Has any Doper except me ever seen a “darkbow” - that is to say, a rainbow at night (my naming)?
I’ve found two references on the web to it - once in aristotle (?) natural history & once in a ship’s log by a 17th century captain. I just wondered how often they are, as I saw one several years ago & none before or since.
It was a very bright full moon that night & it started to rain - I thought “if it were day,there would be a rainbow over there”, turned round & there it was! It was like a normal rainbow but, muted, like burgundy instead of red, almost olive/forest instead of bright green, some of the colours almost weren’t there at all, just a suggestion of themselves.
I don’t know if this is the right thread for this, but I’m sure someone will send it to the right place!
It was a rainbow at night instead of a rainbow during the day. The operative word here is ‘rain’. A darkbow is the antimatter equivalent of a lightbow.
I’ve never seen one. I think there might be something about it in M. Minnaert’s book “The Nature of Light and Color in the Open Air” (Available from Dover Books). Or you might find it in William Corliss’ books (he’s always on the lookout for things the “orthodox” texts miss.)
The optics of a rainbow formed from lunar light would be exactly the same as for a sun rainbow. I AM surprised at the burgundy color you describe, though. I’ve seen red rainbows at sunset, when the other colors have been scattered out of the sunlight. But I’d have thought that a moonbow would have a little more green in it.
A rainbow is formed when light passes through droplets of water in the air and gets refracted. What proably happened was that the full moon provided enough light just after a night rainfall to cause the rainbow. Nothing exciting, nothing new.
Technically, a rainbow created by moonlight is a “moonbow”, but naming something doesn’t explain it.
I can think of two reasons (besides the general low light intensity) for the washed out colors:
For rainbows in general, the larger the raindrops the brighter the colors. You can verify this next time you see a rainbow by contrasting the color intensity at the top of the bow and at the ends. Since raindrops get larger as they fall, the top (where the drops are smallest) is the most washed out and the ends are brightest. You said it had just started raining, so presumably the drops were fairly small.
The moonbow is created from reflected light which wouldn’t have the same spectrum as direct sunlight. The moon absorbs some colors more than others, so the reflected light is different than the incident light. It’s just possible that there may also be polarization effects.
Finally, the band of sky above a primary rainbow and beneath a secondary rainbow (that is, between the bows) is called Alexander’s Dark Band (no relation, AFAIK, to the ragtime band) after an old Greek guy who first described it. It is noticeably darker than the rest of the sky. The light that makes up the bow(s) comes from this area. (It had to come from somewhere, didn’t it?)
In the case of a “darkbow” this would then be Alexander’s Dark Dark Band, wouldn’t it?
I’m afraid not. Alexander’s Dark Band (Named after its discoverer, Alexander of Aphrodisias, the head of the Lyceum sometime in the few hundreds)will be in the same place for either solar rainbows or lunar rainbows – the geometry is the same. (the dark band fills the space between the normal rainbow and the rarer secondary rainbow.)I think your explanation based on the different spectroscopic content of moonlight hits closer to the truth. I suspect there is als some effect due to the fact that it is so dim that your night vision s coming into play, and your peak of night vision shifts more towards the blue end of the spectrum.
Other sources you might want to check are Greenler’s book on Rainbows and Glories, Boyer’s book The Rainbow from Myth to Mathematics, R.A.R. Tricker’s book Introduction to Meteorological Optics, and the ubiquitous Flying Circus of Physics by Jearl D. Walker.
I’ve been trying to get hold of a copy of greenler’s book for ages, but it’s out of print & amazon have had no luck with 2nd hand copies - if anyone can get hold of it – I’d happily pay cost + postage (I’m in the Uk though, but can still send a cheque/postal order in US currency)
Thank you - I thought so too - I’ve seen lots of rainbows & know about the 22degrees rule and so on, although never blase about them because they are too pretty for that! But this was something really special & only darkbow or moonbow fitted it.
Cal - it wasn’t all burgundy just that where red would normally be was a darker shade than usual, di have green & bue in although blues v hard to see against night sky. I’ve never seen an all red one though, although it does sound logical (or at least mostly red, there must have been some other colours left in the light).
Well, I for one think that it would be cool, and I wish I could have been there. It’s funny; folks accuse us rational, scientific types of taking all the wonder out of things like rainbows, but we’re the only ones who ever even notice them! Many a time, I’ve been walking along, gazing up at a rainbow, or clear stars, or a halo, or even an aurora, and people have asked me what I was looking at. Once, when I said “the stars”, a fellow actually asked, “Oh, are the stars out?”. It’s sad, really.
Now that I’m home I could check my library. Tricker isn’t any good on this point, and I don’t have Greenler either. But both Boyer and Minnaert have info on moonbows. Walker lists both of them in his references, along with W.J. Humphreys’ Physics of the Air and two articles that seem relevant:
“Why we Seldom See a Lunar Rainbow” by Humphreys in Science, Vol. 88 p. 496 (1938)
“Frequency of Lunar Rainbows” by C.K. wentworth in Science Vol. 88, p. 498 (1938)