
Originally Posted by
Alex Rubit
The show does, indeed, take place in the same Stargate universe as SG-1 and Atlantis do. It is, however, a couple of years later. The pilot takes place in 2011, and not too long before that the Stargate had gone pubilc. Something that has yet to happen on the TV shows, but there's a good chance it will happen at some point, considering that initially it was meant to happen at the end of Lost City (it was changed when it became clear that there'd be an eight season of SG-1).
There's no official timeline, because basically the timeline is in progress as SG-1 and Atlantis continue to evolve; so we wouldn't want to have any contradictions there.
While Horizon takes place in the same universe, it is at the same time a series of its own, just like it would have to be as a TV series. Meaning that there's not too much of a direct connection to events in SG-1 and Atlantis, other than references here and there.
It's not canon in that the TV shows have to stick to what's happening in the Horizon corner of the Stargate universe. However, all efforts are being made to make it "appear" as canon; i.e. avoiding contradictions, and things like that. So it could very well be canon, since it's a very realistic continuation of the Stargate universe.