Great Debate (astronomy)
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 these nebulae were relatively small and lay within the outskirts of the Milky Way galaxy (then thought to be the entire universe), 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.