Why the Earth has seasons: tilt, not distance

Seasons shape when crops are planted, when animals migrate and what you wear to school. Many people think summer happens because Earth gets closer to the Sun. It is a sensible guess, but it is wrong. Seeing why is a good lesson in testing an idea against evidence.

Why distance is not the answer

Earth's orbit is very nearly a circle, so our distance from the Sun changes only slightly during the year. In fact, Earth is closest to the Sun in early January, which is winter in the Northern Hemisphere.

There is a second clue. If distance caused seasons, the whole planet would have summer at the same time. It does not. When it is summer in Canada, it is winter in Australia. Any good explanation must account for this.

The real cause: a tilted axis

Earth spins on an imaginary line called its axis. This axis is tilted about 23.5 degrees compared with a line standing straight up from the plane of Earth's orbit. The key detail is that the axis keeps pointing in the same direction in space all year. So as Earth travels around the Sun, the Northern Hemisphere leans towards the Sun for part of the year, and six months later it leans away.

Leaning towards the Sun changes two things for a hemisphere:

• Sunlight arrives at a steeper angle, so the energy is more concentrated.
• Days are longer and nights are shorter, so the ground has more hours to warm up.
• In the opposite hemisphere, both effects are reversed, giving winter.

Why the angle of sunlight matters

In summer the Sun climbs high at midday, so its light is concentrated like the straight-down torch. In winter the Sun stays low, so its light is spread out like the tilted torch. Sunlight also passes through more atmosphere at a low angle, which weakens it a little more.

Why day length matters

Longer days mean more hours of heating and fewer hours of cooling at night. In summer, the land and sea gain more energy than they lose each day, so temperatures climb. Near the poles the effect is extreme: in midsummer the Sun may not set at all, and in midwinter it may not rise.

Four key points in the year (exact dates shift by a day or so):

• Around 21 June: June solstice. Northern Hemisphere tilted most towards the Sun; longest day there.
• Around 21 December: December solstice. Southern Hemisphere tilted most towards the Sun.
• Around 20 March and 22 September: equinoxes. Neither hemisphere leans towards the Sun; day and night are roughly equal everywhere.

Facts versus models

It is an established fact that axial tilt causes the seasons. Textbook diagrams, however, are simplified models. They often draw the Sun far too small, the orbit as an obvious oval and Earth far too close to the Sun. Use them to see the tilt, not to judge sizes or distances.

Next time you explain seasons, name both causes, angle of sunlight and length of day, and give the evidence against the distance idea: opposite seasons in the two hemispheres, and Earth being closest to the Sun in January.

Key takeaways

• Seasons happen because Earth's axis is tilted and keeps pointing the same way as Earth orbits.
• The hemisphere tilted towards the Sun gets steeper sunlight and longer days, so it has summer.
• Distance from the Sun is not the cause, because the two hemispheres have opposite seasons.

Check your understanding

Question 1 of 3

Which piece of evidence best shows that distance from the Sun does not cause the seasons?

Choose an answer

The Sun looks the same size all year.Summer days are longer than winter days.The Northern and Southern Hemispheres have opposite seasons at the same time.The Moon also orbits the Sun.

Check answer

Question 2 of 3

In winter, why does sunlight warm the ground less?

Choose an answer

The low Sun spreads the same light over a larger area, and days are shorter.The Sun gives out less energy in winter.Earth is furthest from the Sun in winter everywhere.Clouds always block more sunlight in winter.

Check answer

Question 3 of 3

At an equinox, which statement is true?

Choose an answer

The Northern Hemisphere is tilted most towards the Sun.Earth is at its closest point to the Sun.The Sun does not rise at the equator.Neither hemisphere leans towards the Sun, so day and night are roughly equal everywhere.

Check answer