How Big is the Universe?

Two astronomers tried to answer this question during a live debate in 1920.

At 8:15 pm on Monday, April 26, 1920, two men took to the stage of the Baird Auditorium in the Smithsonian’s National Museum of Natural History in Washington, D.C. Each man had been given 40 minutes to present his views on a topic that was dividing the astronomical community: How large is the universe? The event would be remembered in science history as the “Great Debate.” 

Harlow Shapley of Mount Wilson Observatory in California argued that our Milky Way galaxy makes up the entirety of the universe. Spiral nebulae—which his colleague and rival Edwin Hubble would later identify as galaxies—were just puffs of gas and dust that didn’t contain other stars, said Shapely. Among his evidence, he reported that a colleague had measured rotation in spiral nebulae. If these objects were separate systems of stars, the rotation rate would exceed the speed of light, violating Albert Einstein’s theories of relativity. 

Shapley’s opponent was Heber Curtis, who’d been studying nebulae at Lick Observatory, which is outside San Jose, California. Curtis noted that several novae—stars that suddenly flare to thousands of times their normal brightness—had been discovered in our closest neighboring galaxy, Andromeda. If the nebula was inside the Milky Way, then one small piece of the galaxy would be creating novae at many times the rate of the overall galaxy, which seemed unlikely. In addition, the spirals appeared to be moving too fast to be bound to the Milky Way.

Following the Great Debate, the two astronomers published papers describing their respective theories. It would later emerge that the rotation rates of spiral nebulae cited by Shapley were a measuring error—and the discoveries made by other astronomers, notably Hubble, would ultimately vindicate Curtis’ views.

As for the Great Debate itself, Michael Hoskin, founding editor of the Journal for the History of Astronomy, later wrote that it was more of a polite conversation than a scientific barroom brawl. “Nevertheless, most historians persist in treating these published papers as the verbatim record of a dramatic trial of strength, and so have created an historical romance,” quipped Hoskin. “A referee might have declared ‘no contest.’ ”

Mark Strauss is Air & Space Quarterly’s managing editor.

This article—originally titled "Smackdown at the Smithsonian!"—is from the Summer 2025 issue of Air & Space Quarterly, the National Air and Space Museum's signature magazine that explores topics in aviation and space, from the earliest moments of flight to today. Explore the full issue.

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