The Scale of the Universe

What the Great Debate was, how it was resolved, and why it was important.

A subjective abstract in three short paragraphs.

Published version of the 'Great Debate.'

This is a reprint of the texts of Great Debate published in 1921 in the Bulletin of the National Research Council by Shapley and Curtis.

The 'Great Debate:' What Really Happened

By Michael Hoskin, editor of the Journal for the History of Astronomy. This is a reprint of an article appearing in the Journal for the History of Astronomy discussing reality and myths about the 1920 event. The article includes text from the actual notes used by Shapley and slides used by Curtis.

"The 1920 Shapley-Curtis Discussion: Background, Issues, And Outcome".

This excellent review of the historical context, personalities, and scientific issues relevant to the Great Debate is the written version of the presentation given by noted astronomer and author Virginia Trimble as part of the 75th anniversary debate program.

A long bibliography for the 1920 Great Debate.

This collection of over one hundred articles and books was compiled by Robert W. Smith, a historian working at the Smithsonian National Air and Space Museum.

A short bibliography for the 1920 Great Debate.

Five key books and articles about the Great Debate.

Educational Resources

A lesson plan for teaching undergraduates about the Great Debate. The lesson plan consists of an outline, a lecture, suggested classroom uses for the plan, and 10 review questions.

A Glossary of terms used in the Great Debate

Why the 'Great Debate' Was Important

Although the 'Great Debate' is important to different people for different reasons, it is a clear example of humanity once again striving to find its place within the cosmic order. In the debate, Shapley and Curtis truly argued over the "Scale of the Universe," as the debate's title suggests. Curtis argued that the Universe is composed of many galaxies like our own, which had been identified by astronomers of his time as ``spiral nebulae". Shapley argued that these "spiral nebulae" were just nearby gas clouds, and that the Universe was composed of only one big Galaxy. In Shapley's model, our Sun was far from the center of this Great Universe/Galaxy. In contrast, Curtis placed our Sun near the center of our relatively small Galaxy. Although the fine points of the debate were more numerous and more complicated, each scientist disagreed with the other on these crucial points.

A partial resolution of the debate came in the mid-1920's. Using the 100 inch Hooker Telescope at Mount Wilson, then the largest telescope in the world, astronomer Edwin Hubble identified Cepheid variable stars in the Andromeda Galaxy (M31) . These stars allowed Hubble to show that the distance to M31 was greater than even Shapley's proposed extent of our Milky Way galaxy. Therefore M31 was a galaxy much like our own. In the 1930s, the further discovery of interstellar absorption combined with an increased understanding of the distances and distribution of globular clusters ultimately led to the acceptance that the size of our Milky Way Galaxy had indeed been seriously underestimated and that the Sun was not close to the center. Therefore, Shapley was proved more correct about the size of our Galaxy and the Sun's location in it, but Curtis was proved correct that our Universe was composed of many more galaxies, and that "spiral nebulae" were indeed galaxies just like our own.

Another reason the 'Great Debate' is important is captured nicely in the book Shu, F., 1982, The Physical Universe, An Introduction to Astronomy, (University Science Books, Mill Valley, California) p. 286: "The Shapley-Curtis debate makes interesting reading even today. It is important, not only as a historical document, but also as a glimpse into the reasoning processes of eminent scientists engaged in a great controversy for which the evidence on both sides is fragmentary and partly faulty. This debate illustrates forcefully how tricky it is to pick one's way through the treacherous ground that characterizes research at the frontiers of science."