DAY AT THE POLES. 201 



value as the former, corresponds to the winter solstice 

 (21-22 December). This movement, which is annual, com- 

 plicates itself with the diurnal. It is rendered visible by the 

 displacement of the polar shroud of snow : the portion which, 

 at the moment of the spring equinox, belonged, in the 

 northern hemisphere, to the obscured hemisphere, moves 

 onward, as a result of the inclination of its axis, to become 

 an integral part of the illuminated hemisphere ; while, at the 

 same time, the portion which, in the southern hemisphere, 

 belonged to the illuminated hemisphere, moves onward to 

 become an integral part of the darkened hemisphere. Owing 

 to this displacement, the sun shows itself for six consecutive 

 months above the horizon, for the North Pole ; at the spring 

 equinox it begins to rise, at the summer solstice it attains 

 its maximum elevation, and from the summer solstice it 

 begins to decline. Thus, then, we have in reality a day six 

 months long, of which the morning and the evening are the 

 two equinoxes, its noon the summer solstice. 



In the same periods an exactly opposite order of things 

 prevails in the southern hemisphere. 



Everything, even to the minutest detail, is in this way very 

 clearly explained. Two facts like the touches of a painter's 

 brush suffice to impress the whole upon the mind, namely : 



That, first, one half of the terrestrial surface is constantly 

 illuminated by the sun, while the other half remains in dark- 

 ness; 



That, secondly, the rotation of the earth upon its axis 

 produces day and night, by carrying from east to west the 



