Great Debate (astronomy)

The Great Debate, also called the Shapley–Curtis Debate, was held on 26 April 1920 at the U.S. National Museum in Washington, D.C. 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 center or entirety of the universe), while Curtis held that they were in fact independent galaxies, implying that they were exceedingly large and distant. A year later the two sides of the debate were presented and expanded on in independent technical papers under the title "The Scale of the Universe".

In the aftermath of the public debate, scientists have been able to verify individual pieces of evidence from both astronomers, but on the main point of the existence of other galaxies, Curtis has been proven correct.