The Sentinel is a short story by Arthur C. Clarke, famous for being expanded and modified into the novel and movie 2001: A Space Odyssey. Clarke actually expressed impatience with the common description of it as "the story on which 2001 is based." He was quoted as saying, it is like comparing "an acorn to the resulting oak-tree". 
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 1951, under the title "Sentinel of Eternity". It first appeared in the USA in The Avon Science Fiction and Fantasy Reader published by Avon Periodicals, Inc. in 1951. It was subsequently published as part of a short story collection in Expedition to Earth in 1953. It is also collected in The Nine Billion Names of God and The Lost Worlds of 2001. Despite the initial failure of the story, it changed the course of Clarke's career.
The Sentinel (published 1982) is also the title of a collection of Arthur C. Clarke short stories, including the eponymous "The Sentinel", "Guardian Angel" (the inspiration for his Childhood's End), "The Songs of Distant Earth", and "Breaking Strain".
The story deals with the discovery of an artifact on Earth's Moon left behind eons ago by ancient aliens. The object is made of a polished mineral and tetrahedral in shape, and is surrounded by a spherical forcefield. The first-person 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 million of years (evidenced by dust buildup around its forcefield) the artifact has been transmitting signals into deep space, but it ceases to transmit when, some time later, it is destroyed "with the savage might of atomic power". The narrator hypothesises that this "sentinel" was left on the moon as a "warning beacon" for the possible intelligent and spacefaring life that might develop on Earth.
This quotation illustrates the idea, and its ramifications:
In 2001: A Space Odyssey, the operation of the sentinel is reversed. It is the energy of the sun, falling for the first time on the uncovered artifact, that triggers the signal that creatures from the Earth had taken the first step into space.
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