"The Sentinel" was written in 1948 for a BBC competition (in which it failed to place) and was first published in the magazine Ten Story Fantasy in 1951, 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.
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 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 activated when sunlight touches it for the first time after it was dug up.
Limb is best known for his work on Doctor Who, for which, between 1981 and 1985, he composed the music for the serials The Keeper of Traken, Four to Doomsday, Black Orchid, Time-Flight, Arc of Infinity, Terminus, The Caves of Androzani and Revelation of the Daleks.
Limb also contributed music to the television series The Justice Game, Aliens in the Family, The December Rose, Thinkabout, The Box of Delights, Kevin and Co, Martin Luther: Heretic, Storytime and the Look and Read serials "Fair Ground!", "Dark Towers", "Sky Hunter", "The King's Dragon", "Cloud Burst", "Geordie Racer", "Through The Dragon's Eye" and "Earth Warp". For the latter programme he also composed the cult favourite "Magic E" song as well as the popular education songs "Bill the Brickie", "Dog Detective" and "The Punctuation Song".
He also composed and played "Swirley", a cheerful piece of electronic music that was used as the theme to the BBC's Service Information news bulletins in the late 1970s/early 1980s, and arranged the songs for several series of the BBC Schools programme You and Me, featuring Cosmo and Dibs.
His recording "Passing Clouds" was included on the 1976 LP Out of This World, a compilation of sound effects. This track was used by Prince at the beginning of "Eye No", the opening track of 1988's "Lovesexy". Out of This World was reissued in 1991 on CD as the "Essential Science Fiction Sound Effects, Volume 2"
In 2005, Limb discussed his score for Revelation of the Daleks in "Revelation Exhumed", a special feature on the DVD release of the story.