42 



later some difficulties in the wiy of exactly determining 

 our time from the sun. But while he is seldom used in 

 the Observatory for such a purpose, an exact knowledge 

 of his motion, and some exact way of measuring his posi- 

 tion, is of the utmost importance to the navigators ; and 

 I suppose it is to them we owe the wide-spread opinion 

 that from observations of the sun, all astronomers deter- 

 mine the time. 



Brilliant even through a haze, easily observed even 

 when the ship's deck is rolling, the navigator readily 

 brings the reflected sun and the horizon in contact with 

 the telescope of his sextant. From his tables, with an 

 approximate knowledge of his latitude, he will tell you at 

 just what time the sun will reach a certain altitude, and 

 this will afford him a means of detecting the error of his 

 chronometer on local time. 



But we are chiefly to concern ourselves to-night with 

 the instruments used in fixed observatories for determin- 

 ing time. You are aware that the stars are located on 

 the celestial sphere by a system of coordinates, closely 

 resembling our terrestrial ones of latitude and longitude ; 

 only when applied to the heavens these terms are changed 

 to the more technical ones of Declination and Right Ascen- 

 sion. Now Declinations the astronomer measures with 

 carefully graduated circles, but in measuring Right Ascen- 

 sions the astronomer fixes his instrument in one plane, 

 and notes by his clock, how long after one star passes this 

 plane, another follows it. But he must be able to meas- 

 ure this interval of time with a degree of accuracy which 

 corresponds to the accuracy reached with the graduated 

 circle. Hence the Observatory continues to be the recog- 

 nized critic of the performance of time-pieces, for nowhere 

 else in the arts or sciences is the exact measurement of 

 considerable intervals of time of such vital importance. 



