[Image descriptions: The sun dissapears in the ocean at sunset, emitting a last green ray of light.]
It always confused me why Venus is associated with the color green. I knew that Venus, just like the Sun, and any other celestial object, can give
a green flash when it's almost under the horizon, because the atmosphere refracts light on a different wavelenght at low altitude. The only reason I know this, is because it was the whole premise of the Éric Rohmer movie
The Green Ray. But these green flashes are very short-lived, so why describing the planet by this color? Venus appears white in the sky, so why is she associated with green by Abraham Ibn Ezra, among others?
I found a possible answer to it. Recently, I came across a transcription of a 2014 lecture at the the Royal Astronomical Society, where researchers Peter James and Marinus Anthony Van der Sluijs made a brief mention about why the assigment of Venus' color changed through history. This might had to do with the atmosphere through which we see the light reflected by the planet.
[Richard] Stothers' [(Goddard Institute for Space Studies)] suggestion of heliacal* colours inspires an explanation which also accounts for the Babylonian description of Venus as blue–green instead of white: higher dust levels in the Earth’s atmosphere in antiquity (from volcanic activity and cometary dusting) may have meant that the planets exhibited such ‘heliacal’ colours more frequently, even at higher altitudes. Palaeoclimatologists may be able to determine whether this suspicion is feasible or not.
They go on to give another possible explanation by saying that changes in colors of the planets might have to do with language, and having or not having a distinct word for a particular color.
This theory could also expain, I might assume, why Vettius Valens and Abraham Ibn Ezra describe the moon as green. Maybe the ancients didn't make a linguistical distiction between yellow and green, two colors that are very close to each other in appearance? The moon often has a yellow hue, so maybe this word got translated later on as green? Or maybe our ancestors took the description from traditional older sources, when there might have been atmospheric differences, just like we still keep on associating Venus with green even if she doesn't look that way (anymore)?
On another note: the planet Venus has a green aurora of her own, but this couldn't have possibly be known to the Babylonians - since they had no telescopes. But nowadays, we have the equipment to observe a coming and going of a green glow around the planet Venus. So even in modern times, it's okay to accept that Venus has something for the color green.
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'Heliacal' means 'in relationship to the sun', and it often refers to a star that is visible during twilight and close to the horizon, when the sun is just far enough away to make the star visible.
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