Can you get moonbows?
Sun, 9th Mar 2008
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from the show Naked Science Q&A Show
Sam Smith asked:
I was driving home one night, six weeks ago. There was a full moon and it was a beautiful clear night. There was just a bit of rain coming in from the west. I had to put my windscreen wipers on and it was making a horrible smear across my screen. What I saw at first I thought was muck from the road smearing across my screen, there was this light crescent in the sky. I put my windscreen washers on and it cleared away yet this thing was still there. What I was looking at was basically a very precise, very thin crescent of what looked like light in the sky. I thought it might be clouds or something and then it came to me: it's a moonlight rainbow. Is this actually possible, and if it is, how does it happen?
Dave: Was the moon behind you or in front of you?
Sam: It was in front of me and I was going into the rain.
Dave: I think that is a straight moonbow. It's exactly the same as a rainbow caused by the sun so what happens is, if you shine light on a droplet of water the light will bounce around inside it and when it comes out different colours will come out in different directions. So if you look at different places you'll see a rainbow. The moon is a source of essentially white light so you'll get exactly the same effect. Because it's so dim your eyes won't be sensitive to the colour so you just see a bow of brightness rather than a colour. If you took a very long exposure photograph of it or you had an image intensifier which worked in colours then you would be able to see the colours and it would look just like a normal rainbow.
Sam: I was looking them up afterwards to see if I was imagining things. It seems they're quite rare.
Dave: Yes, proper rainbows are. Normally people just see a ring around the moon itself, maybe 20-30 degrees outside the moon which is normally created by little ice particles. It isn't a true rainbow, it's a similar sort of effect.