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Why the sun is red in colour ?

- Chung Yu Yan (08.10.2001)

Children usually draw the sun as an apple-red disk in their picture. However, the sun is in fact yellow to our eyes. It is because the surface temperature of the sun is around 5780K (i.e. around 5507¢XC), the emitted radiation is peaked around 5000Å which is wavelength of yellow colour. 

However, during the sunrise and sunset, the sun shines at an oblique angle. Sunlight has to travel through a much thicker layer of air. Not only the sun becomes dimmer because of the absorption of light by the atmosphere, but also it looks 'redder'. Since most of the light in blue wavelength has been scattered into other directions. Only the light in red wavelength can reach the ground directly. This effect is more obvious during sunset than sunrise as more suspending particles which have strong scattering effect, are generated by human activities in daytime.


Lau Kai-Ip
Asst. Curator, Hong Kong Space Museum