YOSEMITE NATIONAL PARK, Calif. -- A window of time just opened in Yosemite National Park when nature photographers wait, as if for an eclipse, until the moment when the sun and earth align to create a fleeting phenomenon.
This marvel of celestial configuration happens in a flash at sunset in mid-February – if the winter weather cooperates. On those days the setting sun illuminates one of the park's lesser-known waterfalls so precisely that it resembles molten lava as it flows over the sheer granite face of the imposing El Capitan.
Every year growing numbers of photographers converge on the park, their necks craned toward the ephemeral Horsetail Fall, hoping the sky will be clear so they can duplicate the spectacle first recorded in color in 1973 by the late renowned outdoors photographer Galen Rowell.
"Horsetail is so uniquely situated that I don't know of any other waterfall on earth that gets that kind of light," said Michael Frye, who wrote the book "The Photographer's Guide to Yosemite."
"How many are perched on a high open cliff? Most are in an alcove or canyon and won't get the sun setting behind it. Yosemite's special geography makes this fall distinctive," he said.
Four decades ago, photographers had only to point and shoot to capture another famous Yosemite firefall – a man-made cascade of embers pushed from a bonfire on summer nights from Glacier Point.
But photographing Horsetail is a lesson in astronomy, physics and geometry as hopefuls consider the azimuth degrees and minutes of the earth's orbit relative to the sun to determine the optimal day to experience it. They are looking for the lowest angle of light that will paint Horsetail the colors of an iridescent sunset as rays reflect off granite behind the water. It materializes in varying degrees of intensity for the same two weeks every year.
"If you hit it at just the right time, it turns this amazing color of gold or red-orange," said Frye, a photo instructor with the Ansel Adams Gallery in the park.
Adams photographed the fall, but his iconic black and white images do not capture its fiery quality, and it's unclear whether he ever noted it.