Has anybody else thought about UAP portals/time travelling potentially being anchored to Earth some way?

I’m still trying to understand one thing, if a portal opens up that UAPs come from, is it thought the portal to be anchored to the planet in some way? It doesn’t have inertia, so if the portal would open instantly somewhere, not having the same momentum as earth’s movement, it would look to us as if the portal is moving fast (because the earth is moving fast around the sun + the galaxy moves as well) relative to us. That would apply to time travelling, I would assume? Sure, you can travel to a certain time, but earth wasn’t in that position in the universe at that time, so you would most likely end up being in space far from any celestial body or even earth.