Rotation: Day and Night
≈ 20 minThe Earth spins on its axis — an imaginary line running through the North and South Poles — once every 24 hours. This spinning is called rotation.
The Sun can only light half of a ball at a time. So at every moment, half of the Earth faces the Sun and has day, while the other half faces away and has night. As the Earth turns, your town is carried into the lit half (sunrise), across it, and out of it again (sunset).
The Earth rotates from west to east. That is why the Sun appears to rise in the east, cross the sky, and set in the west. The Sun is not travelling across the sky at all — you are being turned past it.
Worked example — degrees per hour. A full turn is and it takes 24 hours, so the Earth turns
That single number explains time zones: places apart in longitude see the Sun at its highest one hour apart. When it is midday in Johannesburg, the Sun has not yet reached its highest point over Rio de Janeiro.
The common mix-up. Rotation and orbit are two different motions with two different periods:
| Motion | What it is | How long |
|---|---|---|
| rotation | the Earth spinning on its axis | about 24 hours → day and night |
| orbit (revolution) | the Earth travelling around the Sun | about 365¼ days → one year |
What causes day and night on Earth?
What is the name of the imaginary line through the North and South Poles that the Earth spins around?
The Earth turns through a full circle of 360° in about 24 hours. Through how many degrees does it turn in one hour?
Every day, everywhere in the world, the Sun appears to rise in the east and set in the west. What does this tell us?