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The earth spins on an axis that
is tilted 23.3¡ off of its orbit around the sun.
 The above diagram represents the earth and sun on December 21, the winter solstice. On this day, the sun is directly over the Tropic of Capricorn (the pink line). The southern hemisphere has the longest amount of daylight (and thus the shortest night) on this day. On June 21, the effect is reversed. On an Equinox, March 21 and September 22, the earth reaches the half-way point between these two extremes, thus the sun is directly over the equator, and the day and night are of equal lengths on the entire earth. The amount of sunlight on a given hemisphere is the cause of the seasons. |