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 the most distant objects in the Universe, not the nebulae; since the all of these stars are in our own galaxy, Shapley's scenario implied that the Universe was no bigger than our own galaxy!
The the issue was not resolved fully for another decade; as equipment improved, astronomers were able to resolve stars in the spiral nebulae and compare their properties to the stellar counterparts. The distances inferred from these studies implied that the nebulae were indeed other galaxies.
For more information about the Great Debate, check out this excellent site.
In summary, then, that the galaxy is one of a number of such systems in the Universe has been known for about 70 years. This is quite a short time compared to the hundreds of years over which humans have pondered about the Universe!
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