80 TJie Realm of Nature CHAP. 



reaches both poles simultaneously, and as the Earth rotates, 

 every place on the surface is lighted up for twelve hours 

 and plunged in darkness for the other twelve, day and night 

 being equal everywhere. This period is therefore called 

 the vernal or spring equinox, and happens at that point in 



FIG. 17. Diagram illustrating the cause of the seasons. 



the Earth's orbit from which the Sun appears projected on 

 the star-dome in the sign of Aries. This season is spring 

 in the northern and autumn in the southern hemisphere. 



122. Summer Solstice. In three months, the Earth 

 having advanced along one quarter of its path, the equator 

 dips 23^- S. of the plane of the ecliptic when viewed from 

 the Sun, hence the Sun viewed from the Earth appears at 

 noon in the zenith on the parallel of 23 * N. ; and as at 

 this time the Sun is projected on the star-dome in the 

 sign of Cancer, this parallel is called the Tropic of Cancer. 

 This is the highest northern latitude for a vertical Sun, and 

 is called a tropic because the Sun appears to turn south- 

 ward after reaching it. Sunlight reaches 23^- beyond the 

 north pole, and falls short of the south pole by 23^. As 

 the Earth rotates the whole region for 23.^ round the 

 north pole keeps in sight of the Sun, the whole region round 

 the south pole rests in darkness, and the period of daylight 

 diminishes while that of darkness increases over the world 



