Weite - Breaking the Shackles
A Parsec is a unit of length, used to measure the distance between stars. The name parsec stands for “parallax of one second of arc”, and allows calculating distances to nearby stars using simple observation and trigonometric parallax (apparent motion of an object due to change of position of the observer). It is one of the oldest Terran methods for measuring the distances to stars. A parsec is based on a baseline of 1 Astronomical unit.
The Solomani define one parsec to be the distance from the Terra to a star that has a parallax of 1 arcsecond, and equals 3.261 light-years or 206,265 Astronomical units. By coincidence this distance is very close to the maximum distance a jump-1 drive can jump. Because of this, ‘parsec’ has become the standard word used for this distance within the Imperium.
Since Jump distances are calculated in parsecs (3.27 light-years), it has become the scale of a subsector mapping grid. Jump-1, for example, indicates the ability to jump one parsec, or one mapping hex. Astrogation maps are based on this fact and are not accurately scaled maps for astronomical purposes.
Other races, using the orbital distance of their home world and different definitions of an arcsecond would derive different values for a parsec, but these wouldn’t be Imperial parsecs.