When I used to teach a general studies course called The Universe, it was my habit to make a ten-foot wide model of the Milky Way Galaxy on the classroom floor with a box of salt. (For dramatic effect, I asked the students to push back their desks, then began pouring the salt without a hint of what was coming.) Against the dark tiles, salt makes a pretty convincing galaxy.
But the best part of the demonstration came when we started discussing what was wrong with the model.
First, there were not nearly enough salt grains in a pound of salt to represent the number of stars in the galaxy (hundreds of billions). A back-of-the-envelope calculation and the students discovered to their amazement that I should actually have poured 10,000 boxes of salt on the floor!
Which led us to the second thing wrong with the model: the salt grains were way too big to be stars in a ten-foot galaxy. Another back-of-the-envelope calculation. If salt grains were Suns within a galaxy, they should be thousands of feet apart. To get everything into proportion, I should sprinkle my 10,000 boxes of salt out on a classroom floor that is nearly as big as the Moon's orbit.
Still, we had a bit of fun, and I know for a fact that the demonstration stuck. I meet students from decades ago who say, "Remember the time you spilled salt on the floor?"
And now we have the Hubble Ultra Deep Field Photograph that shows galaxies as numerous as salt grains. To have as many galaxies as the Hubble potentially reveals I would need ten thousand boxes of salt, with each grain representing ten thousand boxes of "stars."
It's not easy to get one's mind around these things -- hundreds of billions of galaxies, each with hundreds of billions of stars. The difference between the universe revealed by modern astronomy and the universe of Dante's Divine Comedy is -- well, simply incommensurable. Few of us have taken aboard the implications of the new cosmology. Psychologically, we still live pretty much in the human-centered universe of Dante, watched over by a humanlike deity just up there in the Empyrean Sphere.