References:

Michael Ewert.(No date). Apparent Motion of Planets on the Celestial Sphere. University of Tennessee. (Online). Available: http://cesp10.phys.utk.edu/astr161/lect/retrograde/retrograde.html

Michael Ewert. (No date). Astronomy 161 - In The Solar System. University of Tennessee. (Online). Available: http://csep10.phys.utk.edu/astr161/lect/index.html

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