LONGITUDE. 
dovr, the other at the entrance of their pas- 
sage beliind his body ; whence in eacli 
revolution of the satellite there are fonr 
remarkable appearances, by the observa- 
tion of any one of which the business may 
be done, viz. one at the entrance into the 
shadow, and one at the emersion out of it ; 
one at the entrance behind the body, and 
another at the coming out j but the latter 
of these, viz. the ingress and egress of the 
satellite, into and from under the body, is 
not so much regarded by astronomers as 
the immersion into and out of the shadow, 
because, in the former, the difficulty of 
pronouncing the exact time is very great, 
it requiring, in each observer, eyes equally 
good and strong, and telescopes equally 
large ; but the observation of the former 
of these, viz. the immersion into, and emer- 
sion out of the shadow, is easy and practi- 
cable, because the quick motions of the 
satellites plunge them so quickly into the 
shadow of Jupiter, that it is no difficult 
matter to pronojmee, by any telescope by 
which they may be seen, the exact time of 
their immersion and emersion, as any one 
may soon be satisfied, if he will but try the 
experiment. 
And as each of these happens at the same 
moment of absolute time, if two or more 
persons, in diifereut places, note the time 
of observation, these, when compared 
together, will give the difference of longi- 
tude between the two places of observa- 
tion. And, when we consider the great 
number of these eclipses that happen every 
year, there being more visible in one year 
than there are days in it, and consequently, 
but few nights when Jupiter may be seen, 
(and which is near eleven months of the 
year) but that an eclipse of one or other 
happens, and sometimes two or three in 
a night ; the ease with wliich tliey may 
be made, requiring only a telescope of 
eight or ten feet in length, which may be 
almost managed with the hand; and the 
little likelihood there is of missing the times 
of ingress or egress, they being in a manner 
momentaneous; and lastly, the great exact- 
ness to which they would give the differ- 
ence of longitude, it being certainly as 
exact as the latitude can at present be 
taken ; it is much to be wondered at, that 
the more skilful part of our seamen have so 
long neglected them, and especially in the 
several ports into which they sail. The 
eclipses of Jupiter’s Satellites, and their 
configurations, are given in the nautical 
ephemeris. 
besides these, there is another method 
equally usefitl, expeditious, and certain ; 
and that is, the appulses of the moon to 
certain fixed stars, and their occnltations 
by the interposition of her body ; tor, the 
moon finishing her revolution in the space 
of twenty-seven days, seven hours, forty- 
three minutes, there are but few clear 
nights when tlie moon does not pass over or 
so near to some fixed star, that her distance 
from it, or the time of her visible conjunc- 
tion with it, may be easily observed by the 
telescope, and micrometer only ; and these, 
when compared together, or with the visible 
time computed to the meridian of some 
place, will show ffie difference of longitude 
of those places. 
It is a great objection to the methods 
here described that the agitation of a ship 
at sea prevents their being useful. But the 
invention of Hadley’s quadrant and its mo- 
dern improvements, with the degree of 
perfection to which the moon’s place can 
now be had, by computation, added to the 
great facilities afforded by the nautical al- 
manack and requisite tables, published by 
the commissioners of longitude, and other 
works, particularly Mendoza’s extensive 
Tables, patronized by them, have rendered 
the determination of the longitude at sea, a 
thing of easy and general practice by obser- 
vations of the angular distance of the moon 
from a fixed star. This W'as first proposed 
by John Warner, in his Notes to Ptolemy’s 
Geography, in 1514, and since by others, 
particularly our Sir Jonas Moor, Flamstead^ 
Halley, Bradley ; and in later times,, with 
great diligence, zeal, and ability, by the 
present Astronomer Royal, Dr. Maske- 
lyne. For the processes and computations 
the reader will have recourse to the w'orks 
just mentioned. The principle is simple 
and easy. An observer at sea measures 
the angle between the moon and the sun, 
or a fixed star, wiiile two other observei si 
take their altitudes in order to determine 
the quantities of refraction and parallax. 
The two zenith distances, and the oblique 
distance, constitute a spherical triangle ; of 
which the angle of the zenith may be deter- 
mined, and then by correcting the altitudes 
for parallax and refraction two other zenith 
distances may be had, which are correct, _ 
and with these and the angle at the zenith, 
a new triangle is constituted, ''of which the 
oblique side is the correct distance. By 
Comparing this distance with those in the 
nautical almanack, the time at Greenwich 
is obtained, and the difference between 
this and the time (observed by an altitude 
or otherwise) at the ship, gives the differ- 
