Physics Photo of the Week

January 25, 2013

Stereo Moon - by Don Collins and Tom Overman
These images were taken from two widely separated points on Earth at the same universal time.  Tom Overman, WWC 1984 graduate, lives in Honolulu, Hawaii.  Tom took the left image on December 27, 2012 (the night of full Moon) at 10:00 pm Hawaii time.  Don took the right image from Swannanoa, NC at 3:00 am the following day.  Both events occurred at the same Universal Time (0800 UT 28 December 2012).  This project fulfills an idea that Tom had expressed several years ago to see the stereo effect of imaging the Moon from two different points on the Earth at simultaneous times.  Aside from color, these images look identical.  The color difference is attributed to the difference in humidity at the two sites. 

One method to see the stereo 3-D effect (not recommended) the reader may get rather close to the above images and stare at them cross-eyed (so the right eye is focused on the left image; left eye focused on the right) and merge the images.  It gives the viewer a bad headache.  A much more satisfying result may be obtained by displaying the two images overlapped - one displayed in blue the other in red called a blue/red anaglyph.  The blue/red anaglyph requires blue/red glasses to view the 3-D effect.  Click here to view the anaglyph produced by Michael Collins.  Blue/red glasses may be obtained mail-order from Educational Innovations (  Don has placed a few pairs on reserve in the Martha Ellison Library at Warren Wilson College.

We can see the small shift of the features on the Moon due to the 7000 km separating Swannanoa and Honolulu by blinking the two images (shown at right).  The Honolulu image is the blinked image in which the features are slightly rotated to the left on the page (celestial East) because Honolulu viewed the Moon from much further west on the Earth (left on the images).  Celestial north is the top of the images.  In other words Honolulu sees around the west side of the Moon more than Swannanoa, similar to seeing more of the west side of a building by moving further west.

We measured the shift in pixels between the Swannanoa view and the Honolulu view (6 pixels ± 1 pixel) or 1/550 to 1/750 the diameter of the Moon.  Using geometry, the separation of the two cities, and the fact that the Moon's diameter is 1/4 the diameter of the Earth we have determined that the distance from the Earth to the Moon is between 42 and 57 Earth radii.  This compares favorably with with the accepted value of about 60 Earth radii.  Longer focal length telescopes will give more precise results. 

Tom had also figured out the size and distance of a model Moon if we considered the two cities (Honolulu and Swannanoa) as the right and left eyes of an observer whose eyes are separated by 70 mm.  The model moon would be the size of a ping pong ball located about 3.7 meters from the observer (across a typical residential room). 

Coordinating this project between two widely-separated observers has been lots of fun!  Many thanks to all who helped!  The image of the Moon from Swannanoa was made with a Questar 3 1/2 inch telescope donated to Warren Wilson College by Ralph Brown of Chattanooga, TN.

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

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