SUN AND CLIMATIC VARIATION 41 



cuts the surface along a great circle, is called the equator. The 

 earth, moreover, moves round the sun and describes an ellipse, 

 of which it occupies one of the foci ; the time required for 

 this journey constituting the unit of measurement is called a 

 year. The ellipse described by the earth is called its orbit, and 

 the plane of the orbit the ecliptic. If the earth's axis of rotation 

 were perpendicular to the ecliptic the equator would be 

 situated in this plane, and every part of the globe would then 

 always be illuminated for exactly one half the time occupied 

 by the rotation. The term day is usually limited in its 

 application in ordinary language, and refers not to the 

 astronomical day but to the fraction of that day during which 

 a given place is illuminated. On the hypothesis of a coincidence 

 of the plane of the equator with that of the ecliptic the days on 

 the earth's surface would be equal to the nights. But this is not 

 so. The two planes indicated actually form an angle of 

 23 27' 21". Now it can easily be shown by simple geometrical 

 methods, that on account of this inclination, those parts of 

 the earth, which are situated on the equator are the only ones 

 where the days and nights are of exactly the same length 

 throughout the year. At either Pole, on the other hand, it is 

 night for six months, and also day for six months, the day of 

 one of the Poles coinciding with the night of the other, and 

 conversely. 



For all points of the earth situated on a small circle whose 

 distance from the Pole can be measured by an arc 23 27' 21", 

 there is a consecutive day and night period of 24 hours at the 

 moment when the plane of the terrestrial axis and the 

 perpendicular which passes through the centre of the earth 

 also passes through the centre of the sun. That moment is 

 called the solstice. Polar circles is the name given to the small 

 circles that fulfil these conditions for either Pole. For all 

 points situated between these circles and the Poles, the 

 duration of the day and the night is longer than that of an 

 astronomical day and all these points are situated within the 

 frigid zones. On each side of the equator, a small circle, also 

 situated at a distance of 23 27' 21" from the equator, marks 

 off the region where every point sees the sun twice a year 

 exactly vertical, i.e. at its zenith, that being the moment in 

 which the days and nights in the arctic circles are equal to the 

 astronomical day. This is the torrid zone, and the small circles 



