On this day in 1920 - April 26th - the Shapley-Curtis debate took place on the nature and distance of spiral nebulae.

Even less than a century ago, we didn't know what the "spiral nebulae" were. Were they objects in our Milky Way galaxy and therefore comparatively close? Or were they "island universes", i.e., other galaxies, and therefore at a great distance?


Mona Evans
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