The most famous debate of the 1900s was meant to determine what, exactly, was the center of the universe. The greatest lesson modern people can draw from it is that each of the debaters was both right and wrong.

Science never lacks for controversy. Whether it’s a conflict of ideology, modeling, or interpreting ambiguous data, it seems that most of the people are fighting most of the time. One debate, though obscure now, was known in its era simply as The Great Debate. Astronomers had improved telescopes, pooled data, and were beginning to come up with a model for the universe. Two debaters met, in a highly publicized event, to prove what the universe looked like and where the Earth was situated inside it. They ended up proving something else entirely.

Harlow Shapley was an ambitious man in his mid-thirties who had his eye on the position of director of the Harvard College Observatory – though it turned out he was not a candidate for it at that time. He did not relish the prospect of a public debate that might lose him the chance of advancement. His position was that the Milky Way contained nearly the entirety of the universe, and that anything outside of it was just a bubble of gas that had been blown into space by the radiation pressure of the massive central galaxy.


Heber Curtis was older, and had become rather an expert on the objects that Shapley belived were gas bubbles. He’d made extensive studies of the fuzzy, bright little objects that seemed to lie outside of the plane of the Milky Way. Curtis was convinced that these smudges were whole “island universes,” otherwise known as galaxies. After publishing several stunning images of these objects, he was in the process of becoming the director of the Allegheny Observatory. Winning the debate would gain him nothing. Still, Curtis's reputation stood on the idea that his published work was correct. He couldn't allow someone else to hand the victory to Shapley. He planned to take down Shapley’s single galaxy theory.

The debate was held in April of 1920, both men spoke for about forty-five minutes each. Knowing what we know now, it looks like Shapley was definitely on the losing side, but a closer look shows us how jumbled science is, and how data can cause people to cobble together pictures of the world that are both true and false.


Shapley’s idea of the universe was incorrect. There are galaxies outside of the Milky Way. But he based his assumptions on data that would allow him to think nothing else. His first mistake was believing the observations of a fellow astronomer, Adriaan van Maanen. Maanan claimed to have observed one of these bright clouds – that Curtis claimed were whole galaxies – rotating. Given the size of this supposed galaxy, if a human being could observe it rotating during their lifetime, the outer edges had to be moving faster than the speed of light. That was impossible. They had to be much smaller than a galaxy.

A bigger error, and one made by Shapley himself, was the location of the Cepheids. Discovered by Harvard astronomer Henrietta Swan Leavitt, the Cepheids were a group of stars that varied in brightness, pulsing brighter and dimmer. The longer the period of the pulse, the brighter the star. So if a long-period Cepheid was dim, its dimness had to be due to its distance. If it was bright, it had to be close up. Calculate the distance to any one of these stars, then, and it would be possible to calculate the distance to the rest. The problem was, there was no way at the time to calculate the distance to any one Cepheid. Shapley did some inventive, and rough, calculations, but ended up inadvertently including an extra population of stars that threw his calculations off. When he was done, his data showed that the main cluster of Cepheids, and the dwarf galaxies that housed them, were close to the Milky Way. They were about half the distance of the Milky Way away from the Milky Way. If anything, they were an extension of the Milky Way, and not some other faraway galaxies. In fact, the Cepheids were much farther than Shapley thought they were, but there was no way to be sure of that at the time.

Curtis had spent years studying these supposed gas clouds, and so his most compelling arguments were concrete data. He showed evidence that many of these supposed clouds were shaped the same way the Milky Way was shaped. They gave off the same light that the Milky Way gave off. Then he pointed to Andromeda, and rattled through its history. Astronomers had seen, at several times, bright points of light shoot out from this supposed cloud. Curtis believed that they were novae. Astronomers, he said, had seen more novae in Andromeda than they had in the Milky Way. How, exactly, would a small fringe cloud – which Shapely sometimes said might be no more than a developing solar system – produce all these novae?

Curtis then went on to shakier ground. With hindsight, we see that Shapley’s Cepheid calculations were off, but he had made some incredibly important discoveries within the Milky Way. Shapley had noticed globular clusters in the Milky Way galaxy. By calculating the distance to each, as well as to several other visible features, Shapley had realized how big the galaxy actually was. It’s true his final estimate was off by a factor of three. The Milky Way is 100,000 light years across, while Shapley thought it was 300,000 light years from end to end, but he had come up with a better approximation than anyone else had. Curtis asserted that Milky Way was much smaller, and the distances to other galaxies were much longer. He also disputed Shapley’s claim that the solar system was lodged at the edge of the Milky Way. Although Shapley had shown that the distribution of globular clusters -all in one half of the sky - showed that the Earth was off to the edge of the galaxy, Curtis believed it was lodged comfortably in the galactic center. Although outside the Milky Way, Shapley's facts were wrong, inside of it Shapley had the right data and Curtis was lost in miscalculation.

In the end, neither scientist carried the day. There simply wasn’t enough conclusive data. That had to wait until further observations of Van Maanen’s supposed rotating cloud proved that, although it did move, its rotation wasn’t anywhere near the gallop that Van Maanen believed it was. It also had to wait for Hubble, who put paid to the notion that there was only one galaxy and earned every telescope named after him.

Both scientists went on to academic positions and great discoveries. Curtis engineered, with prisms and plates, a way for astronomers to finally superimpose two images of the night sky and see the difference. He went on to study eclipses. Shapley came up with a little thing called the “liquid water belt” – the zone around a star that would allow an orbiting planet to have liquid water. It’s currently known as the "habitable zone."

What the debate proved conclusively, in a historical context, is the way that looking at certain data can mix in false conclusions with true ones. Both scientists were utterly at odds with each other, both were making false assumptions, but both were also arguing for truths that would forever change the way people saw the universe and their place in it.

Top Image: NASA

Via Astrophysical Data Systems, NASA, NASA, and McCormick Observatory Museum.