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skyandtelescope.org
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John E. Bortle (February 2001). "The Bortle Dark-Sky Scale". Sky & Telescope. Archived from the original on 23 March 2009. Retrieved 2009-11-18.
"Messier 81". SEDS (Students for the Exploration and Development of Space). 2007-09-02. Archived from the original on 2017-07-14. Retrieved 2009-11-28.