I played Traveller many years ago (the GDW version that came in small format paperbacks). I had a number of Sector books (Spinward Marches, for instance), each of which was filled with subsector pages with a hex gride. The size of the hex cells was given as one parsec, as I recall -- which was also the range of a Jump 1 drive, which I recall as the main in-game reason for that map scale.
What never made sense to me, however, was why a unit of measure would be used that was completely foreign to the vast majority of systems and civilizations in the game. The parsec is defined as a "parallax second", the distance that produces one arc-second of parallax on a baseline of two astronomical units -- and an astronomical unit is the radius of Earth's orbit. Earth isn't even known to much of the area covered by the game through much of the game's internal history, so it makes no sense to me to use a distance unit defined by the orbit of a backwater planet -- it'd be like making a unit of measure out of the dimensions of some ruler's body parts (oh, wait...) -- except using a ruler no one ever heard of.
My question is, is there an in-game explanation of why the parsec is used, with that name, for all distances that matter in the original GDW Traveller? I'm not looking for circular definitions like "It's the length of a Jump 1" -- because that doesn't explain why it's called a parsec rather than, say, a "jump unit."