47 



LATITUDE AND LONGITUDE. 



Continued from Page 8. 



2nd. BY Astronomical Observation. We have 

 already seen, that a variety of celestial phenomena may 

 be employed for this purpose, such as eclipses of the 

 Sun, of the Moon, of Stars, by the Moon, or of the Satel- 

 lites of Jupiter; but independant of Lunar and Solar 

 eclipses riot occurring sufficiently often, it is not easy to 

 determine the exact moment either of the beginning or 

 ending of a Lunar eclipse ; because the Earth's shadow 

 through which the Moon passes, is faint and not well 

 defined ; and beside, Solar eclipses are not always visible 

 to different parts of the Earth, at the same time; but 

 eclipses of the Moons of Jupiter happening every 24 

 hours, and the time of their emersion and disappear- 

 ance in Jupiter's shadow, being very rapid, has furnish- 

 ed a tolerably accurate method of determining the 

 Longitude more particularly on shore. The first or 

 nearest satellite to Jupiter, has been chosen for this 

 purpose, because its motion is quicker, and its eclipses 

 consequently more frequent. 



Tables have therefore been constructed by the 

 English Astronomers, fpr shewing the time of such 

 eclipses on the Meridian of ^Greenwich ; which are 

 found in the JStautical Almanac, with a variety of other 

 information of this kind, and published by the Board 

 of Longitude, four years forward. 



It is necessary to understand, that we seldom see 

 the beginning and end of the same eclipse, in conse- 

 cjuence ^T the Earth's jWDgressive motion in its orbit ; 

 it is, therefore, the time of immersion or of emersion that 

 it is necessary to remark ; which compared with the 

 time at Greenwich, by means of the Tames, gives the 

 distance Barter West from that Meridian. 



That we S&dom see the beginnil% and end of the 

 same eclipse, is illustrated by the accompanying dia- 

 gram : 



Fig. A, represents the Sun and the Earth in various 

 parts of her orbit ; 



