Seasons are among the most familiar features of life on Earth, and yet the reason for them is often misunderstood.
The common picture is that summer happens when Earth is closer to the Sun and winter when it is farther away. This sounds reasonable at first, but it is not the real explanation. In fact, Earth is slightly closer to the Sun during Northern Hemisphere winter than during Northern Hemisphere summer. If distance were the cause, both hemispheres would have the same seasons at the same time. They clearly do not.
The real reason is the tilt of Earth’s rotation axis.
Earth spins around an axis that is tilted by about 23.5 degrees relative to the plane in which it orbits the Sun. As Earth moves around the Sun during the year, that axis keeps roughly the same orientation in space. The result is that for part of the year the Northern Hemisphere is tilted more toward the Sun, and for the opposite part of the year it is tilted away from it.
To see why this matters, one has to combine three things: Earth is nearly spherical, sunlight arrives in almost parallel rays, and Earth rotates once per day.
Because Earth is spherical, different places on its surface face the incoming sunlight at different angles. When the Sun is high in the sky, the same incoming sunlight is concentrated onto a smaller area of ground. When the Sun is lower, the same sunlight is spread over a larger area, so each square meter receives less energy. This is one of the key reasons summer is warmer than winter.
Earth’s daily rotation is important because it turns every location through sunlight and darkness. Without rotation, one side of Earth would simply face the Sun continuously and the other side would remain in darkness. What the axial tilt does is change how this daily exposure works over the year. In summer, your hemisphere is tilted so that, as Earth rotates, your location spends more of each day facing the Sun, and the Sun also climbs higher in the sky. In winter, your location spends less time facing the Sun, and the Sun stays lower.
This is why the seasons come from two effects acting together. In summer, sunlight is more direct and the days are longer. In winter, sunlight is less direct and the days are shorter. Spring and autumn are the intermediate cases, when neither hemisphere is strongly tilted toward or away from the Sun.
There is also a secondary effect. When the Sun is lower in the sky, its light passes through more atmosphere before reaching the ground, so more of it is scattered or absorbed along the way. But this is not the main cause of the seasons. The main cause is still the geometry of tilt, spherical surface, and daily rotation.
This also explains why the seasons are opposite in the two hemispheres. When the Northern Hemisphere is tilted toward the Sun and has summer, the Southern Hemisphere is tilted away and has winter. Half a year later, the roles reverse.
It is worth noticing that the oceans, land, and atmosphere smooth the seasons out. The longest day of the year is not usually the hottest day, and the shortest day is not usually the coldest. Earth takes time to warm up and cool down. That is why seasonal temperatures lag behind the astronomical turning points.
So the reason we have seasons is not that Earth moves much nearer to or farther from the Sun, but that Earth’s axis is tilted. That tilt changes both the angle at which sunlight reaches the ground and the amount of time each place spends in sunlight as Earth rotates.
See also: Why do things have color?