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.

Spectacular ... the waterfall flows over the imposing El Capitan. Credit:AP

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 colour 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.