Quarter 3 Astronomer Bio Sources

http://www.britannica.com/EBchecked/topic/546850/Willem-de-Sitter
http://www-history.mcs.st-and.ac.uk/Biographies/Sitter.html
http://en.wikipedia.org/wiki/Willem_de_Sitter
http://www.nndb.com/people/209/000168702/
http://www.infoplease.com/encyclopedia/people/sitter-willem-de.html