Physics Photo of the Week
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
PPOW for Oct 15, 2005)
The cirrus clouds in the atmosphere with the ice crystals frequently
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
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 email@example.com.
here to see all Physics Photo of the Week for 2005
to see all Physics Photo of the Week for 2004.