Physics Photo of the Week

December 12, 2008

Red Sky at Night
This picture of the moon rising above the Swannanoa Mountains on the evening of May 19, 2008 shows a red sky typical of a sunrise or sunset.  We usually don't see the vivid colors at night when our eyes are less sensitive to color.  A color digital camera, however, does not lose its sensitivity to color - it merely must open the shutter for a longer time to compensate for less light.

The color of a red sky at sunrise, sunset, moonrise, etc. is attributed to the phenomenon known as Rayleigh scattering - named after the 19th century physicist John William Strutt (Lord Rayleigh).  The Earth's atmosphere, even if entirely free of pollutants, mist, or humidity is not entirely uniform.  The nature of the molecular composition of air (or gases in general) is such that the density of molecules is not exactly uniform but contains random microscopic fluctuations in density.  Rayleigh proved that these density fluctuations
not only scattered light, but the amount of scattering depends drastically on the wavelength of light.  Blue light with short wavelengths is scattered much more than long wavelength red light.  When we are looking in the direction of the source of light, the blue light has been scattered out of the line of sight.  Only the more penetrating red colors make it through the air to the observer and camera.  Hence sunsets and moonsets are generally red in color.

The Rayleigh scattering also explains why the sky is blue. 
When we look about 90 degrees away from the source of light (Sun or Moon), the blue light components from the source is scattered towards the observer while the red light continues straight and is not scattered towards the observer.  The photo at right was taken a couple of nights earlier, but the Moon was almost as bright.  The moonlit sky is blue in the gaps between clouds.  The moonlit blue sky is not as bright as the daytime sunlit sky, hence the stars in the constellation Lyra are visible.  The picture consists of several 15 second exposures added together.  As a result the clouds are highly streaked.

The moonlight blue sky may be also seen in Physics Photo of the Week on April 22, 2005.



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 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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