LONGITUDE, in navigation, the distance of a ship or place, east or west, from an other, reckoned in degrees of the equa tor. As the discovery of a method to find the longitude would render voyages safe and expeditious, and also preserve ships and the lives of men, the following rewards have been offered by act of par liament, as an encouragement to any per son who shall discover a proper method for finding it out : the author or authors of any such method shall be entitled to the stun of 10,0001. if it determines the longitude to one degree of a great cir cle ; to 15,0001. ir it determines the same to two-thirds of that distance ; and to 20,0001. if it determines the same to one half of the same distance ; and that half of the reward shall be due and paid when the commissioners of the navy, or the major part of them, agree that any such method extends to the security of ships within 80 geographical miles of the shores, which are places of the greatest danger ; and the other half, when a ship, by the appointment of the said commis sioners, or the major part of them, shall thereby actually sail over the ocean, from Great Britain to any such port in the West Indies as those commissioners, or the ma jor part of them, shall choose for the ex periment, without losing their longitude beyond the limits before-mentioned. The French, Dutch, Spaniards, and other na tions, have likewise offered rewards for the same purpose.
Since, by the motion of the earth round its axis, every point upon its surface de scribes the circumference of a circle, or 360°, in twenty-four hours time, it is plain it must describe 15° in one hour, because 3 q0 = 15. Hence the difference of lon gitude may be converted into time, by al lowing one hour for every 15 degrees, and proportionally for minutes ; also dif ference of time may be converted into difference of longitude by allowing for every hour, and proportionally for a greater or less time. Consequently, by knowing the one, we can easily find the other.
Whatever contrivance, therefore, shows the hours of the day, at the same absolute point of time, in two different places, likewise serves to find the difference of longitude between those places. Now, since an eclipse of the moon proceeds from nothing else but an interposition of the earth between her and the sun, by which means she is prevented from re flecting the light she would otherwise re ceive from the sun, the moment that any part of her body begins to be deprived of the solar rays, it is visible to all those peo ple who can see her at the same time ; whence, if two or more different people, at two or more different places, observe the times when it first began or ended, or note the time when any number of di gits was eclipsed, or when the shadow begins to cover or quit any remarkable spot, the difference of those times (if there be any), when compared together, will give the difference of longitude be tween the places of observation.
The longitudes of places may also be determined from the observations of so lar eclipses; but these being encumber 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 Jupiter, 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 under 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.