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.…