MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Schematic showing how the position of the Earth around the Sun gives rise to the seasons. The Earth's spin axis is tilted by an angle of 25 degrees. Summer occurs in the northern hemisphere when, because of this tilt, the planet's north pole is pointed most directly at the Sun. Northern winter occurs when exactly the opposite is true: the south pole is pointed at the Sun, so it is summer in the south and winter in the north. Spring and autumn occur at intermediate positions in the orbit.
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