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Night Blue Sky

Physics Photo of the Week

April 22, 2005 (Earth Day)

Nightime Blue Sky

Notice the nice blue sky, contrasting with the white clouds as viewed from my garden.  Notice also the eerie appearance from all the lights in the valley.  Yes, this photo was taken at about 9:30 at night during the full moon of March 25, 2005.  The full moon, high to the photographer's right, provides enough illumination that a 15 second time exposure with a digital camera is enough to bring out landscape details.  What is more interesting is that the sky is blue similar to a midday sky.  The blue sky in the daytime is caused by the greater scattering of blue light from the sun by the air molecules in the sky (also called Rayleigh scattering).  We also only get blue skies when the atmosphere is relatively free of pollutants - fitting for an Earth Day photo.  At night during a bright moon, the moonlight (also comes from sunlight) is the illuminating source and the Rayleigh scattering of moonlight makes the moonlit sky appear blue.  Visually we don't see the blue color of moonlit skies because our eyes are less sensitive to color when the light is less intense.  Just above the white cloud in the nighttime photo one can barely see the star epsilon Bootis. 

The Photo at the right is the same scene taken in the early afternoon the following day.  Notice that the sky and the clouds are about the same colors as the moonlit photo above, but the foreground colors are much more "natural" without the street lights.  Based on the exposure values, aperture settings, the effective film speeds for the two photos, and the pixel values, the daylight photo is illuminated with about 166,000 times more light than the nighttime photo illuminated by the full moon.  This means the full moon on a clear night is about 166,000 times less bright than the sun on a clear day.  This also means that the full moon reflects about 30% of the solar visible light back to the earth, allowing for the scattering in all directions after the light hits the rough surface of the moon and the distance between the moon and the earth..  This also agrees fairly well with the close-up reflectivity of the lunar surface as about 10%.  The wonders of information and measurements possible with digital cameras!  All photos by Donald F. Collins.

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

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.