A hundred years ago, on 26 April 1920, two young astronomers, Harlow Shapley and Heber Curtis, met for what’s become known as the “Great Debate” on the nature of spiral galaxies. At stake was our understanding of the size and shape of the universe itself. It was an evening’s entertainment for eminent members of the National Academy of Scientists; the topic was chosen instead of Einstein’s new-fangled General Relativity, which most of the audience neither understood nor appreciated.

Shapley cited evidence that certain spiral-shaped nebulae must be relatively small clouds of luminous light within our Milky Way; for one thing, they did not have the colours he expected for stars. Curtis argued from his exquisite photographs that they were indeed distant “island universes” of billions of individual stars, at immense distances from us, as Kant had first suggested in the eighteenth century.