Earth spins on an imaginary line through its center called its axis. One full spin takes about 24 hours.
Step 1: Something to Notice
Watch first. The explanation comes later.
Stand still and watch the Sun for a whole day. It comes up on one side, crosses the sky, and goes down on the other. It looks completely obvious that the Sun is the thing moving.
If the Sun is not moving around us, why does it look exactly as though it is?
Step 2: Find Out
Follow the boundary between the lit and dark halves of Earth as it turns.
Step 1 — Predict
Where does the dark half of Earth always point?
Choose what you think will happen. You cannot see the experiment until you do — guessing first is what makes it worth watching.
Step 3: So Here Is Why
Now the explanation, after you have seen it happen.
Half lit, always
Sunlight only reaches one side of Earth at a time. That side has day; the other has night.
Apparent motion
When you turn, still things appear to move past you. The Sun appearing to cross the sky is called apparent motion.
West to east
Earth rotates west to east, which is why the Sun appears first in the east.
The Sun does not go round us
Day and night come from the Earth rotating once every 24 hours. The Sun appears to cross the sky for the same reason trees appear to rush past a moving car — the observer is what is moving.
The evidence for rotation
A Foucault pendulum slowly changes its swing direction, satellites see the Earth turning beneath them, and hurricanes curve. All are explained by rotation and none by a moving Sun.
Step 4: A Common Mistake
Lots of people think
“The Sun rises, crosses the sky, and sets, so the Sun must be moving around the Earth.”
Step 5: Why the Sun Rises in the East
The same idea somewhere new.
Earth turns from west to east. Because you are being carried east, whatever is east of you comes into view first — so the Sun appears over the eastern horizon and later disappears in the west.
Step 6: Think It Through
Practice makes it stick.
The Time Zones
Problem 1 of 1
When it is midday in London, it is about midnight in New Zealand. How many hours does one full rotation of Earth take?
hours
Rotation and Apparent Motion
1 of 6
How many hours does one rotation of Earth take?
2 of 6
At any moment, what fraction of Earth is in daylight? Enter the denominator, so a half is 2.
3 of 6
Which of these are caused by Earth rotating rather than orbiting?
4 of 6
Label this diagram of Earth in sunlight.
- Jan
- Apr
- Jul
- Oct
- Jan
- Distance to the Sun147.1 million km
- Northern half: sunlight energy37%
- Southern half: sunlight energy93%
Tap a numbered marker on the picture, then tap its name.
5 of 6
Earth rotates from west to east. Enter 1 if true, 0 if false.
6 of 6
Order these events for one place on Earth through a day.
- 1The place faces most directly toward the Sun — midday
- 2The place turns into the dark half — sunset
- 3The place faces directly away from the Sun — midnight
- 4The place turns into the lit half — sunrise
Step 7: Quick Check
Show what you know.
Question 1 of 1
What causes day and night?
Step 8: Explain It in Writing
Claim, evidence, then reasoning.
The question
A friend says the Sun orbits the Earth once a day. Using what you saw in the model, explain why that is not the best explanation.
Words to userotationaxisapparent motion24 hours
Fill in all three boxes. The reasoning box is the one that matters most.
What You Found Out
- Earth rotates on its axis once every 24 hours.
- Half of Earth is always lit and half is always dark.
- The Sun only appears to move across the sky because we are turning.