Physics Photo of the Week

October 2, 2009

Wobbling Moon
The gibbous moon was photographed on two separate nights by members of the Contemporary Astronomy Class.  The first frame, where the moon is less full, was photographed on Monday, Sept. 28 by Marlon Cohn, Dan Faulkner-Bond, and Erin Haggerty.  The other image, when the Moon was more full, was photographed 2 nights later on Wednesday, Sept. 30 by Peter Calfee, Garrett Chaffee, and Kesari Fleury.  The images were then aligned, enhanced, and made into an animated sequence.

Notice how the illuminated portion of the Moon that faces the Earth has grown over the span of 2 nights.  This is because the Moon has advanced in its orbit around the Earth and becomes closer to opposition from the Sun in the fuller image.  Notice also how the craters show up better when they are closer to the "terminator" - the boundary between the illuminated and dark portions of the Moon.  At the terminator, the Sun is "rising" on the surface of the Moon, hence the shadows from the craters are more pronounced.  For parts of the Moon that are well-illuminated, the shadows are not noticeable.  400 years ago, in 1609, Galileo Galilei aimed his primitive telescope at the Moon and discovered the cratered appearance of the Moon's surface.  Galileo may not have been the first person to have observed the Moon through a telescope, but he was certainly the first person to publish his findings in The Stary Messenger in 1610.

Finally, notice that the animation shows a "wobble" to the Moon and a slight variation in the size of the Moon between the two observations.  These are due to a small "back and forth" motion called "Libration" and the variation in the Earth-Moon distance as the Moon orbits the Earth in an eliptical orbit.

These photos were made with a digital camera attached to a Questar telescope.  This telescope was a recent generous donation to Warren Wilson College by Ralph Brown.

The two photos that make up the animated photo above are shown separately below.

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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