The 1920 Shapley-Curtis Discussion: Background, Issues, and Aftermath

No one now living attended the original lectures by Curtis and Shapley, and the scientific and other worlds in which they moved are connected to ours only by the written record and second-hand stories. Depending on which corners you choose to peer into, those worlds can seem remarkably modern or remarkably ancient. As is often the case for classic dichotomies, the wisdom of hindsight reveals that each of the speakers was right about some things and wrong about others, both in choosing which data to take most seriously and in drawing conclusions from those data. Modern (mostly casual) discussions of the 1920 event leave the impression that Shapley was, on the whole, the winner. But the two men's reactions to Hubble's discovery of Cepheids in the Andromeda galaxy make clear that both felt the issue of existence of external galaxies (on which Curtis had been more nearly correct) was of greater long-term importance than the size of the Milky Way (on which Shapley had been more nearly correct). Shapley is much the better known today and is generally credited in text books with the Copernican task of getting us out of the center of the galaxy. Under modern conditions, he would probably also have gotten most of the press notices. Curtis's repeated theme "More data are needed," is remarkably difficult, then as now, to turn into a headline. (SECTION: 75th Anniversary Astronomical Debate)