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Skip to 0 minutes and 7 secondsLight is a funny thing. Nothing in the universe travels faster than light. Travels in straight lines through the emptiness of space. Bounces off planets and moons, or anything else that gets in the way. We see stars in the sky because they make their own light. We see planets in the sky, because light from nearby star bounces off them. The sun is the closest star to us, and we get day and night, because we live on a spinning planet. Our Earth spins once every 24 hours. This is one Earth Day. During the day, we see the sun rising in the east until it reaches its highest point at midday, after which, it sets in the west.

Skip to 0 minutes and 45 secondsBut it's not actually the sun that's moving, it's us. Sunlight reaches the ground, bounces off the trees, buildings, cars, animals, and people, allowing us to see everything. However, on a sunny day, you may have noticed you have a dark outline right behind you. Sunlight can't get through you. So you produce an opaque shadow behind you, where light just can't reach. As the sun moves across the sky, shadows also move, but in the opposite direction. The tree's shadow can help us tell the time without a watch. But what about if we go to Venus? Well, on Venus we would see the sun rising in the west and setting in the east, the opposite of what we see on Earth.

Skip to 1 minute and 23 secondsOur shadows would move from east to west. This is because Venus spins in the opposite direction to Earth. We would also see that our shadows move much more slowly compared to on Earth. The sun moves much more slowly in the Venetian sky. This is because Venus doesn't spin as fast as the Earth. A day on the earth is 24 hours long. A day on Venus is 5,832 hours long. That's a long time to wait while your shadow moves. Should we leave our solar system for a minute, and explore further? That's land on a far away world. We're at a red star called Gliese 581.

Skip to 1 minute and 57 secondsThis world is thought to be rocky like the Earth, and it may have the right temperature for liquid water. If there is water, there might be life. If there is intelligent life, they will also be watching their shadows move during their alien day, however long it is, just like we do.

Night and day

There are key facts and figures to get to grips with when understanding night and day. We decided to create a video for teachers to use in the classroom which pulls them all together and here it is. You can pause the video at certain points to reiterate specific concepts in class.

Here are the main science points mentioned in the video:

  • Light travels in straight lines.
  • It reflects off planets and the Moon - this is why they are visible to us.
  • Stars make their own light.
  • The Sun is a star.
  • The Earth spins on its axis anti-clockwise, once every 24 hours.
  • We see the Sun rise in the east, reach its highest point at midday in the south and then set in the west. It rises again 24 hours later. This is because of the Earth’s spin.
  • A shadow is formed when an object (or person) blocks light - we see the absence of light opposite to the light source.
  • A shadow cast by a stationary object blocking sunlight will move in the opposite direction to the Sun. It will also change size as the Sun rises and sets in the sky.
  • A sundial is a clock that casts a shadow and the direction of the shadow tells you your local time.
  • If the Earth was spinning clockwise we would see the Sun rise in the west and set in the east - like on Venus.

We often use this video along with the resources Spinning Earth and Human Sundials to help explain these tricky concepts.

How would you use this video and the resources?

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This video is from the free online course:

Teaching Primary Science: the Solar System and Beyond

Royal Observatory Greenwich