Every year, the world goes through a familiar rhythm. Days grow longer and warmer, trees fill with leaves, then the light shortens, temperatures fall and, in many places, snow arrives. Ask people why this happens and a surprising number will say that Earth is closer to the Sun in summer and farther away in winter. It sounds logical, but it is not the reason. In fact, Earth is closest to the Sun in early January, in the middle of winter for the Northern Hemisphere. The real cause of the seasons is something more elegant: the tilt of Earth's axis. This explainer shows how that tilt changes the length of the day and the angle of sunlight, and why that matters so much for temperature.
The distance myth
Earth's orbit around the Sun is not a perfect circle but a slightly stretched ellipse. At its closest point, called perihelion, which happens around January 3, Earth is about 91.4 million miles (147.1 million kilometers) from the Sun. At its farthest point, called aphelion, around July 4, it is about 94.5 million miles (152.1 million kilometers) away.
That difference is only about 3 percent. It does change the amount of sunlight reaching Earth slightly, by roughly 7 percent, but it is not enough to cause the seasons. Two simple facts prove it:
- If distance caused the seasons, the whole planet would have summer at the same time. In reality, when it is summer in the Northern Hemisphere, it is winter in the Southern Hemisphere, and vice versa.
- The Northern Hemisphere has winter when Earth is closest to the Sun.
So something else must be going on.
The tilt of Earth's axis
Earth spins around an imaginary line called its axis, running from the North Pole to the South Pole. This axis is not upright relative to the plane of Earth's orbit around the Sun. It is tilted by about 23.4 degrees.
The crucial point is that this tilt stays pointed in nearly the same direction in space as Earth travels around the Sun. The North Pole currently points toward a spot close to Polaris, the North Star, all year long. As a result:
- For part of the year, the Northern Hemisphere is tilted toward the Sun, and the Southern Hemisphere is tilted away.
- Six months later, on the other side of the orbit, the Northern Hemisphere is tilted away from the Sun, and the Southern Hemisphere is tilted toward it.
The hemisphere tilted toward the Sun experiences summer. The hemisphere tilted away experiences winter.
Two effects of the tilt
The tilt changes two things that together control how much solar energy a place receives: the angle of sunlight and the length of the day.
1. The angle of sunlight
When the Sun is high in the sky, its light strikes the ground almost directly, concentrating energy on a small area. When the Sun is low, the same beam of sunlight spreads out over a larger area, so each square meter receives less energy.
You can see this with a flashlight. Shine it straight down at a table, and you get a small, bright circle. Tilt the flashlight, and the light spreads into a larger, dimmer oval. The total light is the same, but it is spread more thinly.
In summer, the Sun climbs higher in the sky at midday, delivering concentrated energy. In winter, the Sun stays low, and its energy spreads out. Low-angle sunlight also passes through more of the atmosphere, which scatters and absorbs some of it before it reaches the ground.
2. The length of the day
The tilt also changes how many hours of daylight a place receives. In the hemisphere tilted toward the Sun, the Sun rises earlier, sets later and spends more time above the horizon. In the hemisphere tilted away, days are short and nights are long.
The effect grows stronger with distance from the equator. Near the equator, day and night stay close to 12 hours each all year. In places like New York or Madrid, summer days last about 15 hours and winter days about 9 hours. Near the poles, the effect becomes extreme: in summer, the Sun never sets for weeks or months, and in winter it never rises.
More hours of daylight mean more time for the ground and air to absorb heat, and fewer hours of darkness to lose it. Combined with the higher Sun angle, this makes summer warm and winter cold.
Solstices and equinoxes
Four points in Earth's orbit mark the turning points of the seasons.
| Event | Approximate date | What happens |
|---|---|---|
| June solstice | June 20 or 21 | Northern Hemisphere tilted most toward the Sun; longest day in the north, shortest in the south |
| September equinox | September 22 or 23 | Neither hemisphere tilted toward the Sun; day and night roughly equal everywhere |
| December solstice | December 21 or 22 | Southern Hemisphere tilted most toward the Sun; shortest day in the north, longest in the south |
| March equinox | March 20 or 21 | Neither hemisphere tilted toward the Sun; day and night roughly equal everywhere |
At the June solstice, the Sun is directly overhead at noon along the Tropic of Cancer, about 23.4 degrees north of the equator. At the December solstice, it is overhead along the Tropic of Capricorn, about 23.4 degrees south. At the equinoxes, it is overhead at the equator. The tropics and the Arctic and Antarctic Circles are defined by Earth's tilt.
The word "equinox" comes from Latin for "equal night," because day and night are nearly equal in length. In practice, they are not quite equal on the equinox, because the atmosphere bends sunlight and because sunrise and sunset are measured from the top edge of the Sun, which makes days slightly longer than nights.
Why the hottest days come after the solstice
If the June solstice brings the most sunlight to the Northern Hemisphere, why are July and August usually hotter than June? The answer is called seasonal lag.
Earth's surface, and especially its oceans, take time to heat up and cool down. Water has a high heat capacity, meaning it absorbs a lot of energy before its temperature rises. After the solstice, the Northern Hemisphere continues to receive more energy from the Sun each day than it loses at night, so temperatures keep rising for several weeks. Only later, when daily energy loss begins to exceed energy gain, do temperatures start to fall.
The same happens in winter: the coldest weeks often come in January and February, after the December solstice. Coastal areas, strongly influenced by the slow-changing ocean, often experience a longer lag than inland areas.
Seasons in the tropics
Near the equator, the Sun is high in the sky all year, and the length of the day barely changes. Temperatures stay relatively warm throughout the year. Instead of four temperature-based seasons, many tropical regions experience wet and dry seasons. These are driven by shifts in wind patterns and a belt of rising air and rainfall near the equator, called the Intertropical Convergence Zone, which moves north and south over the year following the Sun.
Seasons on other planets
Other planets have seasons too, depending on their tilt and orbit.
- Mars has a tilt of about 25 degrees, similar to Earth, so it has seasons. Because its orbit is more elliptical and its year is nearly twice as long, its seasons are longer and less equal in length.
- Uranus is tilted by about 98 degrees, essentially lying on its side. Each pole experiences about 42 years of continuous sunlight followed by 42 years of darkness.
- Jupiter has a tilt of only about 3 degrees, so it has very weak seasons.
Is the tilt always the same?
Over long periods, Earth's tilt slowly changes, varying between about 22.1 and 24.5 degrees over a cycle of about 41,000 years. The direction of the axis also slowly wobbles, like a spinning top, in a cycle of about 26,000 years, called precession. In about 13,000 years, the Northern Hemisphere will have summer when Earth is closest to the Sun, making seasons slightly more extreme there. These slow changes, together with changes in the shape of Earth's orbit, are known as Milankovitch cycles and played an important role in the timing of past ice ages.
The Moon helps keep Earth's tilt stable. Without it, Earth's axis might wobble much more over millions of years, causing more extreme climate changes.
A simple experiment
You can model the seasons at home with a lamp and a ball. Place a lamp in the middle of a dark room to represent the Sun. Stick a pencil through a ball to represent Earth's axis, tilt it slightly and keep the pencil pointing at the same wall as you walk in a circle around the lamp. You will see that on one side of the circle the top half of the ball is lit more directly, and on the other side the bottom half is. That is the cause of the seasons in your hands.
Questions readers ask
Is Earth closer to the Sun in summer? No. Earth is closest to the Sun in early January, during Northern Hemisphere winter. The seasons are caused by Earth's tilt, not its distance from the Sun.
Why are seasons opposite in the two hemispheres? Because when the Northern Hemisphere is tilted toward the Sun, the Southern Hemisphere is tilted away, and six months later the situation reverses.
Why is the summer solstice not the hottest day? Oceans and land take time to warm up. Temperatures keep rising for weeks after the solstice, a phenomenon called seasonal lag.
Do places on the equator have seasons? They have little temperature change but often have wet and dry seasons caused by shifting wind and rainfall patterns.
Would Earth have seasons without its tilt? Without tilt, the Sun's angle and the length of day would stay nearly constant all year at any location. There would be only tiny seasonal changes from the slightly elliptical orbit.
In short
Seasons happen because Earth's axis is tilted by about 23.4 degrees and keeps pointing in the same direction as Earth orbits the Sun. The hemisphere tilted toward the Sun gets higher midday sunlight and longer days, so it warms into summer; the hemisphere tilted away gets low, spread-out sunlight and short days, so it cools into winter. Solstices and equinoxes mark the turning points, oceans make the warmest and coldest weeks lag behind, and distance from the Sun plays only a minor role. It is a simple tilt with enormous consequences for life on Earth.