342 Astronomy. [Lecture 21. 



Mason, so as to determine the distance of the 

 moon from the sun or a fixed star at any given 

 time within little more than half a minute of a 

 degree. This difference from the truth cannot 

 subject us to an error in longitude of much more 

 than a quarter of a degree, or 15 geographical 

 miles. 



It will conduce to a greater degree of accuracy, 

 if the moon's distance is taken from two stars, 

 or jfrom the sun and a star on each side of her 

 as often as opportunity permits : for as the im- 

 perfections of the instrument, as well as unavoid- 

 able small errors which attend the use of it, have 

 a natural tendency to correct each other, the 

 mean result, arising from these different observa- 

 tions, will generally be much nearer the truth 

 than if either of them is taken separately. 



Observations upon the eclipses of Jupiter's 

 satellites, the times of which are recorded in the 

 Nautical Almanac, and in that much more cor- 

 rect Almanac, White's Ephemeris, serve likewise 

 to determine the longitude with considerable pre- 

 cision. But, for a minute explication of these 

 and other methods, the reader will do well to 

 consult Dr. Mackay's work, written expressly on 

 the subject. 



