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Physics Photo of the Week

October 7, 2005

Star Trails at the WWC Farm

Photo and Discussion by Patrick Addabbo

Only 24 hours to complete an entire revolution, yet we don’t feel a thing.  A speed of nearly 1000 miles an hour if standing on the equator, the Earth’s rotation is what gives us night and day, sunrise and sunset.  It is also this rotation that, as seen in this article’s corresponding photo, apparent motion to stars in the night sky.  When photographed as seen here, using a Nikon N75 Camera with 400 speed film set at f/4 for 30 minutes, the stars appear to leave trails in the sky.  Just like the Sun’s path from rise till set, the stars themselves are not moving, but rather it is us moving underneath them. 

To explain for this apparent motion of the stars, or the sky as a whole, one must understand the celestial sphere.  The celestial sphere is the “apparent ‘bowl’ or hollow sphere of the sky,” basically a projection of the Earth’s sphere onto the sky, with the celestial equator and poles being projections of the Earth’s equator and poles.  Imagine the stars being projected onto the celestial sphere with the Earth rotating inside it.  If located at one stationary point on Earth, the sky, or celestial sphere, would appear to rotate around you as the movement of earth is indistinguishable.   The stars then, as they are pasted onto the celestial sphere, appear to move with the sky giving them trails in time exposure photography.

The included photograph taken from the Warren Wilson College farm, on August 30th, from 10:15-10:45, is interesting because of the stars apparent rotation around one star in the left hand side of the image.  This star, Polaris a.k.a. “the North Star,” is located almost exactly on the north celestial sphere, an extension of the Earth’s axis, giving it no apparent motion; instead, all other stars in our northern sky appear to rotate around it.

Photographing star trails is often a guessing game, but is a great way to learn about the night sky.  To produce a successful image, several factors need to be accounted for.  First is light pollution, if there is an abundance of light near the horizon, or a bright moon, this will overpower and washout and star light over any decent length exposure.  Also some sort of foreground should be considered to give the stars some sort of reference point, the silhouette of trees for example will show up just from the light of the stars and will give a basic horizon to the image.  As for the camera itself, as the exposure time lengthens film speed should decrease; this will eliminate excess grain and help prevent washing out star light.  The lens should be set at the widest aperture as depth of field is usually nonexistent and focal length should be set to infinity.  When successful, star trails produce beautiful, yet informative images of our night sky.
References:

Comins, Neil F. (2004). Discovering the Essential Universe. New York: W.H. Freeman and Company.

University of Oregon. Celestial Sphere. Retrieved September 22, 2005, from http://abyss.uoregon.edu/~js/images/celestial_sphere.gif

NASA. (1997, April 01). Speed of the Earth’s Rotation. Retrieved September 22, 2005, from http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/970401c.html

Lodriguss, Jerry. (2005). Star Trails. Retrieved September 22, 2005, from http://www.astropix.com/HTML/I_ASTROP/I06/I0601/I0601.HTM






Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an 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 dcollins@warren-wilson.edu.

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.