Why Do We Have Seasons?

The short answer is axial tilt. The answer worth having is how a tilt turns into a difference in heating, because that mechanism is what exam questions on seasons actually test — and it is also what makes the popular "closer to the Sun" explanation fall apart.

Start with the geometry. Earth's spin axis is not perpendicular to the plane of its orbit; it leans by about 23.4 degrees. Just as important, that lean keeps pointing in the same direction in space as Earth travels around the Sun. So at one point in the orbit the Northern Hemisphere is tipped toward the Sun, and half an orbit later the same fixed tilt leaves it tipped away. The Southern Hemisphere is always in the opposite position. Nothing about the axis changes during the year; what changes is where Earth is in its orbit relative to that fixed lean.

Being tipped toward the Sun changes two things on the ground, and both matter.

The first is the angle of the sunlight. When the Sun is high in the sky, a beam of sunlight strikes the ground nearly head-on and its energy lands on a small area. When the Sun is low, the same beam arrives at a slant and is spread across a much larger area, so each square metre 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 it and the same light becomes a larger, dimmer oval. Summer sunlight is the bright circle. Winter sunlight is the dim oval. Low-angle light also passes through more atmosphere on its way down, which absorbs and scatters a little more of it, though the spreading effect is the larger one.

The second is the length of the day. The hemisphere tipped toward the Sun spends more than half of each rotation in daylight, so days are long and nights are short. More hours of sunshine means more total heating each day and less time to cool off at night. The hemisphere tipped away gets the reverse: short days and long nights. Near the poles the effect becomes extreme, with stretches of continuous daylight in summer and continuous darkness in winter.

Put those together and you have the seasons. Summer is the season of high-angle sunlight and long days; winter is the season of low-angle sunlight and short days. Spring and autumn are the in-between positions, when neither hemisphere is tipped toward the Sun and day and night are close to equal everywhere. The points of maximum tilt toward or away from the Sun are the solstices; the in-between points are the equinoxes.

Now the misconception. The most common explanation people give is that Earth is closer to the Sun in summer. Earth's orbit is slightly elliptical, so its distance does change a little through the year — but the closest point comes in early January, which is the middle of winter in the Northern Hemisphere. If distance drove the seasons, January would be summer for the whole planet at once. Instead, January is winter in New York and summer in Sydney, at exactly the same distance from the Sun. The two hemispheres having opposite seasons on the same day is the cleanest proof that the cause is tilt, not distance. The change in distance is real but small, and its effect on temperature is minor compared with the effect of sun angle and day length.

A useful way to test your understanding is to imagine Earth with no tilt. The axis would stand straight up relative to the orbit, so every place would get roughly the same sun angle and the same day length every day of the year. The seasons as we know them would almost vanish; the equator would stay hot, the poles would stay cold, and only the small effect of Earth's slightly changing distance would remain. Now imagine a much larger tilt: the seasons would become far more extreme, with fiercer summers and harsher winters. The size of the tilt sets the strength of the seasons.

Two details often show up on tests. First, the hottest and coldest weather usually lags a few weeks behind the solstices, because land and especially oceans take time to warm up and cool down — the longest day is not the hottest day. Second, the tropics, the band between about 23.4 degrees north and south of the equator, are the only places where the Sun can ever be directly overhead, and the Arctic and Antarctic circles mark where continuous daylight or darkness becomes possible. Both boundaries are set by the same 23.4-degree tilt.

So when a question asks why we have seasons, the complete answer has three parts: Earth's axis is tilted; the tilt makes each hemisphere lean toward the Sun for part of the year and away for the rest; and leaning toward the Sun means higher-angle sunlight and longer days, which together deliver more energy to the ground. Distance from the Sun is not the reason.