Astronomy. [Lecture 21. 



situated under different meridians, so the clocks 

 and watches of those places, supposing them to 

 be well regulated, will show different hours at 

 the same moment of absolute time ; a difference 

 of 15 degrees in longitude always producing a 

 difference of one hour in the time shown by those 

 machines. 



In the Nautical Almanac, a work printed un- 

 der the authority of the Commissioners of Longi- 

 tude, for the purpose of facilitating astronomical 

 computations, the distances of the moon from 

 the sun, and from certain fixed stars, are ready 

 computed for every day at noon, and every three 

 hours afterwards, for the meridian of Greenwich ; 

 with a rule for finding the time, answering to 

 any given distance whatever. Suppose now that 

 the pupil was at sea, and wanted to find the 

 longitude of the place he was in : he chooses some 

 remarkable fixed star, whose name and situation 

 are known, and finds with a quadrant the angu- 

 lar distance between that star and the moon ; 

 and by a watch, previously regulated for that 

 purpose, the exact time when the observation was 

 made : this being done, he looks into the alma- 

 nac, and finds what time it is at Greenwich when 

 the moon and star have the same distance ; and 

 this time, being compared with the time of obser- 

 vation, will, by allowing 15 degrees to an hour, 

 give the longitude of the place required. The 

 names and places of the brightest fixed stars are 

 to be found in the " Tables requisite to be used 



