44 -A Short History of Astronomy [Ca. 11. 



times of year by quantities so small that they might fairly 

 be attributed to errors of observation. 



This problem was much more difficult than might at first 

 sight appear, on account of the great difficulty experienced 

 in Greek times and long afterwards in getting satisfactory 

 observations of the sun. As the sun and stars are not 

 visible at the same time, it is not possible to measure 

 directly the distance of the sun from neighbouring stars 

 and so to fix its place on the celestial sphere. But it 



FIG. 17. The eccentric. 



is possible, by measuring the length of the shadow cast by 

 a rod at midday, to ascertain with fair accuracy the height 

 of the sun above the horizon, and hence to deduce its 

 distance from the equator, or the declination (figs. 3, 14). 

 This one quantity does not suffice to fix the sun's position, 

 but if also the sun's right ascension ( 33), or its distance 

 east and- west from the stars, can be accurately ascertained, 

 its place on the celestial sphere is completely determined. 

 The methods available for determining this second quantity 

 were, however, very imperfect. One method was to note 

 the time between the passage of the sun across some fixed 

 position in the sky (e.g. the meridian), and the passage of 



