The Greatest Debate Of 20th Century
On April 26, 1920, one of the most important scientific discussions of the 20th century unfolded in the hall of the United State’s National Academy of Sciences in Washington, D.C. The Great Debate, it was the debate between leading astronomers of 20th Century, Harlow Shapley and Heber D. Curtis. The debate was about the nature of spiral nebulae and the scale of the universe. It was the beginning of extragalactic astronomy and which laid the groundwork for our modern understanding of the universe.
Before we stepped on the topic I should introduce our Debater on brief so we should have little idea who are they and why there’s debate was so great.
One the First side was Harlow Shapley (1885–1972) a renowned American astronomer and astrophysicist whose groundbreaking work on the Milky Way and the scale of the universe significantly expanded our understanding of universe . Shapley is best known for his research on the structure of the Milky Way galaxy, particularly his determination of its size and the position of the Sun within it. By studying the distribution and distances of globular clusters, he demonstrated, that the Milky Way is far larger than previously believed and that the Sun is located far from the galaxy’s center, challenging the geocentric views of his time. When he was the director at the Harvard College Observatory (1921–1952), he modernized its facilities and expanded its research programs. He was deeply involved in science communication and the promotion of astronomy, writing extensively for the public and advocating for the importance of science in society. His work fundamentally altered our perception of our own place in the universe.
On the other side Heber Doust Curtis (1872–1942) an influential American astronomer who was known for his pioneering work in the study of galaxies and his contributions to our understanding of the universe’s structure. Curtis is best remembered for his role in the "Great Debate" (which is our today’s headline). Throughout his career, Curtis made significant contributions to observational astronomy, particularly in the study of galaxies, novae, and solar eclipses. He participated in numerous eclipse expeditions, observing phenomena such as the solar corona. Curtis also compiled extensive photographic surveys of galaxies, which provided crucial data for understanding their distribution and characteristics. As the director of the Allegheny Observatory and later the University of Michigan Observatory, Curtis played a key role in advancing astronomical research and education. His work laid the foundation for modern extragalactic astronomy and contributed to shaping our current understanding of the vast universe beyond the Milky Way.
Now we are heading to the early 20th century, a period of time, where the nature of spiral nebulae (now recognized as galaxies) and the size of the universe were among the most hotly debated topics in the field of astronomy. Scientists were lacked the tools to measure distances between these celestial objects definitively, leaving room for competing theories. The debate were mainly defined by the two primary questions First One is that, "What are spiral nebulae?" "Were they’re part of the Milky Way galaxy, or were they independent, distant systems?" And the Second one is, "How large is the universe?",
At the time, Shapley was working at the Mount Wilson Observatory in California. He was known for his groundbreaking work on the size and structure of the Milky Way. Using observations of Cepheid variable stars in globular clusters, Shapley had determined that the Milky Way was much larger than previously thought, with the Sun located far from its center.
Shapley argued that, spiral nebulae were not independent galaxies but rather part of the Milky Way. He believed the universe was dominated by a single, enormous galaxy—our own.
Curtis, based at the Lick Observatory in California, championed the idea that spiral nebulae were "island universes"—entire galaxies like the Milky Way, located far beyond its boundaries.
Curtis also noted the high frequency of novae (exploding stars) observed in spiral nebulae, suggesting they were distant systems similar in scale to the Milky Way.
During the debate, Shapley and Curtis presented opposing views on several key issues, here are some of their arguments;
On the e Nature of Spiral Nebulae;
Shapley’s Argument: Spiral nebulae, such as the Andromeda Nebula, were part of the Milky Way galaxy. He cited the observations of Adriaan van Maanen, who claimed to detect rotational motion in spiral nebulae. If these objects were as far away as Curtis suggested, their apparent rotation would imply impossibly high speeds.
Curtis’ Argument: Spiral nebulae were independent galaxies. Curtis countered van Maanen’s findings, suggesting that no such rotational motion had been reliably observed.
On the Size of the Milky Way;
Shapley’s Argument: The Milky Way was vast, spanning up to 300,000 light-years in diameter. This scale, he argued, could accommodate all known nebulae.
Curtis’ Argument: The Milky Way was much smaller, around 30,000 light-years in diameter, making room for spiral nebulae as separate galaxies beyond its boundaries.
On Novae and Supernovae in Spiral Nebulae;
Shapley said that the brightness of novae observed in spiral nebulae suggested they were relatively nearby and part of the Milky Way.
Curtis argued that novae in spiral nebulae appeared far brighter than those in the Milky Way, implying that they were much farther away.
While the debate generated significant interest, it failed to resolve the issue. Shapley’s actual presentation during the debate was unusually elementary, possibly because he was distracted by an opportunity to become the director of the Harvard College Observatory. This allowed Curtis to present a more convincing case on the night of the debate. As historian Michael Hoskin noted, Shapley’s decision to simplify his argument likely gave Curtis the upper hand.
However, the definitive resolution came in 1923, three years after the debate, when Edwin Hubble made a groundbreaking discovery. Using the 100-inch Hooker Telescope at Mount Wilson Observatory, Hubble identified Cepheid variable stars in the Andromeda Nebula (now called the Andromeda Galaxy). By calculating their distances, he confirmed that the Andromeda Nebula lay far outside the Milky Way, proving Curtis’ "island universe" hypothesis correct. The universe, as it turned out, was vastly larger than Shapley had imagined.
As the legacy of the “Great Debate” it marked a pivotal moment in the history of astronomy. It introduced key concepts that shaped the field of extragalactic astronomy and our understanding of the cosmos to forever. Some of examples are ;
1. The Birth of Extragalactic Astronomy: The debate set the stage for the recognition of galaxies as separate entities, opening the door to studying the structure and evolution of the universe.
2. The Role of Technology in Astronomy: The resolution of the debate underscored the importance of advanced telescopes and precise observational techniques in answering fundamental questions about the universe.
3.A Catalyst for Scientific Progress: By fostering discussion and debate, the event accelerated the search for evidence and inspired groundbreaking discoveries, including Hubble’s work.
Some review of the debate, later remarked:
"The Great Debate was not the end of a question but the beginning of a revolution in our understanding of the universe."
-Edwin Hubble
"The debate showcased not only the power of scientific inquiry but also the human element—the ambition, rivalry, and occasional missteps—that drives progress"
- Michael Hoskin
As of today, Looking back to The “Great Debate” of 1920, it was more than a clash of ideas between two astronomers, it was a turning point in humanity’s quest to understand the cosmos. By challenging long-held assumptions and pushing the boundaries of knowledge, Shapley and Curtis laid the foundation for modern astronomy. Today, their debate serves as a reminder of the power of evidence-based inquiry and the importance of questioning our place in the Universe.
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