Image from Earth of the center (laser points directly at center) of our Milky Way Galaxy, which was once thought to comprise the entirety of the Universe. This image was taken by European Southern Observatory astronomer Yuri Beletsky from Paranal, Chile on 2007 July 21.
(Image Sources: Wikipedia.org, By ESO/Y. Beletsky - https://web.archive.org/web/20081121184421/http://www.eso.org/gallery/v/ESOPIA/Paranal/phot-33a-07.tif.html, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=7398904)
By Glenn A. Walsh
Reporting for SpaceWatchtower
Just a century ago, the size of the Universe, and our place in the Milky Way Galaxy, were still largely not understood. One-hundred years ago today, on Monday Afternoon, 1920 April 26, two prominent astronomers debated these issues during the annual meeting of the National Academy of Sciences.
In what came to be known as the “Great Debate,” the two participants were Harlow Shapley and Heber D. Curtis. The debate lectures were delivered in the afternoon, while a panel discussion on the issues occurred that evening. The following year, the debate was published in the May issue of the Bulletin of the National Research Council, including rebuttals to the arguments in the previous year's debate.
The 1920 debate did not follow a normal debate structure. Rather, the two participants simply gave lectures on their respective viewpoints on the issues, which included responses to the other's viewpoints, but no rebuttals were given during the actual debate. At the time, both were seeking the directorship of prominent astronomical observatories, so part of the motivation of the speakers, particularly Harlow Shapley, was to impress the respective university administrations and boards of trustees.
Harlow Shapley, at the time, had been working at the Mount Wilson Observatory in southern California since 1914. In 1921, he would become Director of the Harvard College Observatory in Cambridge, Massachusetts.
As Harvard College Observatory Director, on Wednesday Evening, 1941 November 19, Dr. Shapley gave the keynote address at the dedication of a rather unique 10-inch Siderostat-type Refractor Telescope, in the public observatory of the original Buhl Planetarium and Institute of Popular Science (a.k.a. Buhl Science Center – Pittsburgh's science and technology museum from 1939 to 1991). A strong supporter of amateur astronomy, in the 1940s he also helped to establish a national organization for amateur astronomers, The Astronomical League.
At the time of the Great Debate, Heber Curtis was working at the Lick Observatory in northern California. Starting at Lick Observatory in 1902, he surveyed nebulae, which would be an integral issue in the Great Debate, particularly the nature of spiral nebulae.
The study of nebulae had been started by Lick Observatory Director James E. Keeler, when Dr. Keeler had returned to Lick Observatory as newly appointed Director in 1898. He had previously been Director of the Western University of Pennsylvania's Allegheny Observatory. However, Dr. Keeler suddenly died in 1900, leading Dr. Curtis to take-on his study of spiral nebulae. Later in 1920, Dr. Curtis would become Director of the newly-built, three-dome Allegheny Observatory, operated by the renamed University of Pittsburgh.
The nature of spiral nebulae observed by astronomers near the end of the 19th century was hotly debated by scientists of that era. Some thought the spiral nebulae were simply nearby gas clouds. Other scientists believed that they were distant “island universes,” or galaxies similar to our own Milky Way Galaxy.
For the 1920 annual meeting of the National Academy of Sciences, a debate was scheduled on the issue (actually, this was one of 14 issues disputed during the debate). Heber Curtis, who had been studying nebulae, was convinced that the spiral nebulae were independent galaxies outside of our Milky Way Galaxy. Further, Dr. Curtis conjectured that the Earth's Solar System was near the center of our rather small Milky Way Galaxy.
Dr. Curtis said, during the debate, "if the spirals are island universes it would seem reasonable and most probable to assign to them dimensions of the same order as our galaxy. If, however, their dimensions are as great as 300,000 light years [as Shapley asserted for our galaxy], the island universes must be placed at such enormous distances that it would be necessary to assign what seem impossibly great absolute magnitudes to the novae which have appeared in these objects."
Harlow Shapley, who spoke first, argued that the spiral nebulae were gas clouds within our Milky Way Galaxy, and that our galaxy was (or nearly was) the totality of the Universe. During the debate, he also stated, "Recent studies of clusters and related objects seem to me to leave no alternative to the belief that the galactic system is at least ten times greater in diameter - at least a thousand times greater in volume - than recently supposed." He also countered Dr. Curtis saying that our Solar System was really far from the center of our much larger galaxy / Universe.
During a graduate work fellowship at Princeton University, Dr. Shapley had been studying globular clusters. As the common observation at that time was that globular clusters were more concentrated in one-half of the sky, he concluded that this part of the sky included the center of the galaxy. By using Cepheid variable stars as a sort-of measuring-rod, he determined that our galaxy was much larger than most astronomers of the time thought, and our Sun was further from the center of the galaxy, as well.
As Dr. Shapley was a younger, ambitious astronomer, Dr. Curtis was a little older and more established in the astronomy field. Dr. Curtis maintained the current opinion of most astronomers that our galaxy was rather small, with our Sun close to the center of the galaxy. He demonstrated that novae in our galaxy were much brighter than novae seen in spiral nebulae, concluding that the spiral nebulae were separate and distinct galaxies in their own right. He disputed Dr. Shapley's larger size of the Milky Way Galaxy, as he did not believe that Cepheid variable stars derived valid stellar distances.
Nothing conclusive came from the Great Debate. The data used by both astronomers were not of a high enough quality to determine the true nature of spiral nebulae.
In later years, it was determined that both scientists were correct in some matters and wrong in other matters. Dr. Curtis was correct that the Milky Way was just one of billions of galaxies in our huge Universe. However, Dr. Shapley was correct that the Milky Way galaxy is much larger than originally believed and our Sun was far from the center of the Milky Way.
By 1924, Edwin Hubble, who had started working at Mount Wilson Observatory in 1919, was able to prove that the spiral nebulae were actually galaxies, far from our own Milky Way Galaxy. Using the newly completed 100-inch Hooker Reflector Telescope (then, the world's largest telescope), he was able to identify Cepheid variable stars (used by astronomers to determine stellar distances) in several spiral nebulae, including Andromeda and Triangulum.
Dr. Hubble was able to demonstrate that these Cepheids were much too far away to be part of our own galaxy, and hence, must be in another galaxy. Consequently, he fundamentally changed the scientific view of the Universe, as well as ending the debate, in Dr. Curtis' favor, which had been the focus of the 1920 Great Debate.
About a decade later, Dr. Shapley was proven correct regarding the fact that our Milky Way Galaxy is much larger than originally believed. And, that our Solar System is far from the center of the galaxy.
Robert J. Trumpler had proven the existence of interstellar absorption. Combined with a better understanding of stellar distances and distribution of globular clusters, it became clear that our Sun was not near the center of the Milky Way Galaxy.
Under the auspices of the George Ellery Hale Lecture series, Dr. Shapley and Dr. Curtis delivered the lectures in the Baird Auditorium of the Smithsonian Institution’s Museum of Natural History, in Washington DC. The National Academy of Science's own building in Washington was not finished until 1924; from then on, annual meetings occurred in the Academy's building.
The 1920 Great Debate became the first of several such great debates regarding current issues in astronomy. To mark the 75th anniversary of the original Great Debate, other such great debates were held in the 1990s, and they continued to occur in the Baird Auditorium. These other great debates occurred in 1995 (Distance Scale to Gamma-ray Bursts), 1996 (again, regarding the Scale of the Universe), and 1998 (Nature of the Universe).
Internet Links to Additional Information ---
Internet links to more detailed information on the Great Debate:
Link >>> http://buhlplanetarium2.tripod.com/bio/greatdebate.html
Heber D. Curtis: Link >>> https://en.wikipedia.org/wiki/Heber_Doust_Curtis
Harlow Shapley: Link >>> http://buhlplanetarium.tripod.com/#hshapley
Edwin Hubble: Link >>> https://en.wikipedia.org/wiki/Edwin_Hubble
James E. Keeler: Link >>> http://johnbrashear.tripod.com/bio/KeelerJ.htm
Harlow Shapley and Buhl Planetarium Assist in Creation of The Astronomical League:
Link >>> http://buhlplanetarium4.tripod.com/AstroLeague.html
Related Blog Posts ---
"100 Years Ago: Shapley Discovers True Magnitude of Milky Way & True Location of Earth in Galaxy." 2018 Jan. 8.
"75th Anniversary: America's 5th Public Observatory." 2016 Nov. 19.
Harlow Shapley gave keynote address at the dedication of the rather unique 10-inch Siderostat-type Refractor Telescope at the original Buhl Planetarium and Institute of Popular Science / Buhl Science Center, Pittsburgh's science and technology museum from 1939 to 1991.
Source: Glenn A. Walsh Reporting for SpaceWatchtower, a project of Friends of the Zeiss.
Sunday, 2020 April 26.
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Glenn A. Walsh, Informal Science Educator & Communicator:
< http://buhlplanetarium2.tripod.com/weblog/spacewatchtower/gaw/ >
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Project Director, Friends of the Zeiss: < http://buhlplanetarium.tripod.com/fotz/ >
SpaceWatchtower Editor / Author: < http://spacewatchtower.blogspot.com/ >
Formerly Astronomical Observatory Coordinator & Planetarium Lecturer, original Buhl Planetarium & Institute of Popular Science (a.k.a. Buhl Science Center), Pittsburgh's science & technology museum from 1939 to 1991.
Formerly Trustee, Andrew Carnegie Free Library and Music Hall, Pittsburgh suburb of Carnegie, Pennsylvania.
Author of History Web Sites on the Internet --
* Buhl Planetarium, Pittsburgh:
* Adler Planetarium, Chicago:
* Astronomer, Educator, Optician John A. Brashear:
< http://johnbrashear.tripod.com >
* Andrew Carnegie & Carnegie Libraries: