Physics Photo of the Week

November 30, 2012

Belt of Venus
Soon after sunset when the skies are exceptionally clear and cloudless, we can see the shadow of the Earth in the east - opposite the sunset.  The dark sky close to the horizon is the part of the Earth's atmosphere in the distance that is already night.  The so-called "Belt of Venus" is the "warm" reddish glow just above the earth's shadow but beneath the blue sky at higher elevations.  The warm glow is the result of back-scattered sunlight from the air molecules in a direction almost opposite the direction of sunlight.  At higher elevations, the region of the blue sky, the sunlight direction of scatter is such that only the blue colors are scattered toward the observer.  In the "belt" region, all wavelengths that reach the distant air are scattered.  In the shadow area there is virtually no light to scatter back to the observer.  Click on the image above to see a higher-resolution image.  The image was taken on Wednesday, November 28, 2012.  Another photo of the "anti-sunset" and the Belt of Venus (including a sequence) was posted on Physics Photo of the Week for January 21, 2005) 

The structure in the foreground is the partially-completed College View Observatory.  The roof (framework only at this stage) is built to roll-off the structure to open up the sky for an astronomical telescope to be installed upon completion.  The image at the right shows a close-up of one of the roof casters.  Click here on the image for a video of the first test rolling of the roof - about one inch of easy movement!


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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