Here are four relative directions: Left, Front, Right, and Back. Perhaps I should have said “forward” and “backward” instead of “front” and “back”, but the words would have been substantially longer. So maybe I should have called this “relative sides”, but I like “relative directions” better. And perhaps I should have added “up” and “down”, but then it wouldn’t have been a four-fold! At any rate, for entities or observers bound by common gravity, up and down are fairly consistent. For all such beings, all down directions should converge to some idealized point in the center of the earth but all up directions should diverge in all directions like the light rays from the sun.
If you compare this four-fold to that of the Cardinal Directions, you can see that these directions are indeed relative to an observer, and depend on how the observer is turned. So they are fixed to the frame of reference of the observer. In comparison, the Cardinal Directions are fixed with respect to a location on the earth, but they vary from location to location, and so are relative to location and fixed for an observer at that location. For example the direction of North at New York City is different from North in Los Angeles, even though they converge at the same point on the earth, the North Pole.
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