THE LATITUDES AND LOXUTl 



I 



navigator requires only to know what is ihe hour at Creenwich at tin- tj.,,, | l( . 

 makes his observation. This he discovers in the following manner- IJ. 

 observes with the sextant the distance of the moon from the S nn, or from 

 some of the most conspicuous stars ; he then, after certain preliminary rai- 

 culations not necessary to detail here, examines in the Nautical Almafac 

 where he learns what the hour is at Greenwich, when it particular 



distances from the sun or the stars. Knowing this, and knowing the hour 

 where he is, the difference of the longitude of a ship and the observatory at 

 Greenwich is known to him. 



Although the moon be of all the celestial objects the best adapted for this 

 observation, it is not the only one which has been resorted to. It may be in a 

 position so near the sun that it cannot be conveniently observed ; in its ab- 

 sence, the navigator may resort to planets which may happen to be visible. 

 These may be used in the same manner and according to the same principles 

 as the moon, but they do not afford a result susceptible of the same accuracy, 

 inasmuch as their motions being slower, he cannot be so certain of their 

 positions. 



The advantage which the lunar method of determining the longitude has for 

 the purpose of the mariner is, that it is always available, when the sky i* un- 

 clouded. There are. however, other methods which are applicable occasion- 

 ally, both by sea and by land, which ought not to be omitted here ; union j 

 these the most frequent, and consequently the most generally available, is tin; 

 eclipses of Jupiter's satellites. Whenever that planet is sufficiently removed 

 from the sun to be visible after night-fall, his moons may be seen with an <;ni;- 

 nary telescope ; indeed, they were discovered at so early a period in the pro- 

 gressive improvement of the telescope, that they must have been first observed 

 with a very inferior instrument of that kind. The periodic time of the first of 

 these satellites, or that which is nearest to Jupiter, being only about 42 hours, 

 and its position and motion being such that it cannot pass beliind Jupit&r with- 

 out going through his shadow, its eclipse must regularly recur every 42 hours. 

 The times of the eclipses at Greenwich are registered in the Nautical Alma- 

 nac, and if they are observed at a distant place, the time at which they occur 

 may be compared with the time at which they would be seen at Green- 

 wich, and the longitude of the place consequently known. In fact these eclip- 

 ses may be regarded as signals which can be seen at the same time from iho 

 two places ; the only difference between them and common signals being lhat 

 their occurrence can be certainly and accurately predicted. It is proper how- 

 ever to observe, that although this method is eminently useful to the geographical 

 traveller, it can scarcely be said to be available in navigation. 



There are other celestial phenomena of occasional occurrence which may 

 also be used for determination of longitudes. Such are solar eclipses, but more 

 especially the occultation of stars by the dark edge of the moon. This latter 

 phenomena is one which admits of very great precision. 



In connexion, with the subject of this discourse, it may not be uninterest- 

 ing or unprofitable to explain the expedient by which the British government 

 enable all navigators leaving the Thames to take with them the precise Green- 

 wich time, which, as we have shown, is necessary for the determination of the 

 longitude of the ship in the absence of the opportunity or ability of practising 

 the lunar method. For a great number of years, the establishment of an easy 

 and certain method of accomplishing this was regarded as an object of great 

 national importance by the English public. At length the object was accom- 

 plished by the expedient now in use, and which we are about to explain. 



The Royal Observatory of England is built on the summit of an elevaied 

 rid^e that overhangs the town of Greenwich, on the right bank of the Thames. 





