ANNUAL MOTIONS. 41 



that is, the tropical, or solstitial points. [They get the 

 name tropical, because at them the sun appears to turn 

 back, and shorten the days that were till then lengthen- 

 ing ; and the name solstitial, because, in consequence of 

 what has been already mentioned, a number of days at 

 those seasons are nearly of equal length.] Those points 

 are at 23 28' north latitude, and the same south. Those 

 points are not fixed ; they have a trifling variation in 

 latitude ; and because the year contains nearly a frac- 

 tional fourth of a day, the actual point will apparently 

 be nearly one fourth of the circumference in longitude 

 further west every year than in the preceding one. 

 There is a beauty and advantage even in that, as it 

 secures an annual distribution of solar heat always 

 over a new succession of lines. None of the motions 

 in the solar system are, indeed, free of fractions when 

 measured in terms of the others; and those fractions, 

 which, to one that considers only the particular case, 

 would be very apt to appear as imperfections, are pro- 

 ductive of the most beautiful and important results. 

 Even in the two portions into which the year is divided, 

 there is an inequality necessarily arising from the 

 elliptic form of the earth's orbit. The line which divides 

 it into those two portions passes through the centre of 

 the sun, at right angles to the larger diameter, and 

 therefore the portion through which the earth passes, in 

 the summer half of the year to the northern hemisphere, 

 is about three days longer in absolute time than the 

 opposite portion. This throws the parallel of the great- 

 est annual action of the sun a little to the north of the 

 equator; and as the northern hemisphere has, in 

 consequence, more summer and less winter than the 

 K 2 



