$ 82] The Seasons, according to Coppernicus 109 



beginnings of the four seasons, according to astronomical 

 reckoning) ; the circle F G H i in each of its positions 

 represents the equator of the earth, i.e. a great circle on 

 the earth the plane of which is perpendicular to the axis 

 of the earth and is consequently always parallel to the 

 celestial equator. This circle is not in the plane of 

 the ecliptic, but tilted up at an angle of 23 1, so that F 

 must always be supposed below and H above the plane of 

 the paper (which represents the ecliptic) ; the equator cuts 

 the ecliptic along G i. The diagram (in accordance with the 

 common custom in astronomical diagrams) represents the 

 various circles as seen from the north side of the equator 

 and ecliptic. When the earth is at A, the north pole (as is 

 shewn more clearly in fig. 42, in which p, p' denote the 

 north pole and south pole respectively) is turned away 



Paries Bores. 



Partes Auftrinae. 



FIG. 42. Coppernican explanation of the seasons. From the 

 De Revolutionibus. 



from the sun, E, which is on the lower or south side of the 

 plane of the equator, and consequently inhabitants of the 

 northern hemisphere see the sun for less than half the day, 

 while those on the southern hemisphere see the sun for more 

 than half the day, and those beyond the line K L (in fig. 42) 

 see the sun during the whole day. Three months later, 

 when the earth's centre is at B (fig. 41), the sun lies in the 

 plane of the equator, the poles of the earth are turned 

 neither towards nor away from the sun, but aside, and all 

 over the earth daylight lasts for 1 2 hours and night for an 

 equal time. Three months later still, when the earth's 

 centre is at c, the sun is above the plane of the equator, 

 and the inhabitants of the northern hemisphere see the 

 sun for more than half the day, those on the southern 

 hemisphere for less than half, while those in parts of the 

 earth farther north than the line M N (in fig. 42) see the 

 sun for the whole 24 hours. Finally, when, at the autumn 



