Harlow Shapley

The debate has often been characterized as centering upon whether spiral nebulae were island universes. However, Harlow Shapley, from the Mount Wilson Observatory in Southern California, preferred to discuss his new and vastly larger estimate of the size of our galaxy. Shapley wrote to a colleague that he was coming to Washington to discuss the scale of the universe and that he did not intend to say much about spiral nebulae because he did not have a strong argument. In the debate, Shapley argued that "Recent studies of clusters and related subjects seem to me to leave no alternative to the belief that the galactic system is at least ten times greater in diameter - at least a thousand times greater in volume - than recently supposed."

Shapley's debate opponent was Heber D. Curtis, from the Lick Observatory in Northern California. His primary interest was the nature of the spiral nebulae. But he realized that "the island universe theory had an indirect bearing on the general subject of galactic dimensions." Curtis acknowledged that "if the spirals are island universes it would seem reasonable and most probable to assign to them dimensions of the same order as our galaxy. If, however, their dimensions are as great as 300,000 light years [as Shapley asserted for our galaxy], the island universes must be placed at such enormous distances that it would be necessary to assign what seem impossibly great absolute magnitudes to the novae which have appeared in these objects."

However, Curtis argued, "it is, of course, entirely possible to hold both to the island universe theory and to the belief in the greater dimensions for our galaxy by making the not improbable assumption that our own island universe, by chance, happens to be several fold larger than the average." Ultimately observations would prove Curtis correct in this suggestion that our galaxy might be larger than most spiral nebulae. But in 1920 Shapley, with his assumption that it was unlikely for our galaxy to be so unusual, held the stronger position among scientists.

Curtis felt that a friendly scrap would clear the atmosphere, and that between handshakes at the beginning and end of the debate each man should use his cudgel to the best of his ability. For Shapley, who was aiming at the directorship of the Harvard College Observatory, the last thing he needed was to be routed in a debate by Curtis, an accomplished speaker. So Shapley preferred to turn the debate into two talks on the same subject from different points of view. He succeeded in changing the format from a debate to a discussion, gladly giving up the opportunity for rebuttal and reducing the time allotted to each speaker to so little that a serious scientific presentation was scarcely possible.

It would be an injustice, however, to attribute Shapley's equivocation at the Washington meeting solely to anxiety over what observers from Harvard might think. His correspondence before and after the meeting shows a reasonable reluctance to take a firm position on flimsy evidence.

Heber D. Curtis
Image courtesy the Astronomical Society of the Pacific.