Physics Photo of the Week

November 13, 2009

Star Trails – Photo and Discussion by Erik Swanson

As we know the Earth rotates on an axis, it is our own axial motion that causes the stars of the sky to change throughout the evening, while they themselves appear to move. This is called diurnal motion, and our rotation is also the cause for the setting and rising of the Sun each day. This motion is fixed in our hemisphere around the north celestial pole, where the commonly known star Polaris, or the North Star, resides almost exactly. The concept of the celestial sphere is useful when thinking of diurnal motion. Imagine the Earth as a tennis ball floating at the center of an inflated beach ball; the tennis ball is also rotating on an axis while the beach ball remains stationary. Dots on the inside of the beach ball represent the stars while the tennis ball rotates, and when stationary on one point of the tennis ball, those dots would seem to rise and set just like the stars in our sky.

This motion is more easily seen when taking continuous long-term exposures with a digital camera.  This photograph was taken on the Blue Ridge Parkway from 9:45 P.M. to 10:15 P.M., on October 8, 2009. The camera was stationary, and took 30 second exposures repeatedly during this half hour, recording the position of the star. Afterwards all of the images were overlaid, and the star trails are a result of this compilation. The weather was not ideal; fog occasionally blocked the stars' images - hence the gaps.

You may notice that as your eyes move left along the picture the concentric star trails get increasingly closer to a center point. This point, while not actually upon the image, is where Polaris and the north celestial pole are located. Were we to see Polaris in this image, it would have nearly no motion throughout the entire series of exposures. The two most noticeable constellations in this image are Cassiopeia and Perseus; see if you can find them!

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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