Like any clash, though, it carried risks. Shapley hoped to win the directorship of Harvard College Observatory. And he knew a defeat by Curtis — whose fearsome oratory skills stemmed from years as a Greek and Latin schoolmaster — could cost him this prestigious appointment.

“There’s no question that Curtis was a gifted speaker,” John Mulchaey, director of the Carnegie Observatories at the Carnegie Institution for Science, tells Astronomy. “Shapley was much more uncomfortable in front of crowds, and that no doubt affected his performance.”



But Curtis also seized the opportunity for a sprightly debate with relish. “A good friendly scrap is an excellent thing, once in a while,” he quipped amiably in a letter to Shapley. “Sort of clears up the atmosphere.”

The topic of the Great Debate

Although little remembered, their Great Debate a century ago hinged on a lingering uncertainty about the extent of our Milky Way and whether it constituted the entire universe or was just one of many “island universes” that we now call galaxies. Former journalist Shapley, who joked that he decided to study astronomy only because he could pronounce it, proved the Milky Way was at least 10 times bigger than previously thought.

Furthermore, he showed our solar system resides not in the Milky Way’s heart, but far from its center.



Shapley believed that “spiral nebulae” like Andromeda (now known to be our closest galactic neighbor) were part of the Milky Way. To regard them as anything more was to admit the cosmos was larger than most astronomers in the early 20th century were willing to accept.

“He was really convinced the universe had to be the Milky Way,” explains Mulchaey, “because his estimate of the Milky Way suggested it was very big. Much too big for there to be much else besides it.”