Physics Photo of the Week

December 18, 2009

2 Solstices

This is a panoramic shot of the horizon near Warren Wilson College looking due east.  Usually a multishot panorama consists of several photos pointing at different angles taken immediately after each other.  However, in this pan shot, the two images were taken almost exactly 6 months apart!  The image on the right was taken very close to the winter solstice on Dec. 24, 2007.  The image on the left was taken the following June 20, 2008. 

Not only do we see the Sun rising in drastically different locations, but the sky color varies greatly.  Both days were obviously very clear, but the summer air usually contains much more water vapor and aerosols that add the red color to the sky.  Actually the water aerosols (microscopic suspended water droplets) scatters the blue wavelengths out of the line of si
ght with the Sun, so the sumer sunrise sky lacks the blue wavelengths.  The lack of moisture in the winter air scatters much less of the sunlight.  Thus the winter sunrise is white and the sky is blue.

The location of the sunrise depends on the season of the Earth in its orbit around the Sun.  Since the axis of the Earth is tilted 23.5 degrees from the orbital plane, the perspective of the Sun changes from due east on the equinox to 23.5 degrees north or south of due East.  The shift in the horizontal position of the sunrise between the winter and summer solstices is a bit larger than twice the 23.5 degrees.  In the picture below the two paths slanted upwards and to the right represent the apparent path the Sun follows in its rising.  That direction depends on our latitude (35.5 degrees at Warren Wilson College).  The solar path is tilted 35.5 degrees off the vertical.  47 degrees is separation of the two rising paths, but on a direction perpendicular to the two paths.  The angular separation along the horizontal is 47 degrees divided by the cosine of the latitude angle.  This results in 58 degrees.  See the diagram belo

Enjoy the winter solstice this coming Monday, December 21.  Have a Merry Christmas, Happy Hannakah, a Joyous New Year and may these holidays be safe for you and your families.  This is the last Physics Photo of the Week for 2009 due to winter break at the college.  We'll see you back on January 22, 2010.

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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