That is a very good point as Lee pointed out.
If you travelled back six months to the precise location that you are currently occupying, the Earth would obviously be on the other side of the sun - slightly uncomfortable without a spacesuit..
Perhaps if time travel were possible, the machine would need to be a spacecraft itself, or maybe it would simply need to be located on a space craft, so you don't end up 'spacing' yourself.
Could you actually predict what would occupy the location of your arrival with any certainty? Or would you just have to pray that you don't arrive inside a star?..Ouch.
There is a bigger, more fundamental problem though.
How do you define any specific location in the universe? Is it the 'exact' position of a given elemental particle? Somewhere in between 2 given elemental particles? Will we ever be satisfied that we have found the smallest unit possible, the most fundamental way of identifying a precise point? This has strong parallels with the problem of identifying any given point in time, given the limitations of our perception.
Add to that the factor of the expanding universe. As the universe is effectively growing, are universal co-ordinates moving apart with this expansion, or they truly static, with new ones being formed?
But if you were to travel back in time, what is it about the destination that are you actually targeting? Simply telling a machine that you wish to travel back to next Thursday is not enough. Are you somehow using the 'history' of a particle to effectively map your destination? If so, would you automatically be drawn to it's 'location' at that 'moment'?
(I don't actually know very much about sub atomic particles and the like, even their 'lifespans', so using them in that way may be nonsense. I think it illustrates my point though, that human 'timescales' are meaningless when tring to operate outside of them..)
Maybe the universe is something like the surface of a balloon. If you blow it up a little and then draw two points on it, they are a certain distance apart. Blow it up further and the distance increases, but they still occupy the same point on the suface. This almost seems paradoxical, but while the distance relative to each other has increased, they are still anchored to the same points relative to the medium in which they exist (ie..the surface). The 'medium'/surface has simply stretched.
We still just don't understand the nature of the medium/universe that we occupy. We tend to assume that space can be defined in simple terms, compared to time, the nature of which which we spend inordinate amounts of 'time' deliberating over. Maybe the two are similarly problematic, or similarly straight forward. Locating a point in space and a point in time might actually be parts of the same overall problem.
