The Great Debate, also called the Shapley–Curtis Debate, was held on 26 April 1920 at the Smithsonian Museum of Natural History, between the astronomers Harlow Shapley and Heber Curtis. It concerned the nature of so-called spiral nebulae and the size of the universe. Shapley believed that these nebulae were relatively small and lay within the outskirts of the Milky Way galaxy (then thought to be the entire universe), while Curtis held that they were in fact independent galaxies, implying that they were exceedingly large and distant. 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 by 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. In the aftermath of the public debate, scientists have been able to verify individual pieces of evidence from both astronomers, but on the main point of the existence of other galaxies, Curtis has been proven correct. Arguments Shapley was arguing in favor of the Milky Way as the entirety of the universe. He believed that "spiral nebulae" such as Andromeda were simply part of the Milky Way. He could back up this claim by citing relative sizes—if Andromeda were not part of the Milky Way, then its distance must have been on the order of 108 light years—a span most contemporary astronomers would not accept. Adriaan van Maanen, a well-respected astronomer of the time, also provided evidence supporting Shapley's argument. Van Maanen claimed he had observed the Pinwheel Galaxy rotating, and that if the Pinwheel Galaxy were in fact a distinct galaxy and could be observed to be rotating on a timescale of years, its orbital velocity would be enormous and there would be a violation of the universal speed limit, the speed of light. Shapley also backed up his claims with the observation of a nova in the Andromeda "nebula" that had briefly outshone the entire nebula, constituting a seemingly impossible output of energy were Andromeda in fact a separate galaxy. Curtis, on the other hand, contended that Andromeda and other such "nebulae" were separate galaxies, or "island universes" (a term invented by the 18th-century