Every morning the Sun seems to rise over one side of the sky, climb slowly up and across, then slide down and set on the other side in the evening. Watch it for a whole day and it really does look as if the Sun is sailing over your head, from one horizon to the other.
But it isn't. The Sun is not flying across the sky. The Sun stays put. The thing that is really moving is you — because the ground under your feet is quietly turning.
The Earth is a giant ball, and it spins — one complete turn, all the way round, every single day. As it turns it carries you, your house, your whole town round with it. That slow, steady spin is the secret behind day and night.
The Sun is far away and shines from just one direction, like a torch held up on one side of the room. So its light can only reach half of the ball at a time. It can never wrap all the way round and light up the back.
The half of the Earth facing the Sun is in bright daytime. At the very same moment, the half turned away is in night. There is always a daytime half and always a night half — they just keep swapping places as the Earth turns. Slide the time of day below and watch the little person carried out of the sunlight, round into the dark side, and back into the morning again.
Watch the person in the box and you can follow a whole day:
The Earth always spins the same way — towards the east. Because you are being carried eastward, the Sun always seems to peek up over the eastern horizon first, drift across the sky, and dip down in the west at the end of the day. It does this everywhere on Earth, day after day, because everywhere is riding the same spin.
Try it with a merry-go-round, or just by spinning slowly on a chair. Turn yourself one way and the whole room seems to slide past you the other way. A lamp on the wall isn't moving at all — but as you turn, it swings into view on one side and out of view on the other. The Sun does the same trick on you: it stays still, and you are the one turning.
Because only part of the ball faces the Sun at any moment, the whole world cannot have midday at the same time. As the Earth turns, midday sweeps slowly across it — first one country has the Sun overhead, then the next, then the next, like a spotlight crawling across the ground.
That is why the world is split into time zones. When the clock says lunchtime where you live, it can already be evening in a country to the east (they turned towards the Sun before you did) and still early morning far to the west (they haven't quite turned to face it yet). Nobody's clock is "wrong" — each place just sets its clocks by where the Sun is in its sky.
Imagine it is the middle of the day where you are — the Sun is high, you're eating lunch. Now picture a friend on the exact opposite side of the world, straight through the Earth from you. They are on the night half. For them it is the middle of the night, dark and quiet, and they are fast asleep in bed.
You could wave, but the whole thickness of the planet is between you. Wait about twelve hours and the Earth will have carried you both halfway round: now you are asleep in the dark, and your friend is blinking awake into their morning Sun. The same spin that gives you your day gives them their night.
Here is the strangest part. You feel completely still as you read this — but if you live near the middle of the Earth, the spin is carrying you along at over a thousand miles an hour. Faster than any aeroplane. Right now.
So why don't you feel a screaming wind, or get flung off? Because everything around you is moving together at the very same speed — the air, your chair, your pet, the trees, the whole Earth. Nothing rushes past you, so there is nothing to feel. Smoothly and silently, without a single bump, the planet carries you round into tomorrow.
Right at the very top and bottom of the Earth — near the North Pole and South Pole — something wonderful and weird happens. For part of the year the Sun goes round and round the sky without ever dipping below the horizon. It just circles overhead, all day and all "night". People call it the midnight Sun: you can play outside in broad daylight at midnight, for weeks on end.
Then, half a year later, it swaps. The Sun sinks below the horizon and stays hidden, so those same places have weeks of darkness called the polar night — the Sun never properly rises. It happens because the Earth is tilted as it spins, so the poles get pointed towards the Sun for one long stretch and away from it for another. Even at the ends of the Earth, it all comes back to the spinning, tilted ball.