On April 26, 1920, in the Baird Auditorium of the U.S. National Museum in Washington, Harlow Shapley and Heber Curtis gave back to back 40 minute lectures on the distance scale of the universe to an audience of National Academy of Sciences members who had just sat through talks on glaciers and zoology. No transcript survives; historians reconstructed the event from Shapley's annotated typescript, Curtis's surviving slides, and letters. This episode explains what each man argued and why both were brilliantly right and spectacularly wrong.

Shapley used Cepheid variable stars in globular clusters as standard candles to argue the Milky Way was 300,000 light years across and was the entire universe, with the Sun far from center and spiral nebulae as nearby gas clouds; a separate Andromeda would have to sit an unthinkable 100 million light years  ...