The thing is that there exists no universal frame of reference. There is no such thing as "space coordinates 0,0,0". Those coordinates would have to be relative to some object in space.
I think the point of GP was that any time travel device you build, calculations/projections you make, will all be built around a frame of reference where the planet IS in a fixed position, as far as you are concerned. Since you don't know any other frame of reference, the machine won't either.