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Weather Fundamentals

The Seasons

How does the motion of Earth around the Sun cause the seasons to occur?

Some people mistakenly think that the seasons are caused by Earth being farther from the Sun in winter and closer to the Sun in summer. This is incorrect; Earth’s elliptical orbit is close enough to a perfect circle that distance is not the reason. In fact, Earth is closest to the Sun in early January and farthest in early July, which is exactly the opposite of our summer and winter seasons. The reason for the seasons has to do with the angle at which sunlight strikes any particular place on Earth at any given time of year. The angle changes throughout the year because the tilt of Earth’s axis differs from the ecliptic. Since the Earth is tilted 23.5 degrees, the Sun’s rays hit the northern and southern hemispheres unequally. When the Sun’s rays hit one hemisphere directly, the other hemisphere receives diffused rays. The hemisphere that receives the direct rays of the Sun experiences summer; the hemisphere that receives the diffused rays experiences winter. Thus, when it is summer in North America, it is winter in most of South America, and vice versa.



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