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 23.5 degrees. Summer begins in the northern hemisphere when, because of this tilt, the planet's north pole is pointed most directly at the Sun (summer solstice). Winter occurs when exactly the opposite: the south pole is pointed at the Sun, so it is summer in the south and winter in the north (winter solstice). Spring and autumn occur at intermediate positions in the orbit, known as the equinoxes.
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