The Great Debate
In April of 1920, Shapley participated in the Shapley-Curtis Debate in Washington, D.C. This "Great Debate" saw Shapley and Herber D. Curtis arguing over the scale of the universe, as well as the nature of nebulas and galaxies. Curtis argued that the Universe is made up of many galaxies similar to ours (these had been identified by astronomers of the time as "spiral nebulae.") Shapley argued that spiral nebulae were clouds of gas and that they and globular clusters occur within the Milky Way galaxy. Shapley argued against Curtis' position that the Sun was at the center of the galaxy, saying that our solar system exists in the outer limits of a very large galaxy. He was correct on this point, although Edwin Hubble played a part in proving Shapley's position about nebulae and globular clusters occurring within our galaxy wrong when he demonstrated that the Cepheid variables in the Andromeda galaxy were much further away than Shapley's proposed extent of the Milky Way and that Andromeda was indeed its own "island universe."