Physics Photo of the Week

May 2, 2014

Ellipsoidal Sunset - Photo by Art Brooks
My friend Art Brooks was visiting Costa Rica on April 8 and took this impressive photograph of the setting Sun. 
Click on the image for a larger view.  This shows a rare ellipsoidal or "squashed" effect caused by the long path length through Earth's atmosphere and the increased density of the air at lower altitudes.  Mr. Brooks took this photo from an elevation of about 5000 feet where he had a good view of the setting Sun.  The increased density of the lower atmosphere is also very evident from the vivid color gradient of the Sun's image.  The lower part of the Sun is redder than the upper part because if the increased distance the lower part has to travel through the Earth's atmosphere.  The larger pathlength in air causes more of the blue color to be scattered away.  This strong gradient doesn't occur until the Sun is barely above the horizon, which is not visible due to the haze.

The density gradient in the Earth's atmosphere also acts like a lens - not to enlarge the Sun, but to "squash" the shape into an oval.  Normal glass lenses have curved surfaces so that the light requires a longer time to pass through the thicker part of the lens and thus gets bent toward the direction of the slower part of the lens.  The larger density of the Earth's atmosphere at the bottom edge of the Sun has the same effect: the light from the bottom limb of the Sun takes longer to reach the observer than light from the top edge.  As a result the light from the Sun's lower limb is bent downward toward the observer.  This doesn't place the bottom of the Sun lower in the picture, but just the opposite: the observer sees the bottom rays come from a higher angle (they have been bent to travel downward toward the observer).  The base of the Sun appears to come from slightly higher than the real position of the Sun.  It is even possible to see the lower limb of the Sun after it has descended below the horizon!


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 dcollins@warren-wilson.edu.

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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