What causes seasons? (Beginner)

I am curious as to why there are seasons.

Seasons occur on the Earth because of the tilt of the Earth's axis. Because the Earth's rotation axis is not perpendicular to the Earth's orbital plane around the Sun, the Sun sometimes shines directly over the nothern hemisphere, and other times shines directly over the southern hemisphere. For instance, the Sun reaches its most northerly position (the Tropic of Cancer) on June 21. This is called the "summer solstice." If you were standing on the tropic of cancer on June 21, the Sun would be directly overhead at noon. This is the first day of summer in the northern hemisphere, and the first day of winter in the southern hemisphere. Because the Sun is shining more directly in the north than in the south, the weather is warmer in the north in June. In December, the opposite happens. The Sun is over the Tropic of Capricorn, and is shining directly at the southern hemisphere. So December is summer in Australia and Brazil, but winter in America, Europe, and Asia.

Some people believe that summer is when the Earth is closest to the Sun. This is not true. In fact, the Earth is farthest from the Sun in early July, and nearest the sun in January. Because these differences in distance are so small however, they hardly affect Earth's seasons at all. On Mars, and distant planets and their moons, however, seasons may be largely affected by their distance from the Sun, an added complication for those modelling atmospheres on Saturn, Uranus, Neptune, Titan and Pluto! On these planets, moons and dwarf planets, the perihelion and aphelion distance can vary by many A.U., where on Earth the differential distance is comparatively negligible.

So when you ask about seasons, know that many places in the solar system have seasons, and that not all seasons are driven by the same process(es)!


This page was last updated on Jan 28, 2019.

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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