[[Image:Andromeda_galaxy.jpg|thumb|250px|Az [[Androméda-galaxis]] [[Ultraibolya sugárzás|ultraviola]] fényben]]
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 galaxy|spiral]] [[nebula]]e and the [[Universe#Size.2C_age.2C_contents.2C_structure.2C_and_laws|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 [[Spencer Baird|Baird]] auditorium of the [[Smithsonian]] [[National Museum of Natural History|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 [[United States National Research Council|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 Galaxy|Andromeda]] and the Spiral Nebulae were simply part of the Milky Way. He could back up this claim by citing relative sizes—if Andromeda was not part of the Milky Way, then its distance must have been on the order of 10<sup>8</sup> [[light years]]—a span most [[astronomy|astronomers]] would not accept. [[Adriaan van Maanen]] was also providing evidence to Shapley's argument. Van Maanen was a well-respected astronomer of the time who claimed he had observed the [[Pinwheel Galaxy]] rotating. 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 clearly be a violation of the universal speed limit, the [[speed of light]]. Also used to back up his claims was the observation of a [[nova]] in the Andromeda galaxy that had temporarily outshone the nucleus of the galaxy itself, a seemingly absurd amount of energy for a normal nova. Thus, the nova and the galaxy itself must be within our own galaxy since, if Andromeda were a galaxy in its own right, the nova would have had to have been unthinkably bright in order to be seen from so far away.