COMMON THINGS THE ALMANACK. 



20th March to 23rd Sept. 

 March . 



June . 

 July 

 August 

 September 



Total . 



1S7 i 



Total . 



. 178 



It appears, therefore, that the one exceeds the other by 

 nine days. 



Now, since in each interval the sun moves over the same space 

 of the heavens, i. e. 180, it follows, that its mean motion during 

 the winter half-year must be faster than during the summer half- 

 year, in the proportion of 187 to 178. 



To explain this fact, it will be necessary to refer to the motion 

 of the earth round the sun, which is the cause producing the 

 apparent annual motion of the sun round the heavens. 



The orbit or path which the earth follows in its course round 

 the sun is not circular, but slightly oval. It may be supposed to 

 be represented by P 3 o p' 9 c, fig. 5, s being the place of the sun 

 nearer to one end, P 2 , of the oval than to the other end, p' . The 

 Fig. 5. s P ee d with which the 



earth would move if its 

 path were a circle, with 

 the sun in the centre, 

 would be uniform ; but 

 in the oval, its distance 

 from the sun varying, 

 its speed will also vary, 

 being greater at less, and 

 less at greater distances. 

 Thus, its speed at P 3 , 

 where it is nearest the 

 sun, is greatest, and at 

 p' 3 , where it is most 

 remote from the sun, 

 least. The speed decreases continually while the earth moves 

 from P 3 to p' 3 , and increases continually while it moves from 

 P', toP 3 . If the oval be divided into equal parts by the line 

 P 3 s p / 3 , the times of moving through each half of it will be 

 equal ; and if it had so happened that the earth should be at 

 these two points, P 3 and p' a , on the days of the equinoxes, then 

 the summer and winter half-years would be exactly equal. 

 But such is not the case. The earth, on the contrary, in 

 40 



