Earth is a giant ball that spins all the way round once a day. The sun shines on whichever half is facing it, and that half has daytime. As your half spins away, the sun sinks below the edge and night falls. The sun never actually moves across your sky — you do, riding the turning Earth. Spin the Earth in the simulator and watch your city roll into the dark.
Most people think the Sun rises and sets by moving across the sky. In fact the Sun sits roughly still while Earth spins eastward beneath you, carrying your town toward it at dawn and away at dusk.
What's actually happening
For most of history the obvious answer was that the Sun travels: it climbs out of the east, arcs overhead, and dives into the west, and the language still agrees — we say sunrise and sunset. The obvious answer is wrong. The Sun sits roughly still at the centre while Earth turns underneath the sky. What looks like the Sun sliding across is really the ground beneath your feet rotating you toward it and then away again.
Earth spins once every 24 hours about an invisible axis through the poles, turning west to east. At any instant one half of the planet faces the Sun and is bathed in daylight; the other half is turned away into its own shadow, and that is night. The boundary between them is a moving line called the terminator. As your town rotates across it at dawn the Sun appears to lift over the eastern horizon; twelve-ish hours later you cross it again at dusk and the Sun appears to drop. You are not watching the Sun move. You are watching the edge of Earth tilt the Sun into and out of view.
The strangest part is the speed you cannot feel. Standing on the equator you are being whipped eastward at about 1,670 kilometres an hour, faster than a passenger jet, yet nothing rattles, because the air, the oceans, and you are all carried along together, with no relative motion to sense. The spin even leaves fingerprints: it deflects winds and storms (the Coriolis effect), it makes the planet bulge slightly at the equator, and it is very gradually slowing down, so a day in the age of the dinosaurs was closer to 23 hours than 24.
Day and night are not the Sun moving but Earth turning under it, so sunrise is really you rotating into view.
- 1Stand in a dark room with a single lamp on across the room as your Sun, and mark the front of your shirt as your home city.
- 2Turn slowly anticlockwise on the spot. When your city faces the lamp it is noon; as you keep turning, the lamp slides toward your shoulder (dusk), then behind you (night).
- 3You never moved the lamp, yet it 'set' and 'rose' — exactly what the spinning Earth does to the real Sun.
Common questions
At the equator the spin carries you eastward at about 1,670 km/h, faster than a jet, but the air, oceans and you are all moving together. With no relative motion to sense, nothing rattles.
Not quite. The Moon's tides drag on Earth's spin, lengthening the day by roughly 1.7 milliseconds per century, which is why timekeepers occasionally add a leap second. In the dinosaur era a day was closer to 23 hours.
In 1851 Léon Foucault hung a long pendulum in Paris and its swing appeared to rotate over hours. The floor was turning beneath it, the first direct laboratory proof that Earth spins.