Why is the sky blue?

How do we explain the blue colour of the sky? The explanation, which anyone with some knowledge of physics would give, is that the light rays from the sun are with a small probability scattered in the atmosphere, and since short wavelengths are scattered more strongly than longer wavelengths, the scattered light, reaching us from above, is dominated by short wavelengths, resulting in a blue impression.

Goethe, in his ”Farbenlehre” says that the sky is blue because we are looking towards the dark space. We are looking at darkness through a slightly turbid medium, the atmosphere, which is illuminated by the sun. Blue is the colour next to black, the colour of absolute darkness. Saturated, deep blue is by necessity a very dark colour.

What kind of explanation is that? In a sense he is right: if the space above the atmosphere were shining, then we wouldn’t be able to observe the weak ”blue” scattered light. For instance, a dark mountain looks blue at a distance, but the areas covered by snow rather look brownish. It is a necessary condition for the blue hue to appear, that the background is dark.

But it is a condition, not a cause. The dark space behind is just there, it doesn’t produce the blue colour we see.

We need to investigate the effective causes in order to explains the function of our visual sense, and to be able to design colour technology. The effective cause behind colour vision is what we call "light", conceived as a kind of radiation. It is natural to focus on light and its interaction with matter, if we are looking for the effective causes behind natural phenomena.

But Goethe was searching for another kind of explanation. His interest was to understand why things are as they are. For him, the formal causes, causa formalis, were as important as -- perhaps even more important than -- the effective causes, causa effectus. ”First of all”, he says, ”we must remember that in dealing with colour we are walking around in the world of images.”

In order to arrive at an understanding of colour, we need to clarify its formal ontology – how things are. We must note down the way colour presents itself under various conditions. We have to find out how different colour phenomena are related, in such a way that they metaphorically speaking "illuminate" each other.

For instance, the sequence of hues, yellow-orange-red, taken on by the setting sun, is so to speak the other side of the coin. In that case we are looking through a turbid medium towards a bright body. The blue of the sky and the yellow and orange of the setting sun are complementary phenomena. Ontologically they are parts of one and the same whole.

Furthermore, Goethe finds it fascinating, that the conditions for the blue of the sky are the same as for the blue of the iris of the human eye.

The different impressions that different hues make on our mind has to do, among other things, with their affinity to light or darkness. Blue is a colour next to black, since "blue brings a principle of darkness with it."

Hence it is also passive: "we love to contemplate blue, not because it advances to us, but because i draws us after it."

"As the upper sky and distant mountains appear blue, so a blue surface seems to retire from us", Goethe concludes in §780 of his Farbenlehre.

© Pehr Sällström, febr. 2006