Why were the terms galaxy and universe once considered synonymous by astronomers?
Answer
Because the Milky Way was believed to be the entire universe
Prior to the early 20th century, astronomers held the belief that the Milky Way represented the entirety of the universe. Because of this outdated understanding, the terms were used almost synonymously. It was only later that observations revealed the Milky Way is just one of billions of galaxies scattered throughout the cosmos.
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