Seasons happen because Earth's axis stays tilted as the planet travels around the Sun.

The longer version

Axial tilt stays fixed in direction as Earth circles the Sun, so each hemisphere leans toward the Sun for part of the year and away for the rest. Leaning toward the Sun brings higher noon angles and longer days, which delivers more energy per unit of ground. Both hemispheres receive the same total sunlight over a full year.

The atmosphere and oceans smooth out the response, so the warmest weeks usually lag the peak sunlight. Near the equator the seasonal swing is small, while the poles endure months of continuous daylight and darkness. Distance from the Sun plays only a minor part.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.