Lesson 1 · Ages 7–11 · 15 minutes
Day and night
Why does the Sun rise and set?
Every morning the Sun comes up in the east, crosses the sky, and goes down in the west. It looks for all the world as though the Sun is moving. It isn’t — or rather, it isn’t moving the way it looks. You are. You’re standing on a ball 12,742 km wide, spinning once a day, and this lesson is about what that actually looks like from outside.
By the end you will be able to
- Explain that day and night come from Earth spinning, not from the Sun moving
- Point to the terminator — the line between day and night — and say which way it travels
- Say why the Sun rises in the east
- Work out roughly what time it is somewhere else in the world
1Half of Earth is always lit
The Sun is a long way off — about 150 million kilometres — so the sunlight reaching us arrives in almost perfectly parallel beams. A ball sitting in parallel light has exactly one half lit and one half dark, always. Not sometimes. Always.
So there is no such thing as “night time” for the whole planet. There is only night where you happen to be standing.
Here is Earth right now, seen from the side so the Sun is off to the left. Press play and the clock runs at one hour every second — a whole day in about twenty-four seconds.
The boundary between the lit half and the dark half is called the terminator. On the ground, standing on the terminator means watching a sunrise or a sunset.
It sweeps across the ground at over 1,600 km/h at the equator — faster than a passenger jet. That’s why sunsets don’t last long.
Which way does Earth spin?
Looking down on the North Pole, Earth turns anticlockwise. That means any spot on the ground is always travelling eastwards — so the eastern horizon drops away from the Sun’s direction first, and the Sun appears to come up in the east. Everyone on the planet, in every country, sees sunrise in the east for the same reason.
2A day above one city
Now let’s stop moving and hover. This scene parks you about 2,500 km above Kathmandu and holds station there while Earth turns underneath. Watch the terminator sweep in, pass over, and leave.
Grab a torch and an orange in a dark room. Shine the torch from across the room, and slowly turn the orange anticlockwise (seen from the top). Stick a small blob of plasticine on it for your house. Count how many times it goes from light into dark — that’s how many days have passed.
What about time zones?
Earth turns 360° in 24 hours, so it turns 15° every hour. That’s the whole idea behind time zones: move 15° of longitude east and local noon arrives an hour earlier. Go right round and you’ve collected 24 hours — which is why there has to be a date line in the Pacific where the day flips over.
★What you learned
- Earth spins on its axis once every 24 hours. That spin, not any movement of the Sun, gives us day and night.
- Half of Earth is lit at all times. It is always daytime somewhere and night somewhere else.
- The terminator is the line between the lit and dark halves — sunrise on one side, sunset on the other.
- Earth turns eastwards, so the Sun appears to rise in the east and set in the west.
- Earth turns 15° per hour, which is why time zones are about 15° of longitude wide.
- Rotation
- Spinning on your own axis. Earth does this once a day.
- Axis
- The imaginary line through the North and South Poles that Earth turns around.
- Terminator
- The moving line dividing day from night on a planet or moon.
“The Sun goes round the Earth.” For thousands of years almost everyone thought so, and honestly the sky looks exactly like it. But if the Sun really orbited us once a day, the stars would have to as well — and the planets would have to loop backwards sometimes. The spinning-Earth explanation is simpler, and it’s the one spacecraft photographs confirm.