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Fitzpatrick, Richard (2 February 2006). "Planetary orbits". Classical Mechanics – an introductory course. The University of Texas at Austin. Archived from the original on 3 March 2001.
"Orbit: Definition". Ancillary Description Writer's Guide, 2013. National Aeronautics and Space Administration (NASA) Global Change Master Directory. Archived from the original on 11 May 2013. Retrieved 29 April 2013.