Centenary of Astronomy's "Great Debate" on the Size of the Universe
hubie writes:
The 26th of April marks the centenary of the Great Debate regarding the nature of the universe. At the end of the 19th Century, the general consensus was that the Milky Way Galaxy was the extent of the known universe. There were these known fuzzy spiral nebular disks that were not stars or planets, or even anything interesting like a comet, and there was quite a bit of speculation earlier in the century whether they were close objects or very very far away, but by the end of the century all the objects in the sky were thought to belong to a single great system. This opinion was summarized by one of the great science writers of the day, Agnes Clerke, who wrote in 1890
The question whether nebuli are external galaxies hardly any longer needs discussion. It has been answered by the progress of discovery. No competent thinker, with the whole of the available evidence before him, can now, it is safe to say, maintain any single nebula to be a star system of coordinate rank with the Milky Way.
At the beginning of the 20th Century the young astronomer Curtis Heber studied these nebulae and his observations suggested that these spiral-shaped objects were further away than the size of the known universe. He believed that the Milky Way, if looked at from a sufficient distance, would appear the same as the spiral nebulae he was observing. As more observations came in, the National Academy of Sciences decided to host a debate on the topic. Heber didn't dispute the size of the Milky Way and argued that the nebulae were the famed "island galaxies". The counterpoint was provided by astronomer Harlow Shapley. Shapley argued that the Milky Way was indeed much larger than the known size (300,000 vs. 30,000 light years) and thus the nebulae were part of the Milky Way.
Twenty-five years ago in celebration of the 75th anniversary of this event, Virginia Trimble wrote an informative historical summary of the debate and noted
As is often the case for classic dichotomies, the wisdom of hindsight reveals that each of the speakers was right about some things and wrong about others, both in choosing which data to take most seriously and in drawing conclusions from those data. Modern (mostly casual) discussions of the 1920 event leave the impression that Shapley was, on the whole, the winner. But the two men's reactions to Hubble's discovery of Cepheids in the Andromeda galaxy makes clear that both felt that the issue of existence of external galaxies (on which Curtis had been more nearly correct) was one of greater long-term importance than the size of the Milky Way (on which Shapley had been more nearly correct). Shapley is much the better known today and is generally credited in text books with the Copernican task of getting us out of the center of the galaxy. Under modern conditions, he would probably also have gotten most of the press notices. Curtis's repeated theme, "More data are needed," is remarkably difficult, then as now, to turn into a headline.
Read more of this story at SoylentNews.