tlie place o f obfervation weft' o f Greenwich.'
which’ the Uongitiwefhs’ Toiund by omenifpnne moon's
.•diftahcetfrdm^ife fdnjtor alfi4ed<ftar be^ifor a fulfcex-
'plauation, we refer the reader.'toTMr. V i s e s ' ! ’Complete
Bfr^tWernyVi jn: jvhjch .works’he.;'>vi'll . find;, all,the
various ■ .calculations | explained ,;af J a rg e f c.and where; b e
vyU alfo foe. three) o ih« methods ofufojfong the ,fongj-
rtude, ; j oiicj iby a folar eclipfe; another, , by atf ipeeulta-
tion o f a liccd . flar, by the moon ; and a third, by the
moon’s tianfit over the.Imcclaia.n, compai e'djw.ilh that o f
a fixed ftar. ] jlhefe are o f too difficult a nature to ad-
in it iof.a popular- cxplaiiatidn.-1 f*
- ’ 2'o.f •' T h e above met hod o f finding, the longitude by
the iniooii^ I-Fi - M ask cl
y s e , who proved thdjatohracy of- it in tw o voyages-,
-one to S/.ifiw/««o,aiHEhecio.ther:tOHBur&<feM, by tb£:|ol-
Idwing irrefragable'»proofs, if t, Onrtlle near agree-
inierit o f fhe; ldngicude, inferredfoptu. obiervaiions .made,
;withiii® few . days: orfenf-s' o fi^Sking land* with .the
•known ' longitude. o f i fuchVlan.ilt)'' !2d , ! From .the' near
ia'greefoe'n'fi'of the' fofigitiide-of tlie rihip tfrom;- ohferya.-
tions foadtion a great ifea»y,.s9«*tU;tj4Py®f -near t® op?
jaitibtiberj'.whenaieoflneSedsils^^lpiofifo^eofojnfo^ES^111-
ingi 3 d,5 h’rton.the near agreement „of the longitude o f
the hup,' deduced frdfojcfoJeryatiobaof ftars on lUti'crent
fides or tfie moon, (fokeu'bn tlie fame night, fort? here
'au.the mofiDrqbable l^ufos^cttor|eOperdtihg different
ways, their effect, Warov ffufb.t!ave. appeared in the reihfcjr
differences \ycre fihfin.a'l, as to warrant hiinrio fay, thdt
by g'uctfi hiiinimnfts.aiKi careful obforvere, th.eylongi.tnde
may'ieimus.fOTwJoj'a-very greatedegreefof accuracy.
‘'/tied- There are two globes,Pohe called the Urrejlrial,
.'.upon which the places of the . earth, are delineated; and
the other called. which all the principal
fixed' 'liars are {hit down, and the figures o f the conllel-
lations. The. terreftrial eW p p e r f e O im a p s o f the
earth,, reprefentjng the relative filiations ,pf aU the places
upon its futfade,; .With .the .tiinC ugfoes|d™l''the different
countries, which can'hbJt|oei properly reprefented tnpon a
map ;' ami thislrenacrs a' teil-cftrial globe very ..neceffary:
for theftlidy o f geography! T he celeftial gmbefetVCT
to.explain all the -phaeiiemona apfing from the dfoimal.
, * The aft J H day begins at noon To that Junc'iS, ,Sh. j'. a9" is, according to rh|)q|mrridn reckoning, June 29, 6h. 5'. ift" in
the morhihg.’ f ’ I
m i motioii