Today in the history of astronomy, two astronomers argue their views of the nature of the Milky Way and other galaxies. | Published: April 26, 2026 The Great Debate between Harlow Shapley and Heber Curtis was held in the Smithsonian Museum of Natural History's Baird Auditorium (shown here about 10 years earlier, in 1910). Credit: Public domain/Wikimedia Commons On April 26, 1920, Harlow Shapley of Mount Wilson Observatory and Heber Curtis of Lick Observatory held a public debate at the Smithsonian Museum of Natural History. In two back-to-back 40-minute presentations, the pair offered their respective explanations of the size and nature of the Milky Way: Shapley was certain that the Milky Way was enormous – roughly 300,000 light-years across – so large, in fact, that it must be the entirety of the universe. He also argued that our solar system was located far from the center of the galaxy, and that “spiral nebulae” were merely gas clouds in the Milky Way.…