The Great Debate

The Great Debate in Astronomy was an argument in 1920 between Harlow Shapley and Heber Curtis over the size of the Milky Way, and in turn, the size of the Universe.


Harlow Shapley believed that all the visible universe was contained in the Milky Way which he thought was 120,000 parsecs in diameter, with the Sun 2/3 of the way from the center to the edge of the Milky Way. To him, all nebulae seen in the sky, were in the disk of the Milky Way and there was nothing beyond the Milky Way.


Heber Curtis (who was director at the Allegheny Observatory) thought that nebulae and stars could exist beyond the Milky Way and that the Universe encompassed more galaxies than just the Milky Way.


Edwin Hubble gave the proof of the size of the Universe being larger than the Milky Way when he found a Cepheid variable star in the Andromeda galaxy. By observing the star, and comparing its brightness to known Cepheids within the Milky Way, he was able to determine that the Andromeda galaxy was about 778,000 parsecs away, much larger a distance than even Shapley thought the Milky Way was.


We now know that the Milky Way is just one of billions of galaxies in the visible universe and is only about 35,000 parsecs in diameter.

The Size of the Milky Way

For centuries, everything we thought about the universe was contained in a tiny package that centered on the Earth, and then the Sun after the heliocentric model was accepted. We only knew about what we could see and assumed that since it was observable, it had to be relatively close. If only we knew.

But first, we should see how the evolution of determining the size of the Milky Way galaxy itself took place.

The first actual attempt to determine the size of the Milky Way galaxy was done by William Herschel. He did a simple thing: he just decided to count all the stars he could see, and assuming that the Sun was the center of the galaxy (galactocentricism - yes, there is a word for it), was able to predict, correctly, that our galaxy was a disk. Of course, he was wrong about where the Sun is located with respect to the disk of the Milky Way.

The shape of our Galaxy as deduced from star counts by William Herschel in 1785; the solar system was assumed near center. (NOTE: The image shown is flipped 180 degrees on the horizontal axis from the original, as first published in the Philosophical Transactions of the Royal Society in 1785; the bifurcated arms of the illustration should be on the left.)

"Herschel-Galaxy" by Caroline Herschel. Licensed under Public Domain via Wikimedia Commons

It wasn't until the 1900s that a more extensive survey of the Milky Way took place to determine the size of the Milky Way. Jacobus Kapteyn (cap-tine) deduced that the Milky Way was similar to Herschel's drawing in that the Milky Way is disk-like, heliocentric, and about 20,000 parsecs in diameter.

Kapteyn's Model of a Heliocentric galaxy

Harlow Shapley immediately published a rebuttal stating that the Sun not at the center, but about 2/3 of the way out into the disk and the Milky Way was about 120,000 parsecs in size. Shapley used the distribution of globular clusters and found them more concentrated around the center of the disk of the Milky Way, instead of near where the Sun is located.

Shapley's Model to Determine the Size of the Milky Way

Who was correct? Actually, both were correct, but also, both were wrong. Kapteyn was around the correct size, but Shapley had the Sun in the correct relative location. This actually led to a debate between Harlow Shapley and Heber Curtis, referred to by astronomers as the Great Debate (or the Shapley-Curtis debate).