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.
Click
here to see the Physics Photo
of
the Week Archive.
Observers are invited to submit
digital photos to:
