Physics Photo of the Week

October 5, 2012

Oblate Sun
As the Sun was setting on the evening of June 5, 2012, Venus was still in front of the Sun.  In addition to the extremely rare occurrence of the Venus transit, the Sun has been distorted out of round by the density gradient in the Earth's atmosphere near the distant horizon.  Viewed from the high elevation of the Craggy Visitors' Center on the Blue Ridge Parkway near Warren Wilson College the optical path length to the lower part of the Sun through the Earth's atmosphere is considerably longer than the optical distance to the top of the Sun.  The atmosphere around the curve of the Earth resembles a lens that is thicker on the bottom than on the top, hence it distorts the Sun to an oval shape.

Compare this image to the earlier image taken when the Sun was considerably higher in the sky shown in the animation.  When the Sun is high in the Sky the difference in optical distance between the upper and lower limb of the Sun is negligible. 

Notice also that the Sun is considerably reddened as it is setting.  This very common effect is also caused by the longer path length of light through the Earth's atmosphere.  The blue colors of the Sun's light are scattered away from the line of sight leaving the red rays to reach the observer.

The Venus transit of June 5, 2012 was featured in a recent Physics Photo of the Week (September 7, 2012)

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature 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

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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