Blog Post 16, The Great Debate

Shapley

Curtis

One of the themes for this week has been depth in astronomy.  When we look up at the night sky, we often forget that we are looking into space, not at space.  What I mean by this is that the sky is not a large sphere surrounding the Earth with all of the stars at the same distance away, it is a 3D space where each object has an RA, Dec, and Distance away form the Earth.  Before computers and the exponential improvement of autonomy equipment, observing distance was a great challenge for Astronomers, and led to many a disagreement.  On of the greater of these disagreements was The Great Debate between Harlow Shapley and Heber Curtis.

Both of these astronomers had observed the Andromeda Galaxy and other spiral galaxies and disagreed greatly upon its size, and the size of the universe.

Shapley liked to think small.  He argued that the Milky Way galaxy made up the entire universe, and as such, the spiral galaxies were simply a nebulae within the Milky Way.  He bolstered his claim using the great distance away that other galaxies must be for the extragalactic theory to work, distances unheard of in 1920.  Furthermore, respected astronomer Adriaan van Maanen claimed that he saw M101 (Pinwheel Galaxy) rotating.  If this were true, the object could not be a galaxy or else its edges would exceed the speed of light due to orbital velocity.

Curtis liked to think big.  He believed that such objects were other galaxies outside of the Milky Way.  His main arguments relied on the fact that the frequency of supernovae in Andromeda far exceeded that of supernovae in other parts of the sky, thus Andromeda must be its own galaxy to avoid an irregular distribution across the sky.  He also used the argument that the doppler shifts of the spiral objects were far greater than anything else in the Milky Way.

It is fairly clear that Curtis ultimately won.  Van Maanen's claims of eye-witnessed spiral rotation were found to be incorrect.  This vastly undermined Shapley's argument and Curtis was victorious.

And they all lived happily ever after,

THE END

Sources:
https://en.wikipedia.org/wiki/Great_Debate_(astronomy)
http://apod.nasa.gov/diamond_jubilee/debate_1920.html
http://antwrp.gsfc.nasa.gov/htmltest/gifcity/curtis.gif
http://antwrp.gsfc.nasa.gov/htmltest/gifcity/shapley.gif