from the days when it was believed the Milky Way was the entire universe, and all those spiral nebulae astronomers were photographing were small, local objects surrounding it. Amazingly, this question was only finally resolved by Hubble in the twenties; there are people alive today who were born before we had learned the true geography and general size of even the visible universe.

We're just starting to explore our universe, and our knowledge of it is as preliminary as our knowledge of the Earth was during the middle ages. It is fitting that Hubble was honored by naming the space telescope after him, although almost unknown to the general public, his proof of an expanding universe of galaxies was the single most important piece of observational science done in the 20th century.

Afterward, the spiral nebulae were then called "island universes", a term I remember reading in astronomy books when I was a kid.

It makes you wonder if our grandchildren will be remarking how naive our beliefs and assumptions today are.