Astronomer sources

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