The color of the sky

Is the sky blue? That’s in the eye of the beholder.

Midsummer, by Carol L. Douglas
The sky, by convention, is blue, but it’s not an even sheet of blue like a recently-painted wall. On a clear day, it’s more turquoise at the horizon, more violet at the zenith. Except when a line of gold or pink sits offshore, or it’s full of moisture and a solid blue-grey. The sky, like the ocean, is big enough to do anything it wants.
Anyone who hangs around with kids has learned the scientific explanation of why the sky is blue. Our sun produces yellow light because its surface temperature is 5,500°C. Still, we don’t have a yellow sky, for the most part. Sunlight reaches Earth’s atmosphere and is scattered in all directions by the gases and particles in the air. Blue light is scattered more than the other colors because it travels as shorter, smaller waves. There’s a lot of that going on, which is a good thing, because it means our planet home is protecting us from deadly high-energy radiation. That’s a nice, pat explanation and it certainly satisfied me when I was little.
A prism refracting and reflecting an incoming beam of uniform white light, after Newton, courtesy Wikipedia. It’s violet, not blue, that’s at the bottom.
It wasn’t until recently that I noticed that this doesn’t match what Isaac Newton discovered about the visible light spectrum back in 1665. Blue is not the shortest wavelength in the visible spectrum; indigo and violet are shorter. So why isn’t our sky violet? For that matter, how do I know that it isn’t violet? Do I believe it is blue because people tell me that?
In reality, the spectrum of skylight, when analyzed, is about equal parts violet and blue. While daylight is actually a complex spectrum, it’s dominated by the color range that falls between 400 nanometers (violet) and 450 nanometers (blue). That falls neatly into the human eye’s response range, which is about 380 to 740 nanometers. So why have we all decided the sky is blue?
Off Ogunquit, by Carol L. Douglas
We have cones in our eyes for detecting color. There are three kinds. Long cones detect yellow at 570 nanometers, medium detect green at 543 nanometers, and short cones detect blue-violet at 570 nanometers. But we perceive up to 10 million colors, because these cones fire off in combination to distinguish the subtleties of color. (This is all happening in our eyeballs, not our brains, by the way.) Blue-plus-violet is perceived by the eye as blue plus an admixture of all colors, or pure white light.
Other animals must see the sky very differently from humans. Most of them only have two types of eye cones. Some birds and honeybees have receptors for ultraviolet light.
Sea Fog, by Carol L. Douglas
More importantly, humans do not always see colors the same. Beyond the obvious issues of colorblindness, some tend to see less blue altogether. But mostly, it’s a question of paying attention.
Next time you go out on a beautiful day, pay attention to how many different shades of blue there are in the sky, depending on where you look. If you don’t feel that violet thrum deep down in your soul, I’ll be surprised.

Debunkery #2: Yes, there was blue in the ancient world.

Lapis lazuli eyes in the 25th century BCE Statue of Ebih-II (eastern Syria).
Today’s misinformationcomes from the same fount that gave us yesterday’s‘four-coned woman.” It’s the idea that the ancients were somehow ignorant of the color blue, as evidenced by the fact that Homer called the ocean the “wine-dark sea.”
Fragment of a fresco from the Bronze Age Palace at Knossos. The blue is kyanos (from which comes our word cyan), a soda/copper frit paste.
Calcium copper silicate was the first synthetic pigment, dating from the Egyptian 4th Dynasty (c. 2575–2467 BCE). Although no Egyptian texts lay out its exact manufacturing process, Vitruvius gave us a formula in his De architectura (c. 15 BCE).
The Egyptians synthesized Egyptian blue because their primary blue pigment, lapis lazuli, could only be found in Afghanistan and therefore was rare and expensive. Lapis has been mined since the 7th millennium BCE, making it one of the oldest known human endeavors.
Carthaginian glass head pendant with cobalt blue hair and eyes, 5th-4th century BCE. Cobalt is another pigment used since antiquity.
By the fourth millennium BCE, the Egyptians already had an established sea and caravan trade network. The Uluburun shipwrecktells us conclusively that the Egyptians were trading blue glass ingots with the Greeks by the late 14th century BC, long before Homer lived.
Blue glass ingot from the Uluburin shipwreck. Chemical analysis indicates that the cobalt blue glass in ancient Egyptian glass vessels and in Mycenaean glass beads were from the same source. Syria in the late Bronze Age was exporting raw glass to both places.
Lapis lazuli’s name derives in part from lāžaward, which is simply the name of the mineral in Persian. From it comes the English word azure, French azur, Italian azzurro, Polish lazur, Romanian azur and azuriu, Portuguese and Spanish azul, and Hungarian azúr. It doesn’t take an etymologist to realize that all these European words come from a common root, one that meant ‘blue’ to its users.
Egyptian blue pyxis, imported to Italy from northern Syria, c 750-700 BCE.

It isn’t a Roman root. The Romans called that blue color caeruleus, deriving from caelum, meaning heaven or sky. The Greeks had a word for blue: kyanos, which comes down to us as cyan. And, yes, the ancient Israelites had a word for blue: tekhelet. This refers to a dye made from a now-unknown marine creature.

On the other hand, the word blue in English derives from a Proto-Germanic word that meant pale, pallid, wan, blue, blue-grey, yellow, discolored and light in color. That is as good a description of the northern sky as anything.
Nobody knows what the marine creature that gave the ancient Israelites tekhelet was, but a piece of wool dipped into Murex-based dye turns green in sunlight, eventually darkening to a blue-violet. This is possibly why the word refers to both blue and green.
So what was Homer—whoever he was—talking about with his references to the “wine-dark sea”? I’ve asked this question before, and my conclusion is that he is speaking of the roiled, opaque, impenetrable ocean.
King Tut’s burial mask (1346 BCE) has lapis lazuli eyelashes, imported from Afghanistan.
Some people have absolutely no poetry in their souls.

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