At the turn of the last century, a little remembered but dramatic debate took place between prominent astronomers Harlow Shapely and Heber Curtis. The debate concerned essentially several key issues: the location of the sun in the Milky Way galaxy, the size of the universe, and whether spiral nebulae were other galaxies.
Throughout the history of science, new discoveries and theories have challenged the prevailing scientific understanding of the world. It is easy to forget the context in which new data and interpretations emerged. The story of heliocentrism is a good example.
Most people are at least vaguely familiar with the story of Galileo and the two competing theories of geocentrism and heliocentrism.
From the time of Aristotle, the theory of geocentrism (that the earth was the center of the universe) had been accepted by the majority of the scientific world, because it fit the observational data of the time—except for the retrograde motion of the planets. In the 1500’s, Copernicus had offered heliocentrism (theory that the sun was at the center of the universe) as a possible resolution to that problem and had developed the math to support it. Still, even at the time of Galileo 150 years later, it is supposed that the observation of stellar parallax necessary for conclusive proof was not achievable with the crude instruments available.
There is evidence that this assumption is incorrect.
Galileo ignored his own data?
Christopher Graney, physics and astronomy professor, has published articles about Galileo in Scientific American and The Journal for the History of Astronomy—both highly regarded journals. In this series of posts for the Vatican Observatory, he praises Galileo for his discoveries and astute observational skills but does not give Galileo a pass when it comes to his honesty. Galileo had in fact recorded his observation of two stars that should have shown parallax but did not. In spite of this, Galileo insisted that he was right.
Fortunately for science, Copernicus and Galileo were right. They were also partly wrong: the sun is not fixed and although at the center of our solar system, it is not the center of the universe nor even of our galaxy.
The Sun is not the Center
In the debate in 1920, the “sun as the center” was no longer the challenger to old ideas, but a “fact” that was to become the relic of an outdated understanding of the universe. According to NASA:
By the time of the Curtis - Shapley Debate in 1920, through the arduous technique of star counts, astronomers had tried to map out the extent of the Milky Way. This work still placed the sun close to the center of a flattened universe of stars...
Shapley’s interpretation of the data, however, was at odds with this view:
Shapley, the prodigy, made a monumentally correct deduction from existing astronomical data that our Sun was not at the center of our Galaxy… like Copernicus before him, [Shapely] single-handedly moved the center of the universe, this time light-years from its old location.
Curtis, the elder more seasoned astronomer, opposed this view. According to his reading of the data, the sun was close to the center of the galaxy, a view held by many astronomers of the day.
The Milky Way is not the only galaxy in the universe
In spite of being wrong about the size of the universe and the sun’s location, Curtis had his own startling contribution to make. NASA writes:
Curtis was able to argue convincingly—for the first time from hard scientific data—that spiral nebulae were external galaxies and that our own Galaxy was only one of a vast number of galaxies, or “island universes.”
From our perspective, it is hard to imagine how drastically this conclusion altered the scientific and public view of earth’s place in the universe.
The events that happened in the first quarter of our century were together much more than a debate—this is a story of humanity's discovery of the vastness of our universe, a story of a seemingly small academic disagreement whose dramatic resolution staggered the world. It is a story of human drama—two champion astronomers struggling at the focus of a raging controversy who's solution represents an inspiring synthesis of old and new ideas.
Like Galileo and Copernicus, Shapley and Curtis were right about some things and wrong about others. Shipley was correct on two counts: the universe was larger than previously supposed (just not as large as he thought) and the sun was not at the center of the galaxy. Curtis, however, was correct that spiral nebulae are indeed other galaxies. The Milky Way was not the universe.
The debates continue
According to Vatican Observatory post “The Great Debate of 1920,” the newest hotly debated topics in astronomy include dark energy, dark matter and the possibility of extraterrestrial life.
As ever, the advancement of science proceeds with new data, new interpretations, validations, new data, new interpretations. Debating these new ideas often involves professional pride, fixed ideas, and an unwillingness to change—no matter the century or the state of science at the time. The need to adjust our limited understanding of this vast cosmos and of our place in it will always be with us.
When I see your heavens, the work of your fingers,
the moon and stars that you set in place
What is man that you are mindful of him,
and a son of man that you care for him?
Yet you have made him little less than the angels,
crowned him with glory and honor. -Psalm 8:4-6
Cover Image Credit: NASA Earth Observatory / Left image (Brahe's model of the Universe where the Sun orbits the Earth) adapted from Tycho Brahe, Astronomiae instauratae progymnasmata (“Introductory exercises toward the restoration of astronomy.”) Right image (Kepler's conclusion that the orbit of Mars was elliptical, not circular) adapted from Johannes Kepler, Epitome astronomia Copernicanae (“Epitome of Copernican Astronomy.”).