Perihelion was when a planet or other celestial body reached its closest to the star it orbited. It was also used by Dr Colin Dove to describe the time that the physical world and a reservoir of evil almost touched, which could be exploited using sensitives to unleash the evil. Liz Shaw later realised that perihelion actually referred to the meeting of Daniel O'Kane and his son Peter Russell, two powerful sensitives, but the plan came to an end when Daniel jumped in front of Russell's knife to save Liz. (HOMEVID: The Zero Imperative)

Some years after Liz left the UK with Patsy Haggard, (HOMEVID: When to Die) when Giles was director of P.R.o.B.e., the organisation had to prepare for an upcoming perihelion. (WC: Shadows of Doubt)

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