202 CAUSE OF THE EQUINOXES. 



illuminated hemisphere, always diametrically opposite to the 

 darkened hemisphere. 



This being thoroughly understood, let us place ourselves 

 in the equatorial plane, so as to embrace at a single glance 

 a quarter of the illuminated and a quarter of the darkened 

 hemisphere. If the equator of the photo-adumbrated sphere 

 perpetually coincided with the terrestrial equator, if, in other 

 words, the earth, in revolving round the sun, invariably occu- 

 pied the plane of the Equator, which, when prolonged, would 

 pass through the centre of the illuminating orb, the diurnal 

 rotation would not cease to divide equally the light and the 

 darkness over the earth's surface, as is shown in Fig. 42 a, 



a b 



FIG. 42. S, the Sun ; EE, the Equator ; N, North Pole ; S, South Pole. 



where S indicates the sun, EE the terrestrial equator, and 

 N S the extremities (or North and South Poles) of the axis 

 which divides the globe into an illuminated and a darkened 

 half. This phenomenon of coincidence exists ; but only for 

 a very brief period, and is only repeated twice a year, that 

 is, at the equinoxes. At all other times, the equator of the 

 photo-adumbrated sphere, in whose plane the orb of light is 

 situated, passes sometimes above and sometimes below the 

 terrestrial equator. 



But this alternate movement of northern and southern declina- 



