What Causes the Seasons to Occur?
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Axis Tilt
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The Greeks believed that the seasons were caused by the goddess Demeter, queen of the natural world, who pined every winter for her missing daughter. These days, we have a few more facts to go on than they did. The Earth tilts on its rotational axis at an angle of 22.5 degrees. That means that the points on which it rotates as it moves through each day, the North and South poles, are at an uneven angle in relation to the sun, rather than a parallel or perpendicular angle. You can see this when you look outside your window at different times of the year: The sun takes a different path across the sky in June than it does in December.
Seasons
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As a result of the Earth's axial tilt, different portions of the Earth receive different amounts of sunlight at different times of the year. The Northern Hemisphere, containing most of Europe, Asia, and North America, receives more sunlight in the summer months of July, August and September than the winter months of December, January and February. As a result, the summer months are hotter and the days last longer, while the winter months are shorter and colder. In the areas of the globe furthest north, some winter days get no sunlight at all. The process is completely reversed in the Southern Hemisphere, which contains Australia and parts of Africa, Asia and South America. While the Northern Hemisphere roasts in July and August, the Southern Hemisphere experiences cold, dark days and have "summer" when the Northern Hemisphere is building snowmen and celebrating Christmas. Seasons are thus flip-flopped on the top and bottom halves of the globe, though both are caused by the same phenomenon of axial tilt.
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Ramifications
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The regularity of the seasons, caused by the Earth's yearly trip around the sun, have allowed mankind to establish accurate calendars and also help explain why certain dates are where they are. The summer solstice, or first day of summer, which is in late June in the Northern Hemisphere, represents the point when the Northern Hemisphere receives the most amount of sunlight at any point during the year. The winter solstice, which is shortly before Christmas in the Northern Hemisphere, is the point where the Northern Hemisphere receives the least amount of sunlight. The situation is reversed with the Southern Hemisphere. Indeed, the Southern Hemisphere actually receives more sunlight during its summer in December, when the Earth is closer to the sun, than the Northern Hemisphere does during its summer in June.
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