//Unless the definition of a parsec is known or provided, how is anyone meant to calculate the distance in light-years?//
I sort of half explained it, Corby (and no, I did not and have not looked it up). It is short for "parallax second". This is from memory, so it may not be spot on:
if you observe a body in space and draw a line to it from the earth, and then observe the same object six months later (when the Earth is on the opposite side to the Sun) and draw another line to it, you can form a triangle (with the Earth's diameter as its base). When the angle at the top of the triangle is two seconds of an arc, the distance to the object will be around 3 light years (can't recall the exact figure). The definition of parsec is when the triangle formed by the radius of the Earth's orbit subtends the angle at one second of an arc.
So you have a right-angled triangle where you know the length of the "opposite" side (the Earth's radius) and using the tangent ratio (opposite/adjacent) you can calculate the length of the adjacent side (the distance from the Earth to the object) when the angle is one second. This turns out to be a bit over three light years (about 18 million million miles). Unfortunately my "four-figure tables" do not go down to seconds of an arc, so I'll have to rely on the figure which others have calculated. But I calculate the tangent ratio (93million/18million million) to be approximately 0.000005167.
I think that's about right. But I may be wrong! :-)