Why we have seasons

The common mistake

Many people assume summer happens when Earth is closer to the Sun. It sounds reasonable, but it is wrong. Earth's orbit is nearly a circle, and the gap between its closest and farthest points is only about 3 percent.

The timing gives the game away. Earth reaches perihelion, its closest point to the Sun, in early January. That is the middle of winter in Europe, North America and most of Asia. If distance ruled the seasons, the whole planet would have summer at the same time, and it does not.

The real reason: a tilted axis

Earth spins around an imaginary line called its axis, and that axis is tipped about 23.4° from straight up relative to the plane of its orbit. The tilt points in nearly the same direction in space all year, toward a spot close to Polaris, the North Star.

As Earth travels around the Sun, this fixed tilt means each hemisphere takes turns leaning toward the Sun. Around June the Northern Hemisphere leans sunward and has summer. Six months later, on the other side of the orbit, the Southern Hemisphere gets its turn.

Why leaning toward the Sun makes it warmer

Two effects work together. First, when your hemisphere leans toward the Sun, the Sun rides higher in the midday sky. High-angle sunlight hits the ground more directly, so the same beam of energy is spread over a smaller patch of ground. Low winter sunlight arrives at a slant and is spread thin, a bit like a flashlight beam tilted across a table.

Second, the days are longer. More hours of sunlight and fewer hours of darkness mean the ground and air gain more heat than they lose. Oceans and land take time to warm up and cool down, which is why the hottest weeks usually come a month or so after the June solstice, not on it.

Solstices and equinoxes

A solstice is the moment when the Sun reaches its farthest point north or south in our sky. The June solstice, on June 20 or 21, brings the longest day of the year in the north and the shortest in the south. The December solstice, on December 21 or 22, does the reverse.

An equinox is the moment when the Sun crosses the celestial equator, the projection of Earth's equator onto the sky. This happens around March 20 and September 22 or 23. On these dates the Sun rises nearly due east and sets nearly due west everywhere on Earth.

The dates shift by a day or so from year to year because our calendar year does not divide evenly into Earth's orbit. Leap years pull the dates back into line.

Day length through the year

Day length changes most dramatically far from the equator. At Istanbul, about 41° north, the June solstice brings roughly 15 hours of daylight and the December solstice about 9 hours 15 minutes. Near the equator, days stay close to 12 hours all year, so seasons there are defined more by rain than by temperature.

Inside the Arctic and Antarctic Circles, at about 66.6° latitude, the Sun can stay up for a full day or more in summer and stay below the horizon in winter. That is the midnight Sun and the polar night.

A small surprise: even on an equinox, daylight is a few minutes longer than 12 hours. Earth's atmosphere bends sunlight so we see the Sun slightly before it truly rises and after it truly sets, and sunrise is counted from the top edge of the Sun, not its center.

Opposite seasons in each hemisphere

Because the hemispheres take turns leaning toward the Sun, their seasons are always opposite. When it is summer in Türkiye, it is winter in Australia, Argentina and South Africa. Christmas in Sydney is a beach holiday.

This also flips the names. The June solstice is often called the summer solstice, but only in the north. Astronomers prefer to name solstices and equinoxes by month so the terms work for everyone.

See it for yourself

You can watch the seasons change without any equipment. Note where the Sun sets on your horizon once a month, using a tree or building as a marker. Over a year the sunset point swings north toward June and south toward December, then pauses near each solstice.

You can also measure the length of a stick's shadow at local noon. The shadow is shortest near the June solstice and longest near the December solstice in the Northern Hemisphere. Never look directly at the Sun while doing this; watch the shadow, not the Sun. If you ever want to look at the Sun itself, use only a certified solar filter that meets ISO 12312-2.