Aristotle wrote about moonbows, and Ben Franklin, and John Muir, and they do, in fact, exist, not just in the pages of dusty old books but out in the real world. Uncommon, certainly, but real. And if you’re anywhere near Yosemite National Park this weekend, you won’t have a better chance to see the elusive night rainbow for the rest of 2010, according to computer modeling that for the first time in all the centuries people have been hunting moonbows accurately predicts when and where they’ll form.

“The rainbow occurs by day, and it was formerly thought that it never appeared by night as a moon rainbow. This opinion was due to the rarity of the phenomenon: it was not observed, for though it does happen, it does so rarely… The colors are not easy to see in the dark… The moon rainbow appears white…” — Aristotle, Meteorologica, circa 340 B.C.

Moonbows are created by the interaction between moonlight and water molecules, in this case the spray from Yosemite Falls. In Yosemite, six elements are required: Bright moonlight, clear skies, dark skies, plenty of mist, correct rainbow geometry, and the moon not blocked by the walls of Yosemite Valley. A team of researchers from Texas State traveled back and forth to Yosemite create and test their moonbow predictions, which show that June 25 and 26 are the best nights for the year.

“Why did not Captain Cook have taste enough to call his great discovery the Rainbow Islands? These charming spectacles are present to you at every turn; they are common in all the islands; they are visible every day, and frequently at night also – not the silvery bow we see once in an age in the States, by moonlight but barred with all bright and beautiful colors, like the children of the sun and rain. I saw one of them a few nights ago.” — Mark Twain in Hawaii, 1866

Yosemite’s moonbows need lots of mist and thus lots of water, which depends on snowmelt–typically by July, sometimes as late as August, winter’s snow is gone and there isn’t enough water to form the necessary mist. If the weather cooperates (forecast), the first subtly colored arc should start appearing after 11:02 p.m. on Friday, when the moon clears the south rim of the valley near Sentinel Dome. The chances for a sighting dwindle but continue through early Monday morning.

“Lunar rainbows or spray-bows also abound in the glorious affluence of dashing, rejoicing, hurrahing, enthusiastic spring floods, their colors are distinct as those of the sun and regularly and obviously banded, though less vivid. Fine specimens may be found any night at the foot of the Upper Yosemite Fall, glowing gloriously amid the gloomy shadows and thundering waters, whenever there is plenty of moonlight and spray. Even the secondary bow is at times distinctly visible. “ — John Muir, 1912

The best times to see Yosemite Falls moonbows in 2010? Here:

• June 25/26 11:02 p.m. to 12:20 a.m.

• June 26/27 11:40 p.m. to 1:00 a.m.

• June 27/28 12:05 a.m. to 1:35 a.m.

And here are a few more pictures, as well as a briefly occurring lunar rainbow in a video from Torres del Paine, Patagonia, Chile.

Tagged as: environment, national parks, science, weather