Sunday, November 16, 2008
NASA's Picture of the Day
Crepuscular rays, in atmospheric optics, also known as sun rays, cloud breaks, sunburst, God's rays, God's Fingers, Fingers of God and Jacob's Ladder, are rays of sunlight that appear to radiate from a single point in the sky. These rays, which stream through gaps in clouds or between other objects, are columns of sunlit air separated by darker cloud-shadowed regions. The name comes from their frequent occurrences during crepuscular hours (those being dawn and dusk), when the contrasts between light and dark are the most obvious. Various airborne compounds scatter the sunlight and make these rays visible, due to diffraction, reflection, and scattering.
So, what's happening in the above photograph?
Although the scene may appear somehow supernatural, nothing more unusual is occurring than a setting Sun [behind the camera] and some well placed clouds. Pictured above are anticrepuscular rays. To understand them, start by picturing common crepuscular rays that are seen any time that sunlight pours though scattered clouds. Now although sunlight indeed travels along straight lines, the projections of these lines onto the spherical sky are great circles. Therefore, the crepuscular rays from a setting (or rising) sun will appear to re-converge on the other side of the sky. At the anti-solar point 180 degrees around from the Sun, they are referred to as anticrepuscular rays. Pictured above is a particularly striking set of anticrepuscular rays photographed in 2001 from a moving car just outside of Boulder, Colorado, USA.