1920: The Shapley-Curtis Debate

Shapley: Our Galaxy is the entire universe. Spiral nebulae are clouds of gas. The sun is not near the center of the Galaxy/Universe, but is in the center of a cluster of stars 50,000 light years from the center. The galaxy is 300,000 light years across.
Curtis: Spiral nebulae are galaxies like our own. The sun is near the center of our Galaxy. The galaxy is less than 30,000 light years across.

Shapley: Our Galaxy is the entire universe. Spiral nebulae are clouds of gas. The sun is not near the center of the Galaxy/Universe, but is in the center of a cluster of stars 50,000 light years from the center. The galaxy is 300,000 light years across.