Art. XIII. — 0)1 Finding the Longitude from Lunar 



Distances. 



By E. J. White, F.R.A.S. 



[Read Thursday, October 10, 1889.] 



About a fortnight ago the newspapers contained an 

 account of the sufferings of the crew of the ship Garston, 

 which had been wrecked, owing it is stated to a fault of the 

 chronometer. On reading this, I was led to reflect on the 

 methods at present in use for finding the position of a ship 

 at sea. The latitude is obtained in so simple and so direct a 

 manner that no notice will be taken of this operation, 

 beyond remarking that the principal desideratum is a night 

 instrument, to allow of altitudes of the stars being observed 

 at other times than twilight, at which time only can the 

 ordinary sextant be relied on for this purpose. The 

 longitude problem is more complex, it resolves itself into 

 two questions — finding the local time, and that of the first 

 meridian, which to English speaking people, and indeed to 

 most others now, is that of Greenwich. The observation 

 for finding the local time is as simple as that for latitude, 

 but the calculation is a little more complex, and it is 

 dependent on the latitude, less so, however, when the body 

 observed is east or west than in any other position. The 

 finding of the Greenwich time is a more serious matter ; 

 it exercised the minds of men for many centuries, and 

 large rewards were offered by the British Parliament to 

 stimulate investigation. As far as navigation is concerned, 

 the methods have been reduced to two — timekeepers, and 

 moon observ\ations, generally known as chronometers, and 

 lunar distances. The chronometer method is so simple that 

 it has now nearly superseded the other (I was informed by 

 a naval officer a little time ago that he had never seen a 

 lunar distance taken for finding the place of the ship, but 

 only for the instruction of the naval cadets) ; but in my 

 opinion, it is criminal to rely upon it for long voyages when 

 less than three chronometei's are carried, for such delicate 

 machines are always capricious, and even the slight rusting 



