The ‘Great Debate’: What Really Happened

On 26 April 1920 in Washington, two prominent astronomers, Harlow Shapley from Mount Wilson and Heber D. Curtis from Lick Observatory, debated the scale of the universe as part of the National Academy of Sciences' meeting. This event, often referred to as "The Great Debate," is frequently misrepresented in historical accounts due to a misunderstanding of the proceedings.

The discussion was initiated by George Ellery Hale, director of the Mount Wilson Observatory, who proposed an evening of talks at the Academy meeting in April 1920, possibly to engage the audience with an annual lecture. Hale suggested that the topic could be either the island universe theory or relativity. He believed that relativity might become irrelevant soon, thus proposing the island universe theory instead.

Despite Curtis's extensive work on nebula photography with the Crossley reflector and his enthusiastic advocacy for the island universe theory, Lick Observatory's director, W. W. Campbell, was selected to defend the theory. Curtis was eventually chosen as the speaker after Hale received a copy of Curtis's published work on nebulae from Campbell.

Although Hale favored relativity, Home Secretary C. G. Abbot expressed concerns about the complexity of the subject, fearing it might overwhelm the audience. Consequently, the debate focused on the scale of the universe, with Shapley and Curtis each allotted 45 minutes to present their arguments. Shapley, a brilliant astronomer at the crossroads of his career, accepted the invitation with wariness, while Hale, as his supervisor, saw the opportunity as a compliment.