A window of time just opened in Yosemite National Park in mid February 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.
Challenge: 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.
To be successful in photographing the watery firefall, it takes luck and timing, and the cooperation of nature. Horsetail Fall drains a small area on the eastern summit of El Capitan and flows only in the winter and spring in years with adequate rain and snow, which is scarce this year. Experts say it doesn't take a lot of water for the fall to light up.
Most important, the southwestern horizon must be clear, and February is the time of year when storm clouds often obscure the setting sun.
When conditions come together, the scrawny Horsetail Fall is the shining star of a park famed for its other waterfalls — raging Yosemite Fall and Bridalveil Fall. But Horsetail is the longest free-falling one, with a drop of 1,500 feet before it hits granite and spills another 500.