The Great Debate
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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. He could back up this claim by citing relative sizes—if Andromeda and such were not part of the Milky Way, then the distance to it be in the order of 108 light years—a span most astronomers would not accept. Adriaan van Maanen was also providing evidence to Shapley's argument. Maanen was a well respected astronomer of the time who said he had observed Andromeda rotating. If Andromeda was in fact a distinct galaxy and could be observed to be rotating, there would clearly be a violation of the universal speed limit, the speed of light.
Curtis on the other side contended that Andromeda and other such nebulae were separate galaxies, or "Island universes". He showed that there were more novae in Andromeda than in the Milky Way. From this he could ask why there were more novae in one small section of the galaxy than the others. This led to supporting Andromeda as a separate galaxy with its own signature age and rate of novae occurrences. He also cited dark lanes present in other galaxies similar to the dust clouds found in our own galaxy and massive doppler shifts found in other galaxies.
Curtis stated that if van Maanen's observation of Andromeda rotating were correct, he himself would have been wrong about the scale of the universe and that the Milky Way would fully encompass it. It later became apparent that van Maanen's observations were completely false—one can not actually see Andromeda rotate during a human lifespan. It is now known that the Milky Way is only one of hundreds of billions of galaxies in the visible universe, proving Curtis the more accurate party in the debate in that respect. On other points the results were mixed (the actual size of the Milky Way is in between the sizes proposed by Shapley and Curtis), or in favor of Shapley (Curtis' galaxy was centered on the Sun, while Shapley correctly placed the Sun in the outer regions on the galaxy).[1]
