The Universe Expands
Speculative Fiction often deals with the concept of "Other Worlds." This, in turn, depends upon the understanding that Earth is a world and that that there are other bodies which are like it.
The modern idea of what the Universe is usually dated from the work of Nicholas Copernicus (1473-1543) whose work on the "Revolution of the Spheres" was published on his death bed. Copernicus was the first to give a fairly accurate view of the Universe in that he saw that Earth revolved around the Sun, and not vice versa. In this, of course, he had been preceded by the Classical Greek Aristarchus of Samos and some medieval Islamic astronomers.
There was much wrong with his model in that the planets revolved on perfectly circular orbits and that the stars were merely points of light on an outermost sphere but it was the first vital step in realising that Earth was a world among worlds.