Why Does Earth Have Seasons?

Drag the controls below. Watch the three pictures react instantly. Try to work out the rule yourself before you scroll down and read it.

Controls

0° (upright)45° (extreme)
Dec solsticeMar equinoxJun solsticeSep equinoxDec solstice
80°SEquator80°N

Right now

What you're looking at

Panel 1 — View from above the solar system. The Sun sits in the middle; Earth travels around it. The faint ghost Earths mark the four seasonal checkpoints. Notice the axis line drawn through every ghost points in exactly the same direction — that line never swings around to follow the Sun. The bright Earth is where your slider has placed it right now.
Panel 2 — Sunlight hitting Earth at local noon. Parallel sunlight arrives from the left. The dot marks your chosen latitude; the angle shown is how steeply the light lands there. Steep (close to 90°) means the same beam is squeezed onto less ground — more energy per square metre. Shallow means that beam is smeared over more ground — less energy per square metre.
Panel 3 — Length of day. A 24-hour clock for your chosen latitude on this date. The yellow arc is daylight, the dark arc is night. More daylight hours means more total time for sunlight to add up, on top of whatever Panel 2 shows about its strength.

The rule

Earth's axis is tilted, and it keeps pointing the same fixed direction in space all year — it does not tip over to follow the Sun. Because of that, as Earth travels around its orbit, first the Northern Hemisphere leans toward the Sun, then six months later the Southern Hemisphere does.

Whichever hemisphere is leaning toward the Sun gets hit by more direct, concentrated sunlight (Panel 2) and gets more hours of daylight (Panel 3). Both effects add up in the same direction, which is why summers are warm and winters are cold — and why the two hemispheres are always in opposite seasons.

Set the tilt slider to 0° and drag through a full orbit: the sunlight angle and day length at your chosen latitude stop changing at all. No tilt, no seasons — even though Earth is still orbiting the Sun exactly as before.

A common myth

It is not because Earth gets closer to the Sun in summer. Earth's orbit is nearly circular, and in real life Earth is actually at its closest to the Sun in early January — the middle of Northern Hemisphere winter.

Distance from the Sun barely changes through the year and affects both hemispheres equally at the same time, so it cannot be what makes one hemisphere warm while the other is cold. Only the tilt explains that.

That is also why Singapore, sitting almost exactly on the equator, barely has seasons at all — try the Singapore preset above and watch how little Panels 2 and 3 move as you drag through the year.