May 23, 2007

PICTURE: It's not a beautiful rainbow, it's a beautiful 'moonbow'

SPECTACULAR GLOW: Commercial photographer Brent Gilstrap captures a moonbow over Yosemite Falls. Ideal conditions for moonbows include clear skies, abundant mist at the base of a fall and the absence of artificial light.

(FOX 59) Aristotle took note of this celestial happening a couple of millenniums back. Ben Franklin bagged a sighting or two, as did Mark Twain. The venerable John Muir, chronicler of Sierra mountaintop and meadow, waxed enthusiastic about the nighttime phenomenon.

The hunt for the elusive "moonbow" has long been a nocturnal lure for dreamy hikers, insomniac seamen and intrepid photo buffs. But in the past, seeing one of these nighttime rainbows — caused when a full moon's rays bounce off the mist of a departing rain cloud or raging waterfall — has been dictated mostly by chance.

No longer.

A team of astronomers from Texas State University in San Marcos has produced a computer model that can reliably predict the date and duration of moonbows at Yosemite Falls, the national park's tallest and most photogenic waterfall.

Their predictions have sent waves of camera buffs and Yosemite Valley visitors trekking up the trail to the plank bridge near the base of the waterfall.

Aside from those who have visited during an overcast night, few have come away disappointed.

"So far as we know, we're the first to predict dates and precise times for when moonbows will appear," said Don Olson, the Texas State astronomy professor who led a team of honors students in the project. "It's great for people who otherwise might have sat around all night waiting to see a moonbow, and for the students it was a nice exercise in calculus, spherical trig and computing."

The team's moonbow table took Brent Gilstrap to the waterfall one recent night.

Gilstrap, who two years ago chucked his computer software career to become a commercial landscape photographer, has been dependably making spectacular shots of moonbows ever since he learned of the Texas State lunar table.

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