1920 -the Great Debate, also called the Shapley–Curtis Debate

[Swede]

In astronomy, the Great Debate, also called the Shapley–Curtis Debate, was an influential debate between the astronomers Harlow Shapley and Heber Curtis which concerned the nature of spiral nebulae and the size of the universe. The basic issue under debate was whether distant nebulae were relatively small and lay within our own galaxy or whether they were large, independent galaxies. The debate took place on 26 April 1920, in the Baird auditorium of the Smithsonian Museum of Natural History. The two scientists first presented independent technical papers about "The Scale of the Universe" during the day and then took part in a joint discussion that evening. Much of the lore of the Great Debate grew out of two papers published by Shapley and Curtis in the May 1921 issue of the Bulletin of the National Research Council. The published papers each included counter arguments to the position advocated by the other scientist at the 1920 meeting Shapley was arguing in favor of the Milky Way as the entirety of the universe. He believed galaxies such as Andromeda and the Spiral Nebulae were simply part of the Milky Way.

http://en.wikipedia.org/wiki/The_Great_Debate