How long has humanity known that we live in a galaxy similar to others in the Universe?
During what percent of human recorded history have people not known that we live in an isolated galaxy of stars similar to the other remote galaxies? We have only known that we live in a galaxy like other spirals that we see for only a relatively short time! A great debate about this topic was held between Harlow Shapley and Heber Curtis in 1920, and is regarded by most as the greatest debate in the modern history of astronomy. The debate was about the scale of the Universe; at the time it was known that there were “spiral nebulae” in the skies in addition to stars, but their nature was very unclear. Curtis argued that the nebulae represented “island Universes”, or galaxies as we know them, and that the Sun itself was part of such a system. If Curtis was right, it meant that the Universe was bigger than anybody at that time imagined it could be. Shapley, on the other hand, argued that the spiral nebulae were simply interstellar clouds near the Sun. The stars in the sky were therefore t
