Physics Photo of the Week
Star Trails – Photo and Discussion by Erik Swanson
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!
Photo of the
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 firstname.lastname@example.org.
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