Astronomers Harlow Shapley (left) and Heber Curtis staged one of the most famous discussions in the history of astronomy on April 26, 1920. They were debating the size of the Milky Way Galaxy and whether the universe extended beyond the galaxy's boundaries. Their discussion is known today as the Great Debate. Curtis was correct in asserting that the universe consists of many other galaxies, not just the Milky Way, but Shapley was right about the scale of the Milky Way. [ESO]