"The Sentinel" is a science fiction short story by British author Arthur C. Clarke, written in 1948 and first published in 1951 as "Sentinel of Eternity", which was used as a starting point for the 1968 novel and film 2001: A Space Odyssey. Publication history "The Sentinel" was written in 1948 for a BBC competition (in which it failed to place) and was first published in the magazine 10 Story Fantasy in its Spring 1951 issue, under the title "Sentinel of Eternity". It was subsequently published as part of the short story collections Expedition to Earth (1953), The Nine Billion Names of God (1967), and The Lost Worlds of 2001 (1972). Despite the story's initial failure, it changed the course of Clarke's career. Anthology The Sentinel (published 1982) is also the title of a collection of Arthur C. Clarke short stories, which includes the eponymous "The Sentinel", "Guardian Angel" (the inspiration for his 1953 novel Childhood's End), "The Songs of Distant Earth", and "Breaking Strain". Story The story deals with the discovery of an artefact on Earth's Moon left behind eons ago by ancient aliens. The object is made of a polished mineral, is tetrahedral in shape, and is surrounded by a spherical force-field. The narrator speculates at one point that the mysterious aliens who left this structure on the Moon may have used mechanisms belonging "to a technology that lies beyond our horizons, perhaps to the technology of para-physical forces." The narrator speculates that for millions of years (evidenced by dust buildup around its force-field) the artefact has been transmitting signals into deep space, but it ceases to transmit when, sometime later, it is destroyed "with the savage might of atomic power". The narrator hypothesizes that this "sentinel" was left on the Moon as a "warning beacon" for possible intelligent and spacefaring species that might develop on Earth. Reception Algis Budrys found "The Sentinel" to be infuriating, saying that "one can raise a formidable reputation for profundity by repeating, over and over again, that the universe is wide and man is very small ... while our instruments show that the universe is wide, they are our instruments and we managed somehow to build them. There is no evidence whatsoever that Man is that goddamned small". Film Adaptation The story was adapted and expanded upon in the 1968 film 2001: A Space Odyssey, made by filmmaker Stanley Kubrick. Kubrick and