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Physics Photo of the Week

September 16, 2005

Sun Pillar
This sun pillar was captured outside my bedroom window on May 9, 2005 before sunrise.  The explanation is fairly simple and sun pillars are supposedly common, but I have seen only about 3 in my lifetime.

The sky has some high cirrus clouds that consist of flat, hexagonal ice crystals.  The  flat platelike ice crystals are oriented horizontally and thus tend to reflect the light from the below-horizon sun.  The reflection is directed toward the observer.   The phenomenon is analogous to the moon-light or sunlight reflected off the ripples on a lake or pond (see PPOW for Oct 15, 2005)

The cirrus clouds in the atmosphere with the ice crystals frequently give rise to haloes surrounding the sun or moon (see PPOW for March 25, 2005).  The haloes, however, are caused by the refraction of light as the light passes through ice crystals.  The sun pillar is simply caused by the reflection of light by the surface of the ice crystals - much simpler than the refraction laws.

The photo above was taken at 6:22 am on May 9, 2005.  Compare the location of the sun pillar in the above photo with the location of the pillar in a slightly later photo taken at 6:49 am on the right - just as the sun is breaking above the mountain ridge.  The sun's path relative to the horizon is slanted - the angle of the slant is due to our location north of the equator.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.