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My kids just made me aware of a rainbow phenomenon I have never heard of before, happening in the sky up above our heads. I have heard of (and seen) double rainbows before, and I was aware that it's also possible for inverted rainbows to be around the light source instead of having the light source be behind the observer. So the initial inverted rainbow around the setting sun was a cool photo op, even if want exactly unusual (7:04 pm):

Rainbow around sun

A few minutes later (7:12 pm), one of the kids pointed out another rainbow, and that's when things got interesting, because this rainbow didn't seem related to the sun at all:

Second rainbow, sun off picture to the lower right

It wasn't until I stared at it a little longer that I noticed that the rainbow around the sun had a twin - and as near as I could tell with how faint everything was, this twin was perfectly tangential to the second rainbow that didn't seem related to the sun at all:

Faint double rainbow, brighter tangential rainbow

For what it's worth (it's kind of hard to see in the pictures, but I was able to confirm this with the naked eye), the red/blue orientation of the tangential rainbow matches that of the double rainbow it intersects; that is, while the double rainbows (I'll say rainbows I(nner) and O(uter) to make it easier) have red on the inside, the (T)angential rainbow has red on the outside, so that the red on T lines up with the red on O, and the blue on T lines up with the blue on O at what appears to be the single point they meet.

If it weren't for the fact that the visible rainbows appear to line up so perfectly, I would assume that what I was looking at was just two unrelated rainbows: one created by the sun, and the other created by some unknown light source that I wasn't able to see (ambient light from the city, perhaps? Though I have a hard time imagining that to be bright enough / focused enough to create a rainbow; and I think it was too early for streetlights to be on, so I'm without explanation there). But the tangential nature of rainbow T has me baffled.

Is it just coincidence that the visual bands line up so perfectly, or is there some kind of known phenomenon that would explain why these rainbows were generated this way?

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    $\begingroup$ A 22° halo topped with an upper tangent arc. Plug your location and date/time into Stellarium or similar to get a solar altitude, then compare with this arc simulator. $\endgroup$
    – rob
    Jul 5 at 23:59

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