Owing to the rotation of the earth, the windsdo not blow due north and south, but are deflected. From the highpressure belts the air flows outwards to the regions oflow pressure. One of the main reasons for differences inpressure is unequal heating of the air. Similar conditionsare experienced in the Southern Hemisphere and theSouth Pole between September 25 and March 21. BetweenMarch 21 and September 23, when the North Pole is tiltedtowards the Sun, the days are longer than the nightsthroughout the Northern Hemisphere and there iscontinuous daylight at the North Pole. At the Equator itselfdays and nights are equal throughout the year. On these dates, except at the Poles,(a) days and nights are equal all over the world and (b)the Sun rises exactly due east and set exactly due west atall places on the Earths surface. On March 21 (Vernal Equinox)and September 23 (Autumnal Equinox) the Sun is over-head at the Equator. The inclinationof the Earths axis together with its revolution round theSun is the cause of the varying length of day and night indifferent parts of the world. There are two other lines, namely, Tropic ofCancer (23O N) and the Tropic of Capricon (23O S).The word tropic means, turning place. The Equator is animaginary line drawn round the Earth midway betweenthe Poles. Position in the course of its revolution about the Sun,and to the inclination of its axis.
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