Science · The Sun, the Earth and the Moon

What causes the seasons?

Is it hot in summer because we are closer to the Sun? No! Discover that the real cause of the seasons is the Earth's axis, tilted by about 23.5°, by looking at the angle of the Sun's rays and the length of the day.

20 min
1 simulation
Mini quiz

Updated: 21 October 2026

Preview

The question

If you ask lots of people "Why is it hot in summer?", you will probably hear "Because the Earth gets closer to the Sun." So what if I told you that the Earth is closest to the Sun at the beginning of January? If the Earth is at its closest to the Sun while it is snowing in Türkiye, what could be causing the seasons?

Try it yourself

No account needed: change the values and check your answer. Some simulations ask for a prediction before they start; it doesn't matter whether it's right or wrong.

Protractor

Set the Earth's axial tilt

Drag the moving arm or turn it 1 degree at a time with the arrow keys. When the angle is at 23°, submit your answer.

Predict first

If the Earth's axis of rotation were not tilted at all, what do you think the seasons would be like?

It doesn’t matter whether you’re right; the simulation opens once you choose.

Lab

Seasons: tilt and the angle of light

Open full page in the Lab →

Fig. 1Seasons: tilt and the angle of light

Slide the date, pick a place or type your own latitude, change the tilt21 June · 73.5°

Date21 June

Place

Your own latitude (north +, south −)

Axial tilt

Your own tilt

Noon Sun height: 73.5°
Daylight: 15 h 0 min
Light per unit area: 93%; relative to light arriving straight down
Distance to the Sun: 152.0; million km

Today is the summer solstice in Ankara.

What if?

Change the axial tilt: at the same place, how do the noon Sun, the length of daylight and polar night change?

Purple: this scenarioDashed: today’s Earth (23.44°)Flat line: the horizon

Noon Sun (over the year): 26.7° – 73.5°
Daylight: 9 h 20 min – 15 h 0 min
Polar night / midnight sun: none
Tropic / polar circle: 23.4° / 66.6°

With today’s tilt the Sun passes overhead somewhere between the two tropics. 40% of the Earth’s surface sees the Sun straight overhead on at least one day a year, and 8% lies inside the polar circles. In Ankara the noon Sun swings between 26.7° – 73.5°, and daylight lasts 9 h 20 min – 15 h 0 min. No polar night: for the Sun to stay down all day here, the tilt would have to pass 50.9°.

In reality: Mars is tilted 25.2°, very close to the Earth: it has seasons too, but a Martian year lasts 687 days, so each season is on average about twice as long.

Assumptions: The orbit, the year and the distance to the Sun stay as they are today; only the axis tilts. Atmosphere, temperature and climate are not modelled, only the geometry of the light. Area shares come from geometry without refraction: the overhead-Sun belt is sin ε, the polar caps 1 − cos ε. The year is scanned at the 365 noons of 2026.

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

If the Earth’s axis were not tilted at all (0°), what would happen to the difference between summer and winter in Ankara?

Gold: sunlight (the patch of light on the ground, daylight on the dial)Purple circle: the chosen latitude; gold dot: the cityEvery number comes from one model: the low-precision formulas astronomers use for the Sun’s position (2026), noon height h = 90° − |φ − δ|, daylight from cos H = (sin(−0.833°) − sin φ sin δ) / (cos φ cos δ), and light per unit area sin h × (1 AU / r)² (atmosphere ignored). Earth is closest to the Sun in January (147 million km) and farthest in July (152 million km); that changes the light by only about 7 percent. In Ankara the light per unit area at summer noon is about twice that at winter noon: what decides it is the tilt, not the distance. The globe’s spin and the passing year are sped up for the display.

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Frequently asked

5 questions

Is summer hot because we are closer to the Sun?

No. Earth is closest to the Sun (about 147 million km) at the start of January, when it is winter in Türkiye. The difference in distance is only about 3 percent, which is not enough to cause the seasons.

What really causes the seasons?

Earth's axis of rotation is tilted by about 23.5°. When one hemisphere is tilted towards the Sun, the rays strike it more directly and the days get longer; together these two effects bring summer. At the same time it is winter in the other hemisphere.

Does Earth's axis keep turning towards the Sun during the year?

No. The axis points in the same direction in space all year round. Because Earth travels around the Sun, this fixed tilt leans the Northern Hemisphere towards the Sun at one time of year and away from it six months later.

Why does slanted sunlight heat less?

When the same amount of light arrives at a slant, it spreads over a larger area, so each part receives less energy. That is also why the lit patch grows bigger and dimmer when you hold a torch at a slant over a sheet of paper.

If the longest day is 21 June, why are the hottest days in July and August?

The ground and especially the seas store heat as they warm up. For a while after 21 June, the energy received during the day is still more than the energy lost at night, so it keeps getting warmer. That is why the hottest days usually come a few weeks after the solstice.