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 distant nebulae were relatively small and lay within the outskirts of Earth's home galaxy, 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.
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 108light years--a span most contemporary 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. Shapley also used to back up his claims 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 side contended that Andromeda and other such "nebulae" were separate galaxies, or "island universes" (a term invented by the 18th-century philosopher Immanuel Kant, who also believed that the "spiral nebulae" were extragalactic). 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 other sections of the galaxy, if Andromeda were not a separate galaxy but simply a nebula within Earth's galaxy. This led to supporting Andromeda as a separate galaxy with its own signature age and rate of nova occurrences. He also cited dark lanes present in other galaxies similar to the dust clouds found in Earth's own galaxy.
Curtis stated that if van Maanen's observation of the Pinwheel Galaxy rotating were correct, he himself would have been wrong about the scale of the universe and that the Milky Way would fully encompass it.
Due to the work of Edwin Hubble, it is now known that the Milky Way is only one of as many as an estimated 200 billion  to 2 trillion or more galaxies, proving Curtis the more accurate party in the debate. Also, astronomers generally accept that the nova Shapley referred to in his arguments was in fact a supernova, which does indeed temporarily outshine the combined output of an entire galaxy. 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's galaxy was centered on the Sun, while Shapley correctly placed the Sun in the outer regions of the galaxy).
The format of the great debate has been used subsequently to argue the nature of fundamental questions in astronomy. In honor of the first "Great Debate", the Smithsonian has hosted three more events.