Science of sunset (and sunrise)

A sunset in Copenhagen

A sloop of amber slips away
Upon an ether sea,
And wrecks in peace a purple tar,
The son of ecstasy.

(Emily Dickinson)

Sunset and sunrise are two magical moments of the day. Thousands of poems have been written and thousand of paintings have been drawn. What is the scientific reason of the incredible colors of the sky during sunsets and sunrises?

Sunset and sunrise are the moments in which the sun actually crosses the horizon. When the sun is slightly below the horizon, the light moves from a less dense to a more dense air and it is refracted through the atmosphere. As a consequence, we see the sky as reddish and we see the sun above the horizon for two more minutes after the actual sunset and sunrise.

Apparent and actual positions of the sun at sunrise

The visible light can be described with wavelengths (see here to have a look to the electromagnetic spectrum) ranging from the blue light (short wavelength) to the red light (long wavelength).

A sunset on the Delta of river Po

During the day we see the sky as blue because the atmosphere scatters only a small portion of blue light. During sunsets and sunrises, the sun get closer to the horizon and the blue light is scattered more and more, allowing her eyes to see the red, orange and yellow colors in the sky.

A sunset in Porto Tolle

The same sunset some minutes later

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