What Causes the Seasons?

Why is it warm in July and cold in January, at least in the UK? Many people, including a surprising number of adults, assume it is because the Earth moves closer to the Sun in summer and further away in winter. It is a reasonable guess, and it is wrong. The real cause is more subtle and more interesting: it all comes down to a tilt.

What causes the seasons

The seasons are caused by the tilt of Earth's axis, which is currently angled at about 23.5 degrees relative to the path of its orbit around the Sun. As Earth travels around the Sun over the course of a year, this tilt stays pointing in the same direction in space. The result is that each hemisphere leans towards the Sun for part of the year and away from it for the rest.

It is the angle of sunlight, not the distance to the Sun, that matters. In fact, Earth is at its closest to the Sun in early January, during the northern hemisphere's winter, which neatly disproves the distance idea.

Why the tilt matters so much

Earth's axis is the imaginary line running through the North and South Poles, around which the planet spins each day. Because this axis is tilted rather than upright, sunlight does not fall evenly on the planet through the year.

The tilt affects the seasons in two connected ways:

• The angle of sunlight. When your part of the world is tilted towards the Sun, sunlight strikes the ground more directly, concentrating its energy over a smaller area and heating it more strongly. When tilted away, the same sunlight arrives at a shallow angle, spreading its energy thinly and warming the ground far less.
• The length of the day. Tilting towards the Sun also means longer daylight hours, giving more time for the ground and air to warm. Tilting away brings shorter days and long nights.

These two effects reinforce each other. Long days of strong, direct sunlight produce summer; short days of weak, slanting sunlight produce winter. This is fundamentally about geometry and the behaviour of light, the same physics of angle and energy that explains how rainbows form.

A year in the life of the tilt

Picture Earth at four points around its orbit.

• Around 21 June, the northern hemisphere is tilted most towards the Sun. It has its longest day and the start of summer, while the southern hemisphere has its shortest day and the start of winter.
• Around 21 December, the situation is reversed. The northern hemisphere tilts away and has its shortest day and the start of winter, while the south enjoys its longest day and summer.
• Around 20 March and 22 September, the tilt is sideways to the Sun, neither hemisphere favoured. Day and night are roughly equal across the planet, and these mark the start of spring and autumn.

Throughout all of this, the tilt itself never changes direction; it is Earth's position in its orbit that changes, altering which hemisphere catches the Sun most directly.

Why the hemispheres are always opposite

A direct consequence of the tilt is that the northern and southern hemispheres experience opposite seasons. When the north leans towards the Sun, the south leans away, and the reverse is true half a year later.

This is why someone in Australia spends Christmas on the beach in high summer, while the UK shivers in midwinter. It is not a quirk or a coincidence but a guaranteed result of a single tilted planet orbiting a single Sun. The two halves of the world are forever six months out of step with each other.

Solstices and equinoxes explained

The turning points of the year have precise astronomical meanings, even though our calendars and weather often treat the seasons more loosely.

The word solstice comes from Latin meaning "Sun stands still", because around these dates the Sun's highest point in the sky changes very little from day to day. Equinox means "equal night". These are examples of how careful observation of the sky leads to firm, testable conclusions, the same evidence-based reasoning described in our guide to the scientific method.

Why does weather lag the calendar?

If midsummer is around 21 June, why is August often hotter than June in the UK? This is because land and oceans take time to warm up and cool down, a delay sometimes called seasonal lag. The ground keeps absorbing more heat than it loses for weeks after the solstice, so the warmest weather tends to arrive after the longest day, and the coldest after the shortest. The astronomical seasons and the weather we feel are related but not perfectly in step. This also explains why meteorologists often define seasons by whole calendar months, which differ slightly from the astronomical dates.

More tilt means more extreme seasons

The 23.5-degree tilt is why the seasons exist at all. A planet with no tilt would have almost no seasons, with sunlight falling much the same all year. A planet with a steeper tilt would have far more extreme seasons. The tilt also explains why high latitudes near the poles experience such dramatic swings, including the midnight Sun of polar summer and the long darkness of polar winter, when one pole leans fully towards or away from the Sun for months at a time. The same orbital and rotational motions that govern seasons play out across all the worlds described in our guide to what a galaxy is.

The bottom line

The seasons are caused by the 23.5-degree tilt of Earth's axis, which stays fixed in direction as the planet orbits the Sun. When a hemisphere tilts towards the Sun, it receives more direct sunlight and longer days, producing summer; when it tilts away, it gets weak, slanting light and short days, producing winter. Because the two hemispheres tilt in opposite directions, their seasons are always reversed. Solstices and equinoxes mark the turning points of this yearly cycle. So the next time someone says it is summer because Earth is closer to the Sun, you can gently set them straight: it is all about the tilt.