632 
A|>p. diftance 
Diff. app. alt. 
NAVIGATION. 
68° 1.9' 50'' N. V. S. 630749 
16 23 o N. V. S. 040604 
Cor. ])’salt. + 45 -9 Diff. 590145 
Log. diff 9-996556 
Log. 5-770959 
obferved at nh. 3m. apparent time. Required the 
longitude. 
At Greenwich that day the emerfion began at 9 57 7 
Obferved emerfion at ftiip - - n 3 o 
Cor. Q’s alt. -f- 322 
Nat. No. 585484 
5 - 7675 t 5 
Diff. true alt. 17 n 51 
N. V. S. 044708 
Truediffmce 68 17 46 
Dift.at iii.hours67 9 17 
at vi.hours 68 32 30 
N. V. S. 630194 
Differ. i° 8' 29® P. Log. 
Differ. 1 23 13 P. Log. 
4197 
3351 
Excefs 
Preceding time 
- 2 28 8 P. Log. 
3 
1 Or 1 
Apparent time atGreenwich 5 28 8 
Sun’s obf altitude 14 l6'0 
Semi-diameter -j- 16 2 
Dip - —- 3’3 
Coiredtion — 34 
Sun’s dec. Nov. 8, at noon 
Equation to 5h. 28' 
16° 37'-2S. 
+ 3 9 
Reduced declination 
16 4i-iS. 
Sun’s true altitude 14 55-5 
-polar diftance 106 41'! 
Ship’s latitude 34 53-0 
- co-fecant 
fecant 
o - oi868 
0-08602 
Sum - 156 29-6 
Half - 78 148 
(Difference 63 193 
- co-fine 
fine 
9 3 o8 99 
9 95112 
28 46-2 
- fine 
19-3 6481 
9-68240 
App. time at (hip 3 50 10 
App. t.atGixen. 5 28 8 
Lon. in time 
x 37 58—24° 295 W. 
To find the Longitude by the Eclipfes of Jupiter's Satellites. 
On the day preceding the evening on which it is pro- 
poled to obferve an eclipfe, look for the time when it will 
happen at Greenwich, in page 3d of the month in the 
Ephemeris. Find the diff. of longitude either by a good 
map, fea-chart, or dead-reckoning. 
Let the watch be regulated by the fun with all poflible 
exa&nefs to the apparent time. Turn the difference of 
longitude into time, and add it to, or fubtraft it from, 
the apparent time, according as it is eaft or welt of Green¬ 
wich ; the fum or difference will be nearly the time when 
the eclipfe is to be looked for in that place. But, as the 
longitude is uncertain, it will be proper to begin 20 or 30 
minutes before. 
Obferve the hours, minutes, and feconds, of the begin¬ 
ning of the eclipfe, called immerfiou, that is, the very 
inffant that the fateliite appears to enter into the lhadow 
of Jupiter; or the emerfion, that is, when it appears to 
come out of the fame. The difference of time.between 
the obferved immerfion or emerfion, and that fet down in 
the Nautical Almanac, being turned into degrees, will 
give the difference of longitude between Greenwich and 
the place of obfervation. 
Thele obfervations made on the firft fateliite, cr that 
which moves neareff to the body of Jupiter, are the moll 
proper for determining the longitude: and here it may be 
obferved, that its emerfions are not vifible from the time 
of Jupiter’s conjundtion with the fun to the time of his 
oppofition to the fun, and that its immerlions are not vi- 
ffble from the time of the planet’s oppofition to the fun 
to the time of its conjundtion. 
The configurations, or the pofitions in which Jupiter’s 
fatellites appear at Greenwich, are laid down every night, 
when vifible, in page the 12th of the month in the 
Ephemeris. 
Ex. Suppofe on July 12, 1813, in Ion. 16 0 34' 4" E. 
by account, an emerfion of Jupiter’s firft fateliite was 
1 
Diff. in time 1 553 
Which, turned into longitude, gives 16 0 28' 15" E. be- 
caufe the time at Greenwich is lefs than at the place of 
obfervation. 
As thefe eclipfes happen almoft daily, they afford the 
moft ready means of determining the longitude of places 
on land, and then the longitude of fea-coafts might be 
better afeertained than they are at prefent; they might 
alfo be applied at fea, could they be obferved with fuffi- 
cient accuracy in a lliip under fail, which can hardly be 
done, fince the lead motion of a telefcope that magnifies 
fufiiciently to make thefe obfervations would throw the 
objedfs out of the field of view'. The eclipfes of Jupiter’s 
fatellites may be well obferved by one of Dollond’s new 
achromatic telefcopes of three feet in length, or by a re- 
fiedling telefcope of 18 or 20 inches focal length. 
To find the Longitude by the Eclip fes of the Moon. 
This is performed by comparing the times of the be¬ 
ginning or ending, as alfo the times when any number of 
digits are eclipfed, or when the earth’s fliadow begins to 
touch or leave any remarkable fpot on the moon’s face. 
Then will the difference of time between the like obferva¬ 
tions made at different places, turned into degrees, be 
their difference of longitude. But thefe eclipfes happen 
too feldom to be of any general u(e at fea. 
To find the Longitude by a Chronometer , or Time-keeper. 
When it is intended to make ufe of a time-keeper, it 
is requifite to examine its rate of going before you leave 
the land, and adjuft it to the meridian of the place from 
which you reckon your longitude. To do this, you 
mult afeertain the apparent time by the fun’s altitude (or 
by fome other method) and apply to it the equation of 
time, taken from page 2 of the Nautical Almanac, ac¬ 
cording to its title of add or fubtratt; the fum or differ¬ 
ence will give the mean time of obfervation: this, com¬ 
pared with the watch, will lhow how much it is too faff 
or too llow ; and, by obferving this difference for feveral 
days fucceffively, you will afeertain its rate of going. If 
you find it gain or lofe a few feconds per day, you mult 
make that allowance on all future obfervations at fea. 
Inftead of comparing the time flhown by the chronometer 
to the mean time at the place of obfervation found as above, 
you may compare it with that mean time reduced to Green¬ 
wich time, by adding to that mean time the difference of 
longitude between Greenwich and the place of oblerva- 
tion, when it is to the vveftward of Greenwich, but fub- 
t radii tig it when to the eaftward ; and by this means you 
will find how much your chronometer differs from Green¬ 
wich time. Having thus regulated your time-keeper, the 
longitude at fea is readily found by it, as will evidently 
appear by the following examples : 
Ex. 1. Suppofe that on Jan. 7, 1818, the apparent time 
was found by an altitude of the fun to be ih. 5m. 9ft 
P. M. when, by a time-keeper well regulated to mean 
Greenwich time, it was 4h. 3m, of. P. M. Required the 
longitude. 
Apparent time - 1 5 9 
Equation of time +0 6 40 
Mean time 
Time per watch 
1 11 
4 3 
49 
o 
2 51 11 equal to 
42 0 47' 45" of weft longitude, becaufe the time at Green¬ 
wich is greater than the time at ftiip. 
Ex. 2. Suppofe that on May 12, 1818, the apparent time 
was found by an altitude of the fun to be 4I1. 3m. 6f. P.M. 
when the time per chronometer is 2I1. P. M. the watch 
being 
