Physics Photo of the Week

September 3, 2010

A meteor, an airplane, or a satellite?
While conducting star photography on April 9, 2010 at about 9:00 pm, a bright light was noticed in the east.  Because the photos used a long time exposure (15 seconds), any movement during the exposure results in a streak of light.  Is this streak a meteor, an airplane, a satellite, or some UFO?  If you look carefully in the top center of the photo, a another faint streak (red) can also be seen. 

Luckily a series of sequential images were made, each image consisting of a 15 second time exposure.  The resulting time lapse is animated in the image below.  Notice that the bright streak can be seen in each of the frames of the animation, but moves between the frames.  The faint red streak at the top can be seen only in the final four frames moving from right to left.  If these streaks were meteors, they would be visible in only one frame, because meteors are visible for only about 1 second.  These streaks move too rapidly to be a distant satellite.  They aircraft - the lower aircraft has its headlights on for an approach to an airport - either Asheville, NC or Greenville-Spartanburg, SC.  The upper aircraft leaves a dotted trail due to the blinking of its running lights.  Even though aircraft move much more slowly than satellites, they appear to move much more quickly because they are much closer to us than satellites.

We can see two other effects in these series.  Within the time span of the animation, the sky becomes progressively darker in each successive image.  Twilight is gradually sinking into darkness.  Notice also that the stars appear to move slightly between the images.  The "motion" of the stars is due to Earth's daily rotation.

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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