1924: Astronomer Edwin Hubble announces that the spiral nebula Andromeda is actually a galaxy and that the Milky Way is just one of many galaxies in the universe.

Before Copernicus and Galileo, humans thought our world was the center of creation. Then (except for a few notable stragglers) we learned that the sun and planets did not revolve around the Earth, and we discovered that our sun — though the center of our solar system and vitally important to us — was not the center of the universe or even a major star in our galaxy.

But we still grandiosely thought our own dear Milky Way contained...

8