Shapley–Curtis Debate on the Scale of the Universe

On April 26, 1920, astronomers Harlow Shapley and Heber Curtis debated the size of the Milky Way and the nature of spiral nebulae at the U.S. National Museum in Washington, D.C. Shapley argued the Milky Way was 300,000 light-years across with the Sun displaced from its center; Curtis countered that spiral nebulae were separate galaxies. Edwin Hubble's 1924 observations confirmed spiral nebulae as independent galaxies, validating elements of both positions and establishing that the observable universe contained billions of galaxies.