Yosemite National Park, Calif. – 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 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 image: 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 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!”
Even with the computer predictions, a little luck is still needed to see a moonbow. A good one requires clear sky, abundant mist at the base of the fall, an absence of artificial light and what Olson calls the correct “rainbow geometry.”
If you can’t get to Yosemite, Olson has a simple solution.
The next time a full moon rises in the night sky, grab a backyard garden hose and turn on the tap.
Keep the glowing lunar face to your back, and a fine mist sprayed skyward will produce an arch of luminescence – a homemade moonbow.
