Why do we have day and night?

1. Every morning the sky brightens and the Sun appears in the east. We call this sunrise. But is the Sun really rising?
2. During the day the Sun seems to travel across the sky from east to west. Shadows swing around, and in the evening the Sun sets in the west.
3. Let's zoom out into space. Earth is a giant ball, and the Sun shines on it from far away. Only the half facing the Sun is lit.
4. Earth spins like a top. As it turns, our house is carried out of the dark side and into the sunlight. That is sunrise! Later it turns away again.
5. When it is midday for you, kids on the opposite side of Earth are asleep in the middle of the night. Somewhere, the Sun is always rising, and somewhere it is setting.
6. One full spin takes about 24 hours. That is one day. At the equator, the ground is carried around faster than a jet plane!

Tap each part of the picture. Some of them move or light up when you find them!

• The Sun. The Sun is a star, a huge ball of glowing hot gas. It is so far away that its light takes about 8 minutes to reach us.
• Day side. The half of Earth facing the Sun is in daylight. People here see a bright blue sky.
• Night side. The half facing away from the Sun is in Earth's own shadow. It is night here, and city lights twinkle in the dark.
• North Pole and axis. Earth spins around an imaginary line called its axis. It goes through the North Pole and the South Pole.
• Sunrise edge. Along this edge, places are turning into the sunlight. People here are seeing sunrise right now.
• Sunset edge. Along this edge, places are turning away from the Sun. People here are watching the sunset.
• Air blanket. Earth's air, the atmosphere, spreads sunlight around. That is why it gets light a little before sunrise and stays light a little after sunset.
• Your home. Your home rides along as Earth spins. It goes all the way around once a day, through sunrise, midday, sunset and night.

Spin Earth with the slider or drag it. Drag the friends to new places on Earth. Watch each window: who sees sunrise, midday, sunset and night?

Experiment: When it is midday for Mia, what time is it for Leo on the opposite side of Earth?

1. Spin Earth until Mia's window shows the Sun high in the sky.
2. Look at Leo's window on the other side of Earth.
3. Keep spinning and watch both friends see sunrise and sunset.

What we found: The Sun can only light half of Earth at once. When Mia faces the Sun at midday, Leo is on the dark side in the middle of the night. As Earth spins, they swap!

Turn on JavaScript to try it yourself.

Drag each friend to a spot on Earth so Rooster sees sunrise, Kai eats lunch at midday and Owl hoots at night. Then press Check.

Earth is a giant ball floating in space, and it is always spinning. It turns around an imaginary line called the axis, which runs from the North Pole to the South Pole. One full spin takes about 24 hours. We call that a day.

The Sun shines on Earth from one direction, so it can only light up the half that faces it. On that half it is daytime. The other half is in Earth's own shadow, and there it is night. As Earth spins, every place is carried into the sunlight and then out of it again.

Because Earth spins toward the east, the Sun always seems to rise in the east and set in the west. It looks as if the Sun is moving across the sky, but really we are the ones moving! It is like riding a merry-go-round and seeing the trees seem to slide past.

Right now, somewhere on Earth people are watching a sunrise, and somewhere else people are watching a sunset. When it is lunchtime for you, it is the middle of the night for kids on the opposite side of the world.

Key words

Rotation

Spinning around. Earth rotates once about every 24 hours.

Axis

The imaginary line through the North and South Poles that Earth spins around.

Terminator

The line on Earth between the day side and the night side.

Hemisphere

Half of a sphere, such as the half of Earth that faces the Sun.

Local solar time

The time of day based on where the Sun is in your sky. At local noon the Sun is highest.

Check your thinking

1. What causes day and night?

   Show the answer

   Earth spinning on its axis. Earth spins once about every 24 hours, carrying each place into and out of the sunlight.

2. If it is noon where you are, what is it on the exact opposite side of Earth?

   Show the answer

   About midnight. The opposite side faces away from the Sun, so it is in the middle of its night.

3. Why does the Sun seem to rise in the east?

   Show the answer

   Earth spins toward the east. Earth spins toward the east, so the eastern sky turns toward the Sun first.

4. City B is 45 degrees of longitude east of City A. When it is 9:00 local solar time in City A, what time is it in City B?

   Show the answer

   12:00. Earth turns 15° per hour, so 45° east means 3 hours later: 9:00 + 3 = 12:00.

5. How long does Earth take to spin around once?

   Show the answer

   About 24 hours. One spin takes about 24 hours, which makes one day and one night.

6. Why can we not feel Earth spinning?

   Show the answer

   It spins smoothly, and everything moves along with us. Like riding in a smooth car, you only feel speeding up, slowing down or turning sharply.