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In 1920, the Academy was the scene of what has come to be called the “Great Debate” over the scale of the universe. The debate took place under the auspices of the George Ellery Hale Lecture series, and consisted of two lectures delivered during the Monday afternoon scientific session of the Academy’s April, 1920 Annual Meeting.
The participants were Heber D. Curtis, then of Lick Observatory, and Harlow Shapley of Mount Wilson Solar Observatory. In brief, the controversy concerned the scale and makeup of the universe. Shapley argued that the universe was comprised of a single galaxy, while Curtis held that it contained many galaxies. In holding these positions, each came to different conclusions regarding the celestial objects astronomers at the time called “spiral nebulae,” the nature of which was still unclear in 1920. Curtis thought that the spiral nebulae were galaxies external to our own, while Shapley disagreed, holding instead that they were clusters made up mostly of gas. On this point, Curtis turned out to be correct, as subsequent data bore out. But Shapley was correct in arguing that our galaxy was larger than previously thought, and for showing that our Sun was not at the center of its galaxy.
It seems clear in retrospect that the “Great Debate” did not produce an overall “winner,” but it did present important issues in our developing understanding of the scale and nature of the universe, and of our place within in it.
Ironically, although the Curtis-Shapley lectures are rightly associated with the Academy and its Annual Meeting, they were in fact delivered in the auditorium of the Smithsonian Institution’s U.S. National Museum. The Academy’s own building, in which all of its Annual Meetings have been held since it was dedicated in 1924, hadn’t yet been constructed.
The complete texts of Curtis’s and Shapley’s arguments, as published in NRC Bulletin No. 11, are available here: http://antwrp.gsfc.nasa.gov/htmltest/gifcity/cs_nrc.html
NASA has a set of web pages devoted to the debate, accessible here: http://antwrp.gsfc.nasa.gov/diamond_jubilee/debate20.html