ioo POPULAR LECTURES AND ADDRESSES. 



any moment of Greenwich time, more accurately 

 than it can be observed at sea, and almost as 

 accurately as it can be observed in a fixed ob- 

 servatory on shore. Hence the error of the clock 

 is known more exactly than we can read its 

 indications at sea, and the accuracy with which 

 we can find the Greenwich time by it, is practically 

 limited by the accuracy with which we can ob- 

 serve the moon's place relatively to the sun, 

 planet, or star. This, unhappily, is very rough 

 in comparison with what is wanted for navigation. 

 The moon performs her orbital revolution in 

 27-321 days, and, therefore, moves at an average 

 rate of o'55 P er hour, or '55 of a minute of angle 

 per minute of time. Hence to get the Greenwich 

 time correctly to one minute of time, or longitude 

 within 15 minutes of angle, it is necessary to 

 observe the moon's position accurately to half 

 a minute of angle. This can be done, but it is 

 about the most that can be done in the way of 

 accuracy at sea. It is done, of course, by measur- 

 ing, by the sextant, the angular distance of the 

 moon from a star, as nearly as may be in the 



