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It would have been possible roughly to estimate the 30-seconds 
exposure-length of this beautiful photo by measuring the star trails. 
If you look closely at the brighter stars in the top right corner and 
especially the top left, you can just see that they are slightly 
elongated. These short trails form concentric circles, with their 
centres near the top of the auroral glow, in the middle of the photo. 
As the earth turns, the sky rotates around a point close to the Pole 
Star. Judging from the trails, it might be one of the stars visible 
just above the cloud. The photographer was clearly facing due north.

http://antwrp.gsfc.nasa.gov/apod/ap060906.html


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Give people facts and you feed their minds for an hour.
Awaken curiosity and they feed their own minds for a lifetime.
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Ian Russell 

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