39] 



Hipparchus 



45 



a star across the same place, and thus to ascertain the 

 angular distance between them (the celestial sphere being 

 known to turn through 15 in an hour), a method which 

 with modern clocks is extremely accurate, but with the 

 rough water-clocks or sand-glasses of former times was very 

 uncertain. In another method the moon was used as a 

 connecting link between sun and stars, her position relative 



FIG. 1 8. The position of the sun's apogee. 



to the latter being observed by night, and with respect to 

 the former by day ; but owing to the rapid motion of the 

 moon in the interval between the two observations, this 

 method also was not susceptible of much accuracy. 



/In the case of the particular problem of the deter- 

 mination of the line of apses, Hipparchus made use of 

 another method, and his skill is shewn in a striking manner 

 by his recognition that both the eccentricity and position 

 of the apse line could be determined from a knowledge of 



