The Flammarion engraving is commonly used to illustrate the concept of the ‘firmament,’ though it should be realised that the engraving is meant as a mockery. Original caption:
The Flammarion engraving is a wood engraving by an unknown artist that first appeared in Camille Flammarion’s L’atmosphère: météorologie populaire (1888). The image depicts a man crawling under the edge of the sky, depicted as if it were a solid hemisphere, to look at the mysterious Empyrean beyond. The caption translates to “A medieval missionary tells that he has found the point where heaven and Earth meet…”
By Anonymous [Public domain], via Wikimedia Commons.