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My question isn't about the arrangement of planets in space (geocentrism / heliocentrism), nor the understanding of the whole universe (we are in a galaxy, there are galaxies etc). My question is: who was the first person to realize that Earth, outside of its atmosphere, is surrounded by vacuum? It seems to be related to the question of air density. Someone, somewhere must have noticed that the air becomes less dense with altitude and thus at some altitude the air may vanish completely.

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