What really causes the seasons (it is not distance from the Sun)

A common belief is that summer happens when Earth is closer to the Sun. If that were true, both hemispheres would have summer at the same time. Instead, when it is summer in Australia it is winter in Canada.

The real explanation is simpler and more elegant, and it accounts for long summer evenings, polar midnight sun and the fact that the tropics barely have seasons at all.

The tilt

Earth’s axis is tilted by about 23.4 degrees relative to its orbit. As Earth travels around the Sun over a year, that tilt stays pointing in the same direction in space. For part of the year the Northern Hemisphere leans towards the Sun; six months later, the Southern Hemisphere does.

A useful way to picture it: imagine walking in a circle around a lamp while holding a globe that is always tilted towards the same wall of the room. On one side of the circle the top of the globe leans towards the lamp; on the opposite side it leans away.

Why tilt makes a difference

When your hemisphere leans towards the Sun, two things happen:

• Sunlight arrives more directly. The same beam of light is concentrated on a smaller area, so it heats the ground more.
• Days are longer. The Sun is above the horizon for more hours, so there is more time to warm up.

When your hemisphere leans away, sunlight arrives at a lower angle and days are shorter, so it is colder.

You can see the first effect with a torch. Shine it straight down at a table and you get a small, bright circle. Tilt it and the same light spreads into a larger, dimmer oval. Each patch of ground gets less energy.

A year, step by step

Following the Northern Hemisphere through the year:

1. Late December. The north leans furthest away. Days are shortest and the noon Sun is lowest.
2. Late March. Neither hemisphere leans towards the Sun. Day and night are roughly equal everywhere.
3. Late June. The north leans furthest towards the Sun. Days are longest and the noon Sun is highest.
4. Late September. Day and night are roughly equal again, and the cycle continues.

For the Southern Hemisphere, the same events happen with summer and winter swapped.

Solstices and equinoxes

The solstices, around 21 June and 21 December, mark the points where one hemisphere leans most towards the Sun. The equinoxes, around 20 March and 22 September, are when neither hemisphere leans towards it, and day and night are roughly equal everywhere.

Near the poles, the effect is extreme. Inside the Arctic and Antarctic circles there are days in summer when the Sun never sets, and days in winter when it never rises. Near the equator, by contrast, the Sun is high and day length changes little through the year, so temperature seasons are weak; many tropical places instead have wet and dry seasons.

Why the hottest days come later

The summer solstice is the day of most sunlight, but it is rarely the hottest day of the year. Land and especially oceans take weeks to warm up, and they keep absorbing more heat than they lose for some time after the solstice. This seasonal lag is why, in much of the Northern Hemisphere, the warmest weeks tend to come in July or August. For the same reason, weather services often use meteorological seasons that begin on the first day of a month, such as 1 June for northern summer, rather than the astronomical dates.

And distance?

Earth’s orbit is slightly elliptical. It is nearest the Sun, at perihelion, in early January and farthest, at aphelion, in early July. The difference in distance is only a few percent, much too small to drive the seasons. It does have a small effect, but it is outweighed by the tilt and by other factors such as the large areas of ocean in the Southern Hemisphere, which moderate temperatures there.

Common misconceptions

• “The tilt makes one hemisphere closer to the Sun.” The difference in distance caused by the tilt is tiny compared with the distance to the Sun. What matters is the angle of the sunlight and the length of the day.
• “The equinox means exactly 12 hours of daylight.” Day and night are close to equal, but the way sunrise and sunset are defined, and the bending of light by the atmosphere, mean daylight is usually slightly longer than 12 hours.

Seasons elsewhere, and over long timescales

Other planets have seasons for the same reason. Mars has a tilt similar to Earth’s, so it has seasons too, though each lasts roughly twice as long because its year is longer. Uranus is tipped over almost onto its side, so for long stretches of its 84-year orbit one pole faces the Sun continuously.

Earth’s tilt is not fixed forever. Over a cycle of about 41,000 years it slowly varies between roughly 22 and 24.5 degrees. These slow changes, together with other gradual shifts in Earth’s orbit, are known as Milankovitch cycles, and scientists link them to the timing of ice ages over hundreds of thousands of years. Exactly how these small orbital changes are amplified into large shifts in climate is still being studied.