YOSEMITE NATIONAL PARK — 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.
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.
What may be his most remarkable moonbow photo came not at the fall's base but across the valley on a cloudless night last year. Gilstrap caught a broad panorama: the entire Yosemite Falls cascading down with a moonbow arching across mist halfway up the sheer cliff face, the whole scene reflected in mirrored waters of a flooded meadow.
On a recent and far less perfect night, the moon was dodging in and out of a gauzy bank of high clouds preceding a spring storm that hit the next day. By 10 p.m. the moonbow was a ghostly arch floating above the creek's granite rubble.
"Honestly, it was pretty lousy until a few minutes ago," Gilstrap yelled above the roar of the waterfall, his parka whipping in the rush of wind and mist coursing down the gorge.
At any one time during the course of the evening, a dozen people paused to take in the nighttime spectacle.
John Wolfarth, visiting from Boston, wasn't disappointed.
"Oh, there it is! There it is! There it is!" he yelled to his friend Kevin Powers. "That's amazing!"
Bruce Wang, a 28-year-old UC San Francisco medical student, had driven four hours from the Bay Area just to photograph the moonbow.
He arrived at 5 p.m., spent hours making moonbow shots with his tripod-mounted camera, then at 11 p.m. packed up to head home.
"Hopefully I'll be home by 2:30 a.m., get a few hours sleep and get to classes by 10 a.m.," he said. "I'm just lucky enough to live close enough to do this. It's a really cool natural phenomenon."
Olson said he had known about moonbows since his days in the graduate program at UC Berkeley and had long been mulling a project to create a table tracking their appearances.
The opportunity presented itself in 2005, when he and Texas State lecturer Russell Doescher arrived in the valley with a team of students.
With the water low, they could venture into the creek bed below Yosemite Falls to make the measurements needed to plot important factors for moonbow observations, including the true horizon of Yosemite Valley's southern cliffs, which can block a moonrise.
Back at the university, Olson, Doescher and students Kellie Beicker, Ashley Ralph and Hui-Yiing Chang from the school's Mitte Honors program crafted a computer model factoring in the various earthbound and celestial coordinates. They conducted real-world tests in 2006 that proved their calculations were spot on.
This year they went public, writing an article for May's Sky and Telescope magazine and posting on the university's website a schedule of moonbows anticipated in the mist of Lower Yosemite Fall.
A little luck is still needed. A good moonbow requires clear sky, abundant mist at the base of the fall, an absence of artificial light and what Olson calls the correct "rainbow geometry."
The predictions have sparked a sort of moonbow renaissance at the park. Well-known photographer Keith Walklet has conducted moonbow workshops and offers photo tips on the Ansel Adams Gallery website at www.anseladams.com/content/newsletter/lunar_rainbow.html.