This is about the famous discussion of astronomy. For the Dream Theater song about stem cell research, see The Great Debate (song). 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 was not part of the Milky Way, then its distance must have been on 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. Also used to back up his claims was a nova that had been observed 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. 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.

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