This question: Which part of 2001 takes place in 2001?
Asks about when the movie 2001: A Space Odyssey (1968) happens, since the discovery of the monolith on the Moon happens 18 months before the events on the spaceship headed for Jupiter. Thus the monolith could be discovered in January to December 2001, and the events 18 months later could happen between August 2002 and and June 2003, or the Monolith could be discovered 18 months before the space events that were sometime between January and December 2001, making the monolith discovered sometime between July 1999 and June 2000. If I did the math right.
So the tv series Space: 1999 (1975-1977) was made by people who were aware of 2001: A Space Odyssey (1968) and who could have noticed that with an 18 month gap between the main sequences of the film they could not both have happened in the year 2001.
So did they decide that the main space sequences in 2001: A Space Odyssey (1968) happened in 2001 and that the monolith was discovered on the Moon in 1999 or 2000, and thus decided to have the Moon ejected from Earth orbit on September 13, 1999, thus showing that even if the two productions happpened in the same mulitiverse they would be in alternate universes?
So was 1999 selected as the year when the Moon was ejected into interstellar space based on the possible discovery of the monolith in 1999 or 2000 in 2001: A Space Odyssey (1968)?