When you see both a primary and a secondary rainbow, look between them. The 8° of sky in that region will be darker than rest of the sky (in that direction, anyway). That dim band is called Alexander's dark band, the Alexander Band, the dark band of Alexander, &c. &c. It is in this band that troubles melt like raindrops.*

It's named for the first occidental natural philosopher to write about it, the peripatetic Alexander of Aphrodisias, circa 200 BCE, as part of his commentary on Artistotle's Meteorologica.

What causes it? A hole in the reflection. See, when sunlight enters a raindrop, it refracts in, reflects off of the inner surface, and refracts out again. Owing to the particulars of concave spherical reflections, the rays entering the top of the drop get scattered back at you, towards the antisolar point, making the sky inside the primary rainbow a little brighter. Rays entering above the drop's equator get concentrated by a single inner reflection and become the familiar 42° rainbow. Rays below the drop's equator get concentrated too, by their double reflection, and become the less common, secondary 50° rainbow. Rays entering near the bottom of the drop get scatterd back towards you, outside the secondary rainbow, but don't have a noticable effect.

A metaphorical way to think of it is this: raindrops part a curtain of reflection, and the concentrated folds make the rainbows. The revealed dark part is what the sky looks like if the raindrops weren't there.

I wondered if any goth bands had adopted this charming name, and conducted a quickie web search in that regard. There is an Alexander's Ragtime Band, but that's probably not derivative. However, both the British indie band Gasgiant and the spooky-indie rocker Jeroan van Aichen produced albums called Alexander's Dark Band. I was hoping either was some sort of Tom-Waits-meets-Sgt.-Peppers-esque concept album, but reviews contraindicate. At least Gasgiant's tracks 1 and 6 (avilable for free download from their website, below) are at least pleasant music to ponder rainbows by.

Timeshredder adds: So far as I know, "Alexander's Ragtime Band" is an unrelated Irving Berlin song which also inspired a 1938 film.


    Sources:
  • http://www.sundog.clara.co.uk/rainbows/adband.htm
  • http://plato.stanford.edu/entries/alexander-aphrodisias/
  • http://www.philiplaven.com/p8e.html
     
  • http://www.floppyrecords.co.uk/gasgiant.html
  • http://www.amazon.com/exec/obidos/ASIN/B00000I972/ref=nosim/ezfolk-20/102-9575463-7262541?dev-t=D2Y5TUCCVJ7DGE
  • *I would love to claim that joke as my own, but I must give credit to Raymond L. Lee, Jr., of the U. S. Naval Academy.

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