Why we have seasons

Ask a room why summer is hot and most hands say the same thing: Earth is closer to the Sun. It is a tidy answer, and it is wrong. Earth is closest to the Sun in January.

The wrong answer first

Earth's orbit is an ellipse, so its distance from the Sun does change through the year. But it changes by only about 3%, far too little to swing the Sun's heating. Worse for the idea, the closest approach falls in January, in the middle of the northern winter. And if distance were the cause, both hemispheres would have summer at the same time. They do not: December is winter in New York and summer in Chile.

Why it matters

The real cause is a single fixed angle, and once you have it, the whole calendar falls out: the solstices, the equinoxes, the Arctic midnight Sun, and the fact that the equator barely has seasons at all.

One tilt, two effects

Earth spins on an axis, the line through the North and South Poles. That axis is tilted by 23.5° from the perpendicular to its orbit, and it keeps pointing the same way in space all year. So as Earth travels around the Sun, in June the Northern Hemisphere leans toward the Sun; in December it leans away; in March and September Earth leans sideways and neither hemisphere is favoured.

Leaning in changes two things at once. The first is the angle of the light. Shine a flashlight straight at a wall and you get a small, bright spot. Tip the beam and the same light spreads over a larger patch, so each part of the wall gets less. In June the Sun stands high in the northern sky, its rays land nearly straight down, and each square metre of ground receives more energy. In December the Sun is low, the same rays smear across a wider area, and the ground heats less.

The second is time. Because the axis is tilted, the Sun in June sits north of the celestial equator, the sky's projection of Earth's equator, and stays above the horizon longer. In the United States it is up for about 15 hours on June 21 and about 9 hours on December 21. So in summer the Sun heats more directly, and it has more hours each day to do it. Both effects run the same direction, and both flip sign for the other hemisphere.

The extremes are fixed by the tilt. On June 21 the Sun is overhead at noon at 23° N, the Tropic of Cancer, and everywhere within 23° of the North Pole, north of 67° N, the Sun never sets. Six months later the roles swap. Near the equator the Sun is up close to 12 hours every day of the year, so the seasons there are marked by rain, not sunlight. A smaller tilt would shrink all of these: at 0° the tropics would sit on the equator and every day would be an equinox.

One last detail: the longest day is not the hottest. Land, air and ocean take weeks to warm, so the northern peak comes in July and August, and the coldest weeks arrive a month or more after the December solstice.

In short

Seasons come from the 23.5° tilt of Earth's axis, not from distance. The hemisphere leaning toward the Sun gets steeper sunlight and longer days; the other gets the opposite. Distance to the Sun changes by only about 3%, and Earth is nearest in January.