Why day and night happen — Science, 7–10 years
Earth spins like a slowly turning ball. The side facing the Sun has day, while the side turned away has night.
Earth's turning
Earth is a round planet that spins around an invisible line called its axis. It takes about 24 hours to complete one turn. As a place on Earth turns into sunlight, it has sunrise and day; as it turns away, it has sunset and night.
Why we need this idea
The changing sky raised a puzzle: does the Sun travel around us every day, or is Earth turning? Watching the same pattern from different places and comparing shadows gave scientists a better explanation. Earth’s rotation predicts day and night without saying that the whole sky must circle Earth.
A model with a lamp
Put a sticker on a ball and place a lamp one metre away as the Sun. Start with the sticker facing the lamp: it is the model's noon. Turn the ball a quarter turn, about six hours: the sticker reaches evening. After another quarter turn it is on the dark side, showing night; a full turn represents about 24 hours.
The Sun is not circling us
It is reasonable to say that the Sun moves across the sky, because that is exactly how it looks from the ground. But the daily path is mainly caused by Earth turning, not by the Sun circling our town. The Sun is moving through space too, yet that is not what creates our ordinary day and night.
Time around the world
Because Earth turns, different places face the Sun at different times. When it is lunchtime in Portugal, it may be early morning in New York and night in Japan. Clocks and time zones organise this pattern for travel, phone calls, schools and online events.
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