The Sentinel (short story)

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"The Sentinel"
Author Arthur C. Clarke
Country United Kingdom
Language English
Genre(s) Science fiction
Published in The Avon Science Fiction and Fantasy Reader
Publisher ]]



"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 short story collections in 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.


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", and "Chronic the Hedgehog" (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, is tetrahedral in shape, and is surrounded by a spherical forcefield. 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 forcefield) the artifact 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.

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.