42 FORMATION OF THE EARTH 



limiting it on each side of the equator are the tropics, or as 

 they are sometimes called, the inter-tropical zone. Between 

 the tropics and the Polar circles extend the temperate zone, 

 where the duration of light and darkness is always less than one 

 terrestrial revolution and where the sun never reaches its 

 zenith. 



At the particular moment in which the plane of projection 

 of the earth's axis to that of the ecliptic is perpendicular to 

 the line joining the centre of the earth to the centre of the sun, 

 the days and nights are equal at all points of the globe, and this 

 is called the equinox. From this moment onwards, we have 

 inequality between the duration of day and night in both 

 hemispheres, the nights becoming longer in one and shorter 

 in the other. In one case we are passing from autumn to winter, 

 and in the other from spring to summer. The culminating point 

 of the hot season, which we call summer, coincides with that 

 particular instant, which lies half-way between the two 

 equinoxes, that is to say, at the summer solstice, and the same 

 holds for the winter solstice. In our hemisphere the hot season 

 coincides with the period in which the earth is approaching 

 the height of its orbit furthest from the sun. It is the duration 

 of the day and not the proximity of the sun which raises the 

 temperature. As the length of the day is the same in the two 

 hemispheres, the summer of the southern hemisphere is a little 

 hotter than in the northern, the earth then being nearer the 

 sun than it is during our summer. The year is consequently 

 divided into four seasons, spring, summer, autumn, and 

 winter. 



These facts are not as definitely fixed as might at first be 

 supposed. The terrestrial axis does not remain parallel to 

 itself. It describes a cone with a sinuous contour round a line 

 perpendicular to the ecliptic, whilst the orbit itself is revolving 

 in its own plane. These combined movements cause the line 

 of equinoxes passing through the centre of the orbit to turn, 

 in the plane of this orbit, 62" per year, just as if it were going to 

 meet the earth, thus gradually advancing the period of the 

 equinoxes. The procession of the equinoxes requires 21,000 

 years to make one complete rotation. The duration of the 

 seasons itself varies periodically, and under conditions at 

 present prevailing in our hemisphere the spring-summer 

 period lasts eight days longer than the autumn-winter period. 



