Big question
Why do we have day and night?
Day and night happen because the Earth spins on its axis roughly once every 24 hours, so different sides face toward or away from the Sun. The half facing the Sun has daylight, while the half turned away is in darkness, night.
One Spinning Ball, Two Halves
The Earth is shaped like a slightly squashed ball, and it constantly spins around an imaginary line running through its centre from the North Pole to the South Pole, called its axis. At any given moment, sunlight can only shine on the half of this ball that is currently facing the Sun, exactly the same way a lamp can only light up the side of a football that faces it, leaving the other side in shadow. As the Earth turns, the boundary between the lit half and the dark half, called the terminator, sweeps steadily across the surface, bringing sunrise to places it moves onto and sunset to places it moves away from.
Roughly 24 Hours, Not the Sun Moving
It looks, standing on the ground, as though the Sun rises in the east, crawls across the sky, and sets in the west, but the Sun barely moves at all compared with the Earth. What is really happening is that you, standing on the spinning Earth, are being carried around to face the Sun and then away from it again, once every 23 hours and 56 minutes for a full spin relative to distant stars, close enough to the familiar 24-hour day. Ancient astronomers, watching only from the ground, understandably assumed the Sun was moving around a still Earth, and it took centuries of careful observation to work out that it is actually the other way round.
Why Day Length Still Varies
Even though every full spin takes about 24 hours everywhere on Earth, the number of daylight hours within that spin is not the same everywhere or all year. This happens because Earth's axis is tilted, about 23.5 degrees, rather than standing perfectly upright compared with its path around the Sun. Depending on the time of year, this tilt means one hemisphere leans more toward the Sun and gets long days with short nights, while the other hemisphere leans away and gets short days with long nights, which is also the underlying reason the seasons exist.
Try it yourself
Shine a torch at a ball, like an orange or a football, in a dark room, and slowly spin the ball. Watch how only the side facing the torch is lit, and how the bright and dark halves change as you turn it, just like day and night on Earth.
Questions children ask
How long does it take the Earth to spin around once?
The Earth takes about 24 hours to complete one full spin on its axis, which is why a full day-night cycle lasts roughly that long. More precisely, one spin relative to distant stars takes about 23 hours and 56 minutes.
Does the Sun really move across the sky?
The Sun appears to move across the sky, but that motion is actually caused by the Earth spinning underneath it, not the Sun travelling around Earth. Standing still on a spinning planet makes distant objects look like they are moving.
Why are days and nights not always equal length?
Days and nights are unequal in most places for most of the year because Earth's axis is tilted, which makes one hemisphere face the Sun more directly at certain times of year. Only around the equinoxes are day and night close to equal everywhere.