It's a question that parents get asked every day, and it's a great one.
Some people think the sky is blue because sunlight reflects the sea into the sky. But the sky is also blue inside the continent, and there is no place for the ocean nearby!
Others think it's because of water molecules in the air, but the sky is also blue in extremely dry places, such as in deserts.
The blue sky over the Sahara Desert in Libya
So, what is the real reason?
The cause of the blue sky speaks to the way sunlight and the atmosphere interact.
If you've ever played with a prism or seen a rainbow, you should know that sunlight is made up of different colors of light, and the name "roy g. biv" helps us easily remember these colors: red, orange, yellow, green, blue, indigo. (Note, roy g. biv is taken from the color initials: red, orange, yellow, green, blue, indigo, and violet.)
Rainbow on the outskirts of Melbourne, 2017
These visible lights make up only a very small part of the electromagnetic spectrum, which also includes ultraviolet, microwave, and radio waves. This means that these invisible waves cause skin sunburn, allow us to heat food, and allow us to hear radio broadcasts, all of which are different forms of light.
Spectrum, wavelength versus life-size comparison.
Light propagates in waves of different lengths: there are some wavelengths that are shorter, the color of the light is closer to blue, and there are some wavelengths that are longer, and the color of the light is closer to red. When sunlight reaches our atmosphere, air molecules scatter blue light and let red light pass through. Scientists call this phenomenon Rayleigh scattering.
Visible light spectrum. Each color of light varies from red to blue, and the distance between the waves is getting shorter and shorter.
When the sun hangs high in the sky, it reveals its true color: white. At sunrise and sunset, we can see a redder sun. This is because sunlight is passing through a thicker atmosphere. In the process, both blue and green light are scattered, while redder rays pass through and dye the clouds beautiful red, orange and pink.
On the winter solstice day of 2017, red sunlight illuminates melbourne's evening clouds
Rayleigh scattering also affects the moon we see. As the Moon passes through the Earth's shadow during a total lunar eclipse, blue and green light scatters through the Earth's atmosphere, and red light can pass through. Earth's atmosphere acts like a magnifying glass, refracting red light on the moon. This casts the moon with a strange dark red color at this time.
For this reason, many cultures, including the Aboriginal people of Australia, associate a total lunar eclipse with blood.
May 15, 2003, a dark red moon during a total lunar eclipse
Rayleigh scattering plays the same role on other planets. Did you know that the sky on Mars is also blue? (When no storm lifts red dust into the air, it is indeed blue!) (Translator's note, in fact, it is yellow-brown during the day, only gray-blue at sunset, and the Martian air is thin, there is not much Rayleigh scattering, but it is dusty, mainly Based on Mie scattering, so the sky color is related to the scattering angle.) ）
The Martian sky photographed by the Viking spacecraft on August 29, 1976
So finally, where did the sky start?
This is a tricky question. A bird leaps over us from an altitude of 50 meters and it looks like it should be in the air. The same is true of aircraft, but they fly at an altitude of more than 10,000 meters.
The "sky" is simply the atmosphere we look up at the bottom of the atmosphere. Most of the atmosphere extends upwards by about 16 kilometers, which is where most of the Rayleigh scattering occurs.
If you've ever seen a video of a rocket going into space, you can see that as the rocket passes through the atmosphere, the blue sky gradually fades into a black background.
Watch the space shuttle launch. As the space shuttle moves above Earth's atmosphere, you can see the sky turn from blue to black.
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