Curious Kids: Why is the sky blue and where does it start?

This is an article from Curious Kids, a series for children. The Conversation is asking kids to send in questions they’d like an expert to answer. All questions are welcome – serious, weird or wacky!

Why is the sky blue and where does it start? – Oliver Scott, age 7, Wombarra.

This is something that parents get asked every day. And it’s a great question, Oliver!

Some people think the sky is blue because of sunlight reflected off the ocean and back into the sky. But the sky is blue even in the middle of the countryside, nowhere near the sea!

Others think it’s because of the water in our atmosphere. But the sky is blue in places that are extremely dry, like the desert.

A blue sky over the Sahara desert in Libya. Wikipedia

So what’s the real reason?

The sky is blue because of the way sunlight interacts with our atmosphere.

If you’ve ever played with a prism or seen a rainbow, then you know light is made up of different colours. The name “ROY G. BIV” helps us remember these colours: red, orange, yellow, green, blue, indigo, and violet.

A rainbow over my house in suburban Melbourne, 2017. Duane Hamacher

These colours make up just a tiny portion of the electromagnetic spectrum, which includes ultraviolet waves, microwaves, and radio waves. This means the invisible waves that cause sunburns, allow us to heat-up our leftovers, and let us listen to the radio are all forms of light.

The spectrum of light, showing the wavelength with objects of comparable size. NASA

Light moves as waves of different lengths: some are short, making bluer light, and some are long, making redder light. As sunlight reaches our atmosphere, molecules in the air scatter the bluer light but let the red light pass through. Scientists call this Rayleigh scattering.

The spectrum of light we can see. Each colour from red to blue looks has a shorter distance between the waves. NASA

When the Sun is high in the sky, it appears its true colour: white. At sunrise and sunset, we see a much redder sun. This is because the sunlight is passing through a thicker layer of our atmosphere. This scatters the blue and green light along the way, allowing the redder light to pass through and illuminate the clouds in a beautiful array of red, orange, and pink.

Red sunlight illuminating the clouds at sunset outside Melbourne during the 2017 winter solstice. Duane Hamacher

Rayleigh scattering can affect how we see the Moon. When the Moon passes through the shadow of the Earth during a total lunar eclipse, blue and green light is scattered in the Earth’s atmosphere, letting red light pass through. Our atmosphere acts a like a magnifying glass, refracting (bending) the red sunlight onto the Moon. This can give it an eerie dark red hue.

For this reason, many cultures - including some Australian Aboriginal groups - associate lunar eclipses with blood.

The dark red colour of the Moon during a total lunar eclipse on 15 May 2003. Frank Schulenburg/Flickr, CC BY-SA

Rayleigh scattering works on other planets, too. Did you know that the sky on Mars is also blue? (When there are no big storms kicking red dust into the air, that is!)

A photo of the Martian sky from the Viking spacecraft on August 29, 1976. NASA/JPL.

And finally, where does the sky start?

This is a tricky question. A bird flying 50 meters above us looks like it’s in the sky. But so do aeroplanes, and they fly more than 10,000 metres overhead.

“The sky” is just our atmosphere as we see it from underneath. A majority of our atmosphere extends about 16 km upward, and this is where most of the Rayleigh scattering happens.

If you’ve ever seen video of a rocket going into space, you can see the blue sky fade away to a black background as it climbs above the atmosphere.

Watch a space shuttle launch. You can see the skies turn from blue to black as the shuttle moves above the Earth’s atmosphere.

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