Why We Have Seasons Seasons are produced by the inclination of the earth's axis of rotation, with the yearly revolution around the sun. During the earth's yearly revolution around the sun, the northern and southern hemispheres alternate their incline towards and away from the sun. This unequal heating of the earth's surface causes the weather to change in approximately three-month periods. When the Northern Hemisphere is inclined toward the sun from about March 21st to September 23rd the earths surface is heated causing the seasons of spring and summer to occur. Meanwhile in the Southern Hemisphere the effects of fall and winter occur. The first three months are the season of spring and th