Why is the sky blue (and what happens to it at bedtime)?
Look up… the sky is big and blue! But the sun is bright and gold — so where does all that blue come from? Here is a secret: the sun's light is really all the colors hiding together, and the sky is full of tiny specks of air. Now it is bedtime, and the sun is sinking low, low, low. Ready to guess… will the sky stay blue, or will it turn?
After you watchWhy is the sky blue (and what happens to it at bedtime)?
The short answer
The sky is blue because the sun's light is made of all the colors mixed together, and the tiny bits of air bounce the blue color around the whole sky. At sunset the sun is low, so its light crosses much more air, the blue bounces away before it reaches you, and the sky turns orange and red.
Try this next
- What color is the sky when the sun is high at lunchtime? Look straight up on a clear day, away from the sun — is the blue deeper overhead or down near the ground?
- What happens to the orange after the sun goes all the way down? Watch the sky at bedtime and see how long the orange stays once the sun has dipped below the hills.
The whole story
How it works
Sunlight looks plain and gold, but it is really all the colors hiding together. The sky is full of tiny specks of air. When sunlight hits them, the blue gets bounced off in every direction far more than the other colors, so blue fills the whole daytime sky. At bedtime the sun sits low, so its light has to cross a long, long path of air. The blue bounces away on that long trip, and mostly orange and red are left to reach your eyes, which is why the sky turns warm at sunset.
What people get wrong
Lots of little kids think the blue sky and the orange sunset are two completely different things, or that the sky is blue because it copies the blue sea. Really it is one trick: tiny air specks bounce blue light around. By day you catch that bounced blue everywhere; at bedtime the blue has bounced away on its long trip, so only orange is left.
The catch
The bouncing only works because the air is full of tiny specks. Up where there is almost no air, like out in space, there is nothing to bounce the light, so even with the sun shining the sky stays black instead of blue.
Questions kids ask
Why does the sky turn orange at bedtime?
At bedtime the sun is low, so its light has to cross a long path of air to reach you. The blue bounces away on that long trip, leaving mostly orange and red to come through.
Is the sky blue because it copies the sea?
No. The sky is blue because tiny specks of air bounce blue sunlight in every direction. It happens over deserts and mountains too, far from any sea.
Why is the sky black in space even when the sun is shining?
Out in space there is almost no air, so there are no tiny specks to bounce the sunlight around. With nothing to bounce it, the sky stays black even with the sun shining.
Talk about it
- Ask them: the air didn't change between lunchtime and bedtime — so where did the blue go when the sky turned orange?
- Ask: the sun's light looks plain gold, but a rainbow shows it's really all the colors. Where do you think all those colors were hiding?
For grown-ups
This is Rayleigh scattering: air molecules scatter short (blue) wavelengths far more strongly than long (red) ones, so by day the whole sky glows with scattered blue. Near sunset the light's path through the atmosphere is much longer, so the blue is scattered out of the direct beam and the transmitted light is dominated by red and orange. For a 4–6-year-old we keep it concrete: sunlight is all the colors hiding together, tiny air specks bounce the blue around, and on the long bedtime path the blue bounces away so only orange is left.
Keep going
What else makes you wonder?
- The sky was blue all day, then it turned orange at bedtime. Where did all the blue go?
- If tiny air specks make the sky blue, what color would the sky be on a place with no air at all?
- Clouds are made of the sky too — so why don't they look blue like the rest of the sky?