>

We could deploy planets and other celestial objects as references. Their displacement will be an issue since all systems and individual celestial objects keep moving but much advanced navigation systems should solve that
This is a friendly reminder to read our rules.
Remember, /r/Showerthoughts is for showerthoughts, not "thoughts had in the shower!"
(For an explanation of what a "showerthought" is, please read this page.)
Rule-breaking posts may result in bans. >

Obviously, by the time space travel becomes commonplace there will already be an alternate coordinate system. Space travel couldn't become commonplace without one. >

Not so. The spinning of a planet determines these directions. If you take your right hand, make a thumbs-up sign, and orient it so that your fingers curl in the direction the planet is spinning, your thumb will point towards the north pole and the other directions will be consistent with earth.
From that we can get latitude angles without any arbitrary decisions. The only piece that is arbitrary is where you start numbering longitude. We can set some standard for that pretty easily though. For example, we could say that the point of highest elevation on the planet is longitude 0. That would give a complete coordinate system for any planet >

Also if you hold a wire with your right hand, and thumb pointing in the direction of current flow, your curled fingures match the rotation of the magnetic field generated by the flowing current through the wire.