http://www.astro.virginia.edu/research/observatories/26inch/history/curtis.php
http://en.wikipedia.org/wiki/The_Great_Debate
http://www.daviddarling.info/encyclopedia/C/Curtis.html
http://en.wikipedia.org/wiki/Heber_Doust_Curtis
http://apod.nasa.gov/diamond_jubilee/debate_1920.html
http://www.astrosociety.org/pubs/mercury/30_03/seasons.html
http://incubator.rockefeller.edu/?p=266
http://www.britannica.com/EBchecked/topic/147181/Heber-D-Curtis
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