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 every 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 m the titres of in gress or egress, they being in a anner momentancous ; and, lastly, the great ex actness to which they would give the dif 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 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 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. Alas 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, and than 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 ze nith, a new triangle is constituted, of which the oblique side is the correct dis tance. By comparing this distance with those in the nautical almanack, the time at Greenwich is obtained, and the differ ence between this and the time (observed by an altitude or otherwise) at the ship, gives the difference oflongitude. Though this computation with tables, which give every tenth second, is not operose, it is much abridged by the formula given in the said works.
Time-pieces are likewise rendered so perfect at present, that they afford the most inestimable assistance to mariners. See CIIRONOMETER and HOROLOGY. Our John Harrison, between the years 1726 and 1762, first vanquished the great diffi culty, and was rewarded with 20,0001. from the English government. Very libe ral encouragement has since been given to other artists, such as Arnold, Earn shaw, and others.