Doctor Moon

A Doctor Moon was an artificial moon that maintained the computer systems on the planet it orbited through wireless connection. One such moon was built by Felman Lux, Strackman Lux's grandfather, to protect CAL, his youngest daughter who became the Library's computer and was given an imaginary world in which to play while in control of the Library. He a virus checker designed to monitor CAL's systems and her spiritual wellbeing. He continued to perform these functions during the Vashta Nerada crisis. His presence stopped the Doctor's sonic screwdriver working when nightfall arrived and his physical body came in range.



In Charlotte's imaginary world, Doctor Moon was human and was treating her. He advised her to save the others in the Librarys until she overloaded and switched him off. When the Tenth Doctor saved River Song into CAL's imaginary world, the Doctor Moon was back online, protecting CAL, River and the rest of the team, except for Strackman Lux, who survived the events. (DW: Silence in the Library / Forest of the Dead)

Doctor Moon's function in the virtual world was apparently to keep both Charlotte and others downloaded into it from realizing the unreality of the world they were in, possibly to help them keep sane. The virtual world was modeled on early 21st Century Earth (or something similar to it). He did inform Charlotte “she had forgotten again” and that the “world of her nightmares” (reality) was real, perhaps in preparation of her uploading the survey team.

In the virtual world he could control people's memories, including CAL's, using the simple sentence “And then …” followed by whatever command he wished, from forgetting something to remembering something. Unlike CAL, who could be considered to have gone mad thanks to the Vashta Nerada incident, Doctor Moon was still quite sane, though very logic bound. However, his manipulation of the memories of Donna Noble seemed pointless as she was soon taken out of the computer.