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The Question

(Submitted June 11, 1997)

Why is the sky blue and why is the Sun red at sunrise and sunset (taking into account the properties of interstellar dust)?

The Answer

Actually, it is not interstellar dust that is responsible for the color of the sky during the day and the Sun in morning and evening. It is the scattering within the Earth's atmosphere that is responsible.

Visible light is the region of the electromagnetic spectrum with wavelengths from 0.00000035 meters (violet light) to 0.00000075 meters (red light). Light usually travels in a straight line path, but it can be scattered by particles such as the nitrogen molecules in the Earth's atmosphere. This means that portions of the light energy are sent off into different directions. These directions are determined by the size of the wavelength of the light and the size of the particles doing the scattering. For the Earth's atmosphere, the effect is strongest in the shortest wavelengths; the effect is very small at longer wavelengths. The scattering is dominant for the wavelength that we see as blue, and this is why the sky appears blue.

When the Sun is close to the horizon, such as at Sunrise and Sunset, the light must pass through a much longer path in the atmosphere; more of the blue light is scattered out of this direction, leaving only the red light. This is why the Sun appears red at Sunrise and Sunset.

Cheers,
Padi Boyd
for the Ask an Astrophysicist

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