Physics Photo of the Week

October 19, 2012

My digital camera (Canon XSi with a 300 mm zoom lens) caught Neptune on September 9, 2012.

Neptune was first observed and confirmed in 1846 by Johann Galle and Heinrich d'Arrest.  With a sky of millions of stars, how does an astronomer know which star-like spot of light is a distant planet?  First astronomers must know where to look.  Urbain Le Verrier in the preceding several years made careful calculations from the positions of Jupiter, Saturn, and Uranus (discovered in 1781), that there must be a small gravitational perturbation from another planet.  Le Verrier was able to calculate the position of this 8th planet.  Galle and Arrest, pointed their visual telescopes at the predicted place for the new planet.   They carefully sketched by hand the relative locations of stars in the field on different nights and saw that one of the "stars" had moved - indicating a planet orbiting the Sun and wandering among the stars. 

Technology of the 21st century has enabled amateurs to photograph Neptune and Uranus (both not visible without optical aid).  Digital cameras, particularly those with exposure control, can photograph the starfields of the night sky with enhanced sensitivity.  Computer software (The Sky - a commercial product - eventually will become available for a nominal fee for tablet computers) lets the user find the location of practically any astronomical object on any date and time from anywhere on Earth, and print paper maps.  A similar on-line free tool is skymaponline.  Another tool is software that can blink successive images.  A photograph made on another night - after aligning all the stars to match the first photograph - can be alternately displayed on a computer screen.  The animation here shows a "blink" of the star field between Sept. 9, 2012 and Oct. 11, 2012.   Similar techniques were used to track Pluto (PPOW October 15, 2010).  Pluto, however, is about 100 times fainter than Neptune, a telescope was required.

In these pictures of Neptune, the camera was fixed on a tripod.  Exposures of 30 seconds and 20 seconds cause all the stars' images to show as short streaks due to the rotation of the Earth.

Due to fall break at Warren Wilson College, there will be no Physics Photo of the Week next week.  Look for the return of Physics Photo of the Week on November 2, 2012.

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.

Click here to see the Physics Photo of the Week Archive.

Observers are invited to submit digital photos to: