Seasons : I Wish I Knew

The short version

We have seasons because the Earth is tilted. As it travels round the Sun, first one half leans towards it and then the other. The half leaning towards the Sun gets longer days and more direct sunlight, so it is summer there and winter on the other side of the world.

I always thought summer happened when the Earth was closer to the Sun, and winter when it was further away.

It is the other way round for us in the UK. The Earth is closest to the Sun in early January.

The Earth is tilted

The Earth does two things at once. It spins once a day, which gives us day and night. It also travels once round the Sun every year, about 365 and a quarter days, which gives us the seasons.

The spin is around an imaginary line through the North and South Poles, called its axis. It leans about 23.4 degrees from upright: a line at right angles to the flat path Earth takes around the Sun.

Over the course of a year, the axis keeps pointing in almost the same direction in space. So in June the northern half of Earth leans towards the Sun, and in December it leans away.

One tilt, two summers

The Earth at four points around the Sun, with its axis tilted the same way each time

When the north leans towards the Sun, the UK has summer. At that moment the south leans away, so Australia has winter. Half a year later it swaps. That is why Christmas is hot in Sydney.

The tilt stays almost fixed over a year, but the Sun's position relative to it changes. That is what gives us the seasons.

Seasons happen because the Earth is closer to the Sun in summer.

Earth is closest to the Sun in early January, during UK winter. It receives slightly more solar energy then, but the 3.4% distance change cannot explain why the two hemispheres have opposite seasons.

Why leaning towards the Sun means warmer

Leaning towards the Sun does two things, and both warm you up.

First, the days get longer. In midsummer the UK has about 16 and a half hours of daylight, so the ground has more time to heat up. In midwinter it has under 8, and the ground has more time to cool.

Second, the Sun climbs higher in the sky. High sunlight lands on the ground almost head-on, so each patch of ground gets a concentrated dose. Low winter sunlight arrives at a slant and is spread over a bigger area, so each patch gets less. Torch a wall from straight on, then from a low angle, and you can see the same effect.

Hours of daylight in London

Midsummer, about 21 June 16.6 hours

Spring and autumn, about 20 March and 23 September 12.1 hours

Midwinter, about 21 December 7.8 hours

The four turning points of the year

The dates move by a day or so from year to year. The names are for the northern half of the world.

Spring equinox

(about 20 March)

equal day and night

Day and night are about the same length, and the days are getting longer.

Summer solstice

(about 21 June)

longest day

The north leans furthest towards the Sun, so this is the day with the most daylight.

Autumn equinox

(about 23 September)

equal day and night

Day and night are about the same length again, and the days are getting shorter.

Winter solstice

(about 21 December)

shortest day

The north leans furthest away, so this is the day with the least daylight.

23.4°

The Earth's tilt. A small lean from upright, yet it is enough to give us every season.

Near the equator, and near the poles

The tilt matters less the closer you are to the equator. Places near it have days that stay close to 12 hours all year, so they do not get four seasons. Many have a wet season and a dry season instead.

Near the poles it matters most. Inside the Arctic Circle the Sun does not set at all around midsummer, and it does not rise for weeks around midwinter. These long bright and dark spells make the seasons especially noticeable there.

Why July is hotter than June

The longest day is in June, but the hottest weeks in the UK usually come in July and August. The land and the sea are still soaking up heat for weeks after the solstice, and each day they take in more than they lose. The sea warms and cools slowly, so it holds on to the heat. The warmest time arrives once the heat coming in has finally been matched by the heat going out. The same delay explains why January and February are often colder than the shortest day in December.

Sources:

Daylight and seasonal dates are approximate. Figures are for the UK unless stated.

Key takeaways

The Earth's axis is tilted by about 23 degrees.
The axis points in almost the same direction over one year.
Summer is when your half leans towards the Sun.
Distance from the Sun plays almost no part.
The two halves of the world have opposite seasons.

Now you know.

The Earth is tilted, and it stays tilted all year.

As it goes round the Sun, each half takes a turn leaning towards it.

That lean gives longer days and higher sunlight, which is what we call summer.

Seasons