FYI, I created a new tag, trajectory. I thought I should ask it not if I was too lame to ask before creating, so there wouldn't be too late to delete it in case.
Is this tag necessary or is tag orbit enough? (there's not even so clear for me the difference between the two worlds)
It's perfectly reasonable to have both, in my opinion. They aren't really the same thing. Orbit might be considered of course as a type of trajectory (i.e. orbital trajectory), but what's usually implied with the latter is non-orbital trajectories, such as parabolic-, hyperbolic-, ballistic-,... So yeah, I say we keep them both, see if they later need further differentiation.
By the way, that question you tagged with trajectory, I'm not sure it really needs it, since it already uses orbit and you're specifically asking about orbital trajectories. There might be a point in making orbit a synonym of trajectory tho, but that's probably best asked as a different question, or just asked of community managers moderating the site now through flags on your question. @JonEricson is experienced and knowledgeable enough on the subject to simply do it with respect to what's best for the community, if prompted to it by a flag, I have no doubt. ;)
