TimeAndSpace.Science
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Seasons

Why do we have seasons?

Not because Earth gets closer to the Sun. When it is summer in the United States it is winter in Australia, on the same planet, at the same distance. Seasons happen because the axis is tilted 23.4° and keeps pointing the same way all year.

Sunlight, arriving parallelNSsun overhead hereTropic of CancerEquatorTropic of Capricorn
The northern half leaning into the light: longer days, higher Sun, summer. Half a year later the same picture upside down.

The test that kills the distance story

Earth is actually closest to the Sun in early January — northern winter. If distance ran the seasons, the whole planet would warm and cool together. It does not. One hemisphere’s summer is the other’s winter. The cause has to be a direction, not a distance.

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Two effects, one lean

The lean does two things at once: it makes the day longer, and it raises the noon Sun. More hours, and each hour more concentrated on the ground. Neither is a change in how much light the Sun puts out.

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On the hub: Day and night map · Sun–Earth–Moon simulator · Big questions

Related questions

What is Earth’s axial tilt? Earth’s spin axis is not upright. It leans about 23.4° from the vertical to its orbit. That lean is why we have tropics, polar circles, and seasons — and it barely changes from one year to the next.

What is the Tropic of Cancer? It is the circle at 23.4° north. The Sun is straight overhead there once a year, on the June solstice, and never north of it. That number is Earth’s tilt, not a climate zone someone drew.

What are the polar circles? The Arctic Circle and Antarctic Circle sit at 66.6° north and south — 90° minus Earth’s tilt. Inside them the Sun can stay up all day in summer, or stay down all day in winter.

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