Why Earth has day and night — Earth science, 7–10 years
Earth is always turning, so sunlight reaches different places at different times. Earth science, 7–10 years.
The turning Earth
The Sun lights only the half of Earth that faces it. As Earth spins, your place moves into sunlight and then away from it, like a spot on a slowly turning ball. Day and night are caused by this turning, not by the Sun travelling around Earth each day.
The problem of different times
People noticed that daylight did not arrive everywhere at once. If the Sun went around a flat, still Earth, this pattern would be hard to explain. Earth’s rotation gives a simple reason: places facing the Sun have day, while places turned away have night, so morning travels across the planet.
A globe and a lamp
Put a sticker on London and shine a lamp at a globe. First, turn the sticker toward the lamp: it is in daylight. Turn the globe half a turn, about 180 degrees: the sticker now faces away and is in darkness. Keep turning; each full turn brings another day and night.
The Sun seems to move
It is reasonable to think the Sun moves across the sky because that is exactly what we see from the ground. But the daily path is mostly an effect of our turning viewpoint. Like sitting on a merry-go-round, you can feel still while the scene seems to slide past you.
Time around Earth
Day and night help us plan sleep, work and travel. Because different places face the Sun at different times, clocks show different local times around Earth. A video call can be morning for one person and evening for another, even though both clocks are correct.
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