• The sky is blue thanks to the atmosphere that surrounds the Earth and diffuses sunlight, according to our partner The Conversation.

  • A funny consequence of this scattering is that the Sun itself looks less blue to us than it actually is.

  • The analysis of this phenomenon was carried out by Daniel Bloch, research director at CNRS, physicist, specialist in optics, lasers and nanotechnologies at Sorbonne Paris Nord University.

In reality, Gabriel, this might surprise you, but the sky is not blue: it is… black!

Well, half the time, when it's night (especially in the country, and without the moon).

And from space, the sky looks black too.

The sky is only blue because there is the Sun, and its light which is… white, even if the Sun is often represented as yellow when we draw it.

We often - wrongly - color the sun in yellow © Shafin_Protic / Pixabay

The sky is only blue because there is an atmosphere around the Earth: the layer of gas that surrounds our planet.

This atmosphere diffuses light.

The light rays generally go in a straight line.

However, milk or clouds are only white (or gray) if there is a source of light (at night, clouds are also black).

Clouds and milk diffuse light, like the walls of a lighted room, and like the sky.

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What is the color of the scattered light?

With a candle placed behind a finger, it appears illuminated, but in red.

It is our body which diffuses preferentially in red.

For the diffusion by the atmosphere, the diffusion takes place preferentially in blue, and less and less towards green, yellow and red (the order of the rainbow).

A funny consequence of this diffusion by the sky of the blue light emitted by the Sun, is that the Sun appears less blue than it really is, and more red and green than it is: the red-green mixture seems rather yellow to our eye.

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When it sets, the almost horizontal sun crosses a longer distance, even losing its green: it seems red to us, in a sky with pretty colors.

For the same reason (the length of atmosphere crossed) but in contrast, at high altitude, with a high sun, the sky is an unusual, deep blue, because atmospheric diffusion is reduced.

The white light of the Sun appears yellow, even orange or red, depending on the length of the atmosphere crossed © Daniel Bloch

Why then does the Moon, or the stars, not appear yellow to us?

In fact, when it is on the horizon, we sometimes have a red moon.

Especially our eye saturates the colors differently, the intensity of the Sun being incomparable to that of the stars or the Moon.

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This analysis was written by Daniel Bloch, research director at CNRS, physicist, specialist in optics, lasers and nanotechnologies at Sorbonne Paris Nord University.


The original article was published on The Conversation website.

Declaration of interests

Daniel Bloch does not work, advise, own shares, receive funds from any organization that could benefit from this article, and has not declared any affiliation other than his research organization.

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