In my limited understanding, it feels you are missing Umbrans point. Everything is relative, so there is no absolute point of reference. Thus you can choose any frame of reference you want and work from there.
So you could say that the landing point on the Moon or Mars is point zero / the centre of frame of reference and thus unmoving. But then relative to the moon, the earth, sun etc are moving, so you still need to map out a path to get there noting that the earth and space shuttle will be moving relative to the moon so can't go in a straight line.
Or can take Earth as fixed point, and then have to deal with the moon moving relative to the earth, but would still end up with same path required.
Or use Sun as point of reference, and still need to work out path working out how relative movements work, and end up with same path required.
Ultimately I think for simplicity sake, wr often take centre of galaxy as point of reference. Other galaxies are moving further away or in some cases closer, the sun is orbiting centre of galaxy, we are orbiting sun etc. That all allows newtonian physics to work well and things go from there.
We could take Earth as centre, but in that case the Sun doesn't orbit earth, but instead sketches a strange pattern in relation to an unmoving Earth. Thus we take a frame of reference that makes sense of orbits easier. But by relativity, we dont have to, we just need to be aware that just choosing Earth as centre, doesn't make the universe orbit Earth, just all our describing of the movements of bodies not on Earth would be described relative to Earth.
Your example earlier of moving 100 feet above Earth is an interesting one on this basis. Is that relative to Earth, in which case stay 100 feet above a rotating Earth. Or is it relative to the Sun, in which case the Earth may rapidly disappear and return in 365 days. Or is it relative to the centre of the Galaxy, in which case the whole solar system may disappear to return eons later.
So if setting a time travel device, and Paris is its centre of revenue, it doesn't need to take into account Paris is moving a lot relative to every thing else, as it isnt moving relative to itself.
It does though that challeng for time travel, is ensuring the device is enforcing a frame of reference as such, and maybe time travel hasn't worked well so far as they arent getting whatever msths or science required right, and so they do appear in open space.
The Difference though is Einstein. You are discussing moving through space, but not time.
Most are looking at the Newtonian method and travel. In this, you are worried about one direction. How to get from point A to Point B. We will call this one Vector. They are viewing Time as a second Vector...so two Vectors on how to get from Point A to Point B.
In truth...it will be more complicated than that.
In order to move through time you need to be put outside of the current time. This by default also separates you from other things (such as gravity) which have a connection to Time. By default this will also unhook you from space, as you would. You will need entry for Time and Space and Exit from Time and Space. All relative to your starting position. Not only that, but instead of one vector (as in travel on our planet here) you will need entry for both Space and Time (2) and Exit vectors for both (2).
The center then, would be the point you started at...not the end destination, but the start destination. You would be relative to that location, as that's the focal point of entry, and you could have a direct line from that point to where you separated yourself out of time and space.
The focal point is no longer "Paris" if you would, of your entry.
Similar to how I pointed out at one of my earlier posts. If you started in Tokyo, and then expected to go 50 years into your past when you were in Paris, you would only end up in Tokyo 50 years in the past. Your focal point to breaking time would be the point of entry (where you are) relative to your point that you are going to in time.
In that way, you'd need some sort of references (or coordinate system) of where Paris is in relation to Tokyo or your present location. Without any reference on how to get to Paris, you'll simply never get there. You have no directions and no idea how to get there. You have two points that you are aware of and one destination point thus far. Your first point is your location (Tokyo). Your second is the present time you are at.
Everything after that is up in the air. You are aiming for 50 years in the past. What's the focal point for that? Without any other references that you know of, it will still be Tokyo...saying you can navigate the time somehow (which we haven't even touched on how tricky that could be, it may be just as tricky as going to Paris).
Without directions on how to get to Paris, you'll just end up at the same location but 50 years in the past (if you travel back 50 years). Thus, even if you want to be in Paris, as you are the focal point (your time and space where you start, so Tokyo at the Present), without directions no where to go from that spot, you aren't going to get where you are going. If you end up going back in Time you are just going to end up in the same spot, 50 years prior.
You are right, regardless of whether it is on Earth or another location, it is relative. It's why you need some system of coordinates, or directions, or some way in order to direct WHERE you are going in Time and Space in addition to a point in Time.
You cannot just go back in Time and expect you'll end up at a certain destination without directions on how to get to that destination...and it's not going to be as simple as if you were just doing it in the present. In stead of just one vector you are instead handling 4 (or more) vectors in traveling through Time (because it is no longer simply just a distance you are traveling through, you have to account for time differences, the change in time, the change in space, entry and exit for both). You are no longer just dealing with a singular vector of direction, but 4 dimensions which you have to navigate through.
However, it appears Umbran is arguing against this idea, and as such I pointed out what I understood he was claiming (so if it is a misunderstanding on my part, he can clarify. I think it is possible we are saying the same thing but talking past each other as well...but it is unclear to me).
