The Earth. 341 



with the Nautical Almanac ;" together with the 

 methods made use of for obtaining their true 

 distances from the moon at the time of observa- 

 tion. For it is to be observed, that the distance 

 found by the quadrant is not that which is to 

 be used in determining the longitude, but the 

 distance as it would appear to a spectator placed 

 at the earth's centre. This is the distance as it 

 is computed for Greenwich ; and in order that 

 they may agree, it must be determined in the 

 same manner for the place of observation. 



The last method of finding the longitude, 

 which is founded upon observations of the moon, 

 is, by the general consent of astronomers, the 

 best that has yet been discovered. And though 

 it may not be easily practised by every common 

 mariner, yet by a person of skill and abilities the 

 operation will be performed in a few minutes. 



In the first place, then, it may be observed, that 

 the moon's daily motion in her orbit being about 

 13 degrees, her hourly mean motion is about 

 half a degree, or one minute of a degree in two 

 minutes of time ; so that, if an error of one mi- 

 nute is committed in calculating the place of the 

 moon, it will produce an error of two minutes in 

 time, or half a degree of longitude. 



The late Professor Mayer, of Gottingen, fol- 

 lowing the theory of Newton, formed a set of 

 lunar tables which gave the moon's place in the 

 heavens to a great degree of accuracy ; and these 

 were afterwards improved by Mr. Charles 



