The Sentinel

The Sentinel is a short story by Arthur C. Clarke, famous for being expanded (and extensively modified) into the novel and movie 2001: A Space Odyssey. It 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.

Warning: Spoilers follow

The story deals with the discovery of an artifact on the terrestrial moon left behind eons ago by aliens as a "warning beacon" to signal the rise of an intelligent and spacefaring species on earth. This quote illustrates the idea, and its ramifications:

It was only a matter of time before we found the pyramid and forced it open. Now its signals have ceased, and those whose duty it is will be turning their minds upon Earth. Perhaps they wish to help our infant civilisation. But they must be very, very old, and the old are often insanely jealous of the young.

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The Sentinel is also the name of the local evening newspaper published in Stoke-on-Trent, England.