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"... it changes in tone depending on it's angle to the sun ... the effect works ... even when the sun is below the horizon."

This sounds like magic. Have you observed this effect yourself and do you have an explanation for it or a reference to one?

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I'll have to check it out, next chance I get, with a polarizer. One thing that could be exploited is that the sunlight scattered by the ionosphere (why the sky is blue) is polarized, so the polarization axis could be the clue.

According to your link, that's the case. It only works when the sun is just below the horizon so there is still scattered light in the sky.

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