LATITUDE AND LONGITUDE. 49 



Fig. B, represents Jupiter and his Satellites with 

 their shadows. 



Now, whilst the Earth passes on from C to F, we 

 shall only perceive the immersions of Satellite 1, and 

 only the emersions whilst it passes from G to M ; 

 and when the Earth is at O, we cannot strictly speaking 

 see either the emersions, or immersions of them, because 

 the body of the planet hides or obscures the Satellites 

 before they enter the shadow, or after they emerge 

 from it ; and when the Earth is at P, of course the Sun 

 then hides Jupiter Satellites and all. 



It is possible we may see the emersions and immer- 

 sions of Satellites 2, 3, 4, whilst the Earth is between 

 D and E, or between R and N. The Satellites of 

 Jupiter being only visible through a Telescope of con- 

 siderable magnifying powers, renders this method of 

 determining longitude a very difficult operation at sea, 

 from the motion of the ship and other causes. The 

 most practical method of -finding the longitude at sea 

 by celestial observation, is by observing the angular 

 distance between the Moon and Sun, or the Moon and 

 certain stars, at any absolute moment of time, usually 

 called a Lunar Observation. 



For this purpose > the true angular distances of the 

 Moon from the Sun, or certain fixed or well known 

 stars, are set down in the Nautical Almanac for the 

 beginning of every third hour of Greenwich time : the 

 intermediate times being ascertainable by proportion. 

 Hence the distance being taken between any of those 

 objects with a sextant or some adequate instrument, 

 and the corresponding time found at Greenwich by 

 means of the Almanac, and compared with the ships 

 time gives the Longitude. 



In taking a Lunar observation, four persons have 

 been employed ; a principal observer to take the angu- 

 lar distances of the objects, two assistants to take their 

 altitudes at the same time, and a fourth with a watch, 

 to mark the times when the observations are made. It 

 has also been common to have five or six sets of obser- 

 vations in succession, and take the mean of the whole ; 

 VOL. i. 1833. G 



