The astronomer Heber Curtis was born #OTD in 1872. In his “Great Debate” with Harlow Shapley, he famously argued that each of the countless spiral nebulae observed by his colleagues was an entire galaxy like our Milky Way,
Image: Special Collections, UC Santa Cruz

A thread on the Great Debate between Curtis and Shapley, which will have its 100th anniversary next year. You have probably known people who were born into what most scientists thought was a much smaller universe. https://twitter.com/mcnees/status/1121778215775870976?s=21

It was the observations of Hubble et al, building on work by Vesto Slipher and Henrietta Swan Leavitt, that would eventually settle the Great Debate. https://twitter.com/mcnees/status/947857373313499136

(There is an apocryphal story about Shapley receiving a letter from Hubble while Cecilia Payne-Gaposchkin was in his office. Upon reading the letter, which laid out the evidence for Hubble's results, Shapley said to her “Here is the letter that destroyed my universe.”)

Here's a neat anecdote: Apparently, Curtis noticed a "curious straight ray" connected to the nucleus of M87 (NGC 4486 in his log book). But he did not publish the photograph. That's too bad!
Article/image: https://www.lindahall.org/heber-curtis/ 

We now know that the ray is a jet associated with the supermassive black hole at the center of M87. That early photo, taken in 1918, would be a nice complement to this more recent piece of evidence for the black hole.
Image: Event Horizon Telescope Collaboration