EARTH'S AXIS AND MOTIONS. 23 



the distances are wonderful ; almost as wonderful as 

 the eye that can see them, and the mind that can mea- 

 sure the distance beyond which the nearest of them 

 must be situated. 



As the position of the earth's axis has its own posi- 

 tion parallel in all points, and forms, with the largest 

 diameter of the orbit, an angle different from a right 

 angle, the position of the axis with regard to the line 

 joining the sun and earth, which line, from centre to 

 centre of course, is the middle of the illuminated half 

 of the earth, must be continually changing. When it 

 is in any extremity of the larger diameter, the one 

 pole must be 23 28' (say 23 less than a right 

 angle from the line joining the centres of the earth 

 and sun, and the other pole must be 23^ more 

 than a right angle), therefore the angular difference 

 will be 47. Consequently 47 round the one pole, 

 that is a segment measuring 47 across, and hav- 

 ing the one pole in the centre, must be illuminated 

 during the whole revolution ; an equal portion round 

 the other pole must be without illumination ; and the 

 parallel of greatest illumination must be 23 nearer the 

 illuminated pole than the parallel midway between the 

 poles. 



But, as the earth rolls on, the angular difference of 

 its two poles, with reference to the line joining the 

 centres, must gradually diminish; along with it the 

 illuminated portion round the one pole, and the dark 

 one round the other, must diminish also ; and the pa- 

 rallel of greatest illumination must gradually approach 

 the parallel midway between the poles. When one 



