The Great Debate was the 1920 astronomy event that questioned whether spiral nebulae lay within the Milky Way or were separate galaxies. It was held on 26 April 1920 at the U.S. National Museum in Washington, D.C., between Harlow Shapley and Heber Curtis.
Shapley argued that spiral nebulae were relatively small objects within the Milky Way. Curtis argued that they were independent “island universes,” enormously distant systems comparable to the Milky Way. The disagreement involved the scale of the universe, the distances of nebulae, nova brightness, and claimed measurements of spiral rotation.
The debate did not immediately settle the issue. In 1921, Shapley and Curtis presented expanded arguments in separate papers under the title “The Scale of the Universe.” Edwin Hubble’s later distance measurements of Cepheid variables in Andromeda established that it was a separate galaxy, vindicating Curtis on the central question. The event is also commonly called the Shapley–Curtis Debate, but “Great Debate” is the standard canonical name.