After having turned round 366 times on its axis, our planet has terminated its orbicular course, and is in the same position relatively to the sun as at its starting-point; it has then accomplished its _year_. This daily rotation of the earth round its axis produces the succession of days and nights, and, in the same way, its annual revolution round the sun causes the alternations of the seasons. If the axis of the earth, that is the ideal line which passes through its two poles, were perpendicular to the plane of its annual orbit, it is evident that the portion of the globe lighted by the sun would invariably extend from one pole to the other, and that in both hemispheres the days and nights would always consist of twelve hours each. But this is not the case. The earth performs its revolutionary movements in an inclined position; its ideal polar axis is sloped about 23° 28′ from a perpendicular to its plane, and this position is so far maintained that as regards the comparatively rapid succession of days and seasons it may be looked upon as invariable. This obliquity of axis causes continued changes in the phase presented to the sun. The portion of the earth illumined by the rays of the sun varies every day; for, although the planetary axis may appear to maintain its extremity in a fixed position as regards some point in infinite space, in respect to the sun it presents a constantly varying degree of inclination, in consequence of the continual motion of the earth. Twice during the course of the year it so happens that the solar rays fall perpendicularly upon the equator of the earth; at every other period in the annual revolution, sometimes the Northern and sometimes the Southern Hemisphere receives the greatest amount of light.