A nice, sunny, clear, autumnal blue sky is something most of us enjoy. A blue sky is a special part of the Earth, and one we can thank our combination of star - the Sun, and atmosphere for.
Subscribe now for unlimited access.
or signup to continue reading
Called Rayleigh scattering, the blue sky is due to light bouncing or scattering from the molecules in our air. The size of the nitrogen and oxygen molecules which make up most of our atmosphere are much smaller than the light wavelength.
The shorter the wavelengths of light, the colours of light, the more they get scattered. Blue is on the shorter end of the visible spectrum - the colours of light our eyes can see. Green colours are in the middle, and red are on the longer end, meaning blue light scatters or bounces more, and red light less.
The other consequence is that the more atmosphere sunlight travels through, the more it scatters. When the sun is higher in the sky, it doesn't have to travel through as much atmosphere, so the blue light doesn't scatter away as much. The light that does scatter, scatters around in the sky, filling up the sky with blue light.
The human eye is also more sensitive to detecting blue light rather than red light, so even though the sky is filled with blue light and red light, we detect the blue light more.
As the sun gets lower in the sky, there is more atmosphere for the light to get through, bouncing more of the blue light away, leaving just the red light - giving us the orange and red colours at sunset or sunrise.
If the molecules or air particles change, the colour of the sky can change as well. If you think about the bushfires and the colour of the sky, or when there is back burning, the colour of the sky changes. The smoke molecules are bigger and denser, scattering or blocking the blue and green colours, leaving only the red.
This affect can be seen on other planets.
Mars has a very thin atmosphere, about on per cent that of earth's. This means there is not a lot of molecules to scatter light, allowing all of the colours to get through. However, the dust on the ground turns the sky a brown and red colour - similar to dust storms on earth.
At sunrise and sunset, sunlight must travel a much longer way, even through a very thin atmosphere. This path is similar in density to our sky during the day, meaning the blue light scatters and bounces around, filling the sky, and making it look blue - like our daytime. This means sunrise and sunset on Mars is blue, not red.
On the moon, there essentially is no atmosphere. so no molecules to scatter or bounce the light around. This is what on the moon, it looks like you are in space - as there is no atmosphere above being filled with colours of light from the sun.
The dense atmosphere of Venus, filled with sulfuric atmosphere and carbon dioxide gives it a yellowy-orange appearance.
- Brad Tucker is an astrophysics and cosmologist at Mt Stromlo Observatory and the National Centre for the Public Awareness of Science at the ANU.
