Why seasons happen

On 3 January 2026 Earth was 147.1 million km from the Sun, about 5 million km closer than in July, and it was winter in Seoul, London and New York.

The common wrong answer

Ask a group of people why summer is warm and many will say that Earth is closer to the Sun in summer. It sounds sensible, since a fire feels hotter when you step closer. The numbers say otherwise. Earth's orbit is very nearly a circle. In 2026 the closest point, called perihelion, came on 3 January at about 147.1 million km, and the farthest point, aphelion, came on 6 July at about 152.1 million km. The north was coldest when Earth was closest.

There is a second clue. If distance caused the seasons, the whole planet would have summer at the same time. Instead, December is summer in Sydney and Buenos Aires while it is winter in Seoul. Something must treat the two halves of Earth differently, and that something is the tilt of Earth's axis.

A tilted axis that keeps its direction

Earth spins around an axis through the North and South Poles, and that axis is tilted 23.44° from upright compared with the plane of the orbit. The important part is that the axis keeps pointing the same way in space, towards the North Star, all year. Watch the model while you play through a year. In June the north end of the axis leans towards the Sun; six months later, on the other side of the orbit, the same north end leans away from it. Nothing about the Earth changed. Only its position around the Sun did.

When your hemisphere leans towards the Sun, two things happen together. The Sun climbs higher in the sky at noon, and it stays up longer, because a larger part of your circle of latitude is on the daylit side. In the model, the yellow arc on the chosen latitude is daytime and the blue arc is night. At 37° N on the June solstice the yellow part is clearly longer, and the readout gives 14 hours 42 minutes of daylight. On the December solstice it shrinks to 9 hours 37 minutes.

Why a high Sun heats more

Shine a torch straight down on a table and you get a small, bright circle. Tilt the torch and the same light spreads into a long, dim oval. Sunlight works the same way. A beam that arrives from straight overhead lands on its own width of ground; a beam that arrives at 30° above the horizon is spread over twice as much ground, because 1 ÷ sin 30° = 2. Each square metre then gets half the energy.

At 37.5° N the noon Sun stands about 75.9° high on the June solstice and only about 29.1° high on the December solstice. The ratio sin 75.9° ÷ sin 29.1° is about 2.0, so at noon each square metre of ground receives about twice as much sunlight in June as in December, and for about 5 hours longer each day. Those two effects together are the seasons. The change in distance is real but small: sunlight at perihelion is only about 7% stronger than at aphelion, because (152.1 ÷ 147.1)² ≈ 1.07.