LONGITUDE. 



ed with the considerations of parallaxes, 

 are not near so proper as those of the 

 moon ; and each of these happening but 

 rarely, another excellent expedient has 

 been thought of, and that is the eclipses 

 of Jupiter's satellites. 



Now as neither Jupiter nor any of his 

 attendants have any native light of their 

 own, but shine with a borrowed light 

 from the sun, it happens that each of 

 these, in every revolution about J upiter, 

 suffers two eclipses, one at their entrance 

 into the shadow, the other at the en- 

 trance of their passage behind his body ; 

 whence in each revolution of the satellite 

 there are four remarkable appearances, 

 by the observation 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 com- 

 ing out ; but the latter of these, viz. the 

 ingress and egress of the satellite, into 

 and from tinder the body, is not so much 

 regarded by astronomers as the immer- 

 sion into and out of the shadow, because, 

 in the former, the difficulty of pronounc- 

 ing the exact time is very great, it re- 

 quiring, in each observer, eyes equally 

 good and strong, and telescopes equally 

 large ; but the observation of the former 

 of these, viz. the emersion into, and 

 emersion out of the shadow, is easy and 

 practicable, because the quick motions of 

 the satellites plunge them so quickly into 

 the shadow of Jupiter, that it is no diffi- 

 cult matter to pronounce, by any tele- 

 scope by which they may be seen, the 

 exact time of their immersion and emer- 

 sion, 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 different places, note 

 the time of observation, these, when com- 

 pared together, will give the difference 

 of longitude between the two places of 

 observation. And when we consider the 

 great number of these eclipses that hap- 

 pen eveiy year, there being more visible 

 in one year than there are days in it, and 

 consequently, but few nights when Jupi- 

 ter may be seen, (and which is near ele- 

 ven months of the year,) but that an 

 eclipse of one or other happens, and 

 sometimes two or three in a night ; the 

 ease with which they may be made, re- 

 quiring only a telescope of eight or ten 

 feet in length, which may be almost ma- 

 naged with the hand ; and the little like- 

 lihood there is of missing the times of in- 

 gress or egress, they being in a manner 

 momentaneous ; and, lastly, the great ex- 



actness to wliich they would give the dit' 

 ference of longitude, it being certainly as 

 exact as the latitude can at present be ta- 

 ken ; it is much to be wondered at, that 

 the more skilful pai-t 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 nau- 

 tical ephemeris. 



Besides these, there is another method 

 equally useful, expeditious, and certain ; 

 and that is, the appulses of the moon to 

 certain fixed stars, and their occultations 

 by the interposition of her body ; for, the 

 moon finishing her revolution in the 

 space of twenty-seven days, seven hours, 

 forty-three minutes, there are but few- 

 clear nights when the moon does not 

 pass over or so near to some fixed star, 

 that her distance from it, or the time of 

 her visible conjunction with it, may be 

 easily observed by the telescope, and mi- 

 crometer only ; and these, when com- 

 pared together, or with the visible time 

 computed to the meridian of some place, 

 will show the 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 modern 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 almanack and requisite ta- 

 bles, published by the commissioners of 

 longitude, and other works, particularly 

 Mendoza's extensive Tables, patronized 

 by them, have rendered the determina- 

 tion of the longitude at sea a thing of 

 easy and general practice, by observations 

 of the angular distance of the moon from 

 a fixed star. This was first proposed by 

 John Warner, in his Notes to Ptolemy's 

 Geography, in 1514, and since by others, 

 particularly our Sir Jonas Moor, Flam- 

 stead, Halley, Bradley ; and in later times, 

 with great diligence, zeal, and ability, by 

 the present Astronomer Royal, Dr. Mas- 

 kelyne. For the processes and computa- 

 tions, the reader will have recourse to 

 the works just mentioned. The princi- 

 ple is simple and easy. An observer at 

 sea measures the angle between the moon 

 and the sun, or a fixed star, while two 

 other observers take their altitudes, in or- 

 der to determine the quantities of refrac- 

 tion and parallax. The two zenith dis- 

 tances, and the oblique distance, consti- 

 tute a spherical triangle ; of which -the 

 angle of the zenith may be determined, 



