286 FERGUSON'S LECTURES. 



LECT. Capricorn every day, for half a year ; in the middle of 

 wjlich nalt > he crosses tne equinoctial at the beginning 

 of Aries : and at the <! of it he arrives at Cancer. 



The sun's motion in the ecliptic is not perfectly equa- 

 ble, for he continues eight days longer in the northern 

 half of the ecliptic, than in the southern : so that the 

 summer half year, in the northern hemisphere, is eight 

 days longer than the winter half year ; and the contrary 

 in the southern hemisphere. 74 



Tropic*. The tropics are lesser circles in the heaven, parallel 

 to the equinoctial ; one on each side of : t, touching the 

 ecliptic in the points of its greatest declination ; so 

 that each tropic is 231 degrees from the equinoctial, one 

 on the north side of it, and feae other on the south. 

 The northern tropic ouches the ecliptic at the begin 

 ning of Cancel, the southern at the beginning of Capri- 

 corn ; for which reason the former is called the tropic of 

 Cancer, and the latter the tropic of Capricorn. 



Polar r- The polar drdto in the heaven, are each 23J degrees 

 from the poles, all around. That which goes round the 

 north pole, is called the arctic circle, from O.OKTOS, which 

 signifies a bear; there being a collection or groupe of 

 stars near the north pole, which goes by that name 

 The south polar circle, is called the antarctic circle, from 

 its being opposite to the arctic. 



The ecliptic, tropics, and polar circles, are drawn 

 upon the terrestrial globe, as well as upon the celestial. 

 But the ecliptic, being a great circle fixed in the heavens, 

 cannot properly be said to belong to the terrestrial 

 globe ; and is laid down upon it only for the conveni- 

 ence of solving some problems. So that, if this circle 

 on the terrestrial globe was properly divided into the 

 months and days of the year, it would not only suit the 



Note 74. The term tropic is derived from a Greek word, which 

 signifies to turn, as the sun recedes from these circles immediate!) 

 after the longest day in the summer, and the shortest in winter. 



