I started off this exercise with a question: "what if the unit of jump drive distance was similar to the Vilani parsec, instead of the Terran one?". It may not be apparent, but the parsec is actually a somewhat subjective distance, and each race will have its own definition of it.
A Terran parsec is best defined on this wiki page, but essentially is the distance of the long side of a right-angle triangle which has a short side length equal to one AU, and an angle of 1/3600 degree. (see the diagram at the bottom of the wiki page).
So there are two units in there that are subjective - the unit of distance (AU), and the unit of angle (one arc-second, which is 1/3600 degree).
Given that the Vilani were the first into space, it seems reasonable that their units would be standard for a very long time. But since they are from a different planetary system, it also seems reasonable that their units of measurement would be different too.
Angle
The measurement of angles using degrees is an older methodology than measuring them using radians, even though the latter is based on more objective standards (One radian is the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle (ref)). Apparently the origin of the degree is that "Ancient astronomers noticed that the stars in the sky, which circle the celestial pole every day, seem to advance in that circle by approximately one-360th of a circle, i.e., one degree, each day. (Ancient calendars, such as the Persian Calendar, used 360 days for a year.)" See: http://en.wikipedia.org/wiki/Degree_(angle) .
Looking at the Vland system though, we run into a problem: the DGP Vilani and Vargr book claims that Vland orbits its F8 V primary star at a distance of 1.6 AU, that Vland's rotation period is 32 hours (114498 seconds, to be precise) and that its orbital period is roughly 360 of its days. Unfortunately, this is complete nonsense - in Earth terms, that corresponds to 477.075 earth days, and for it to complete an orbit at 1.6 AU in that time means that the star has to be 2.4 solar masses, which means it has to be an A3 - A4 V star which cannot possibly support advanced life (according to the mass table in CT Book 6).
Using CT Book 6's definition of the mass of an F8 V star (1.144 Sols), we find that the orbital distance required for a planet to have an orbital period of 477.075 earth days is actually very close to 1.25 AU (= 360.12 Vilani days). This coincidentally is not too dissimilar to the mass of a real F8 V star, so at least this part of the CT stellar mass table isn't too unreasonable. At this distance, and given the star's luminosity of 2.126 Sols, the planet's blackbody temperature would be 300 K (compared to Earth's 278 K), which means it'd be significantly warmer than Earth if it had an atmosphere exactly like our own (but it wouldn't be uninhabitable).
So assuming we shuffle the rest of the system around to accommodate this change, we now have Vland orbiting its 1.144 solar mass F8 V primary star in 360.12 of its (almost) 32-hour long days. Which (getting back to the point) is pretty much a perfect fit for the definition of a degree - ancient Vilani astronomers would have noted that their "fixed stars" rotated by a specific amount every day, and that amount would be 1/360th of a circle, which is the same as a Terran degree.
Therefore it is not unreasonable to assume that the Vilani and Solomani definitions of "degree" would have been the same. It is also not unreasonable that they would have come up with similar subdivisions of a degree - the minute of arc (1/60th degree) and the second of arc (1/60th of an arcminute, which is 1/3600th of a degree).
Distance
Now that we've established Vland's orbital distance from its star, it's a simple matter to determine the size of its "Astronomical Unit" - it'd just be the distance that the planet orbits its star, which is 1.25 AU.
The Vilani Lightyear (Vly)
Of course, the Vilani lightyear (Vly) is also different to the Terran one, because while the speed of light is the same, the year length is different. Vland's orbital period of 360.12 Vilani days is equal to 477.23 Terran days, which is 1.3066 Terran years. So one Vilani lightyear = 1.3066 Terran lightyears.
The Vilani Parsec (Vpc)
Now that we've established angle and distance, it's a simple matter to define the Vilani parsec (Vpc). The angle, by happy coincidence, is the same - 1/3600th of a degree is the same in Vilani as it is for Terrans. The "short side" distance of the triangle is Vland's orbital distance, which is 1.25 AU. So after all this, we see that the Vilani definition of the parsec is 1.25 times that of the Terran definition, which is 4.077 Terran lightyears (Tly) - this is equal to 3.1203 Vilani lightyears (Vly).
The point
Which brings me to the point of this exercise, which turned out to be more fiddly than originally anticipated. My original assumption was that a Vilani parsec would just be 1.6 times that of a Terran parsec, but that was thrown out of the window when I found that the Vilani orbital period was completely miscalculated in the DGP book. So now we're left with 1 Vpc being equal to 1.25 Terran parsecs, which isn't actually that much bigger than the latter. However, if J1 was actually equal to 1 Vpc, then that would mean that Alpha Centauri (at 4.3 Tly) would be just over J1 from Earth (1.055 Vpc) - reachable in a single jump and a little M-drive. Barnard's Star would be 1.46 Vpc - still problematic, but better than 1.83 Tpc.
Had a Vpc been equal to 1.6 Tpc, then Barnard's Star would have been 1.15 Vpc, which could have possibly solved the "problem" of empty hex jumps because that 0.15 Vpc would have been easier to traverse using M-drives, and you wouldn't need bases or substellar objects to do it. But alas, it wasn't to be.
Oh well. That's probably utterly useless to anyone, but make of that what you will...
ADDENDUM: The point of this post isn't to restart the EHJ trainwreck. As far as I'm concerned there isn't a "problem" with EHJs - I just wondered what the effect of using a Vpc as a base unit of jump distance would be and if it would circumvent that issue.
A Terran parsec is best defined on this wiki page, but essentially is the distance of the long side of a right-angle triangle which has a short side length equal to one AU, and an angle of 1/3600 degree. (see the diagram at the bottom of the wiki page).
So there are two units in there that are subjective - the unit of distance (AU), and the unit of angle (one arc-second, which is 1/3600 degree).
Given that the Vilani were the first into space, it seems reasonable that their units would be standard for a very long time. But since they are from a different planetary system, it also seems reasonable that their units of measurement would be different too.
Angle
The measurement of angles using degrees is an older methodology than measuring them using radians, even though the latter is based on more objective standards (One radian is the angle subtended at the center of a circle by an arc that is equal in length to the radius of the circle (ref)). Apparently the origin of the degree is that "Ancient astronomers noticed that the stars in the sky, which circle the celestial pole every day, seem to advance in that circle by approximately one-360th of a circle, i.e., one degree, each day. (Ancient calendars, such as the Persian Calendar, used 360 days for a year.)" See: http://en.wikipedia.org/wiki/Degree_(angle) .
Looking at the Vland system though, we run into a problem: the DGP Vilani and Vargr book claims that Vland orbits its F8 V primary star at a distance of 1.6 AU, that Vland's rotation period is 32 hours (114498 seconds, to be precise) and that its orbital period is roughly 360 of its days. Unfortunately, this is complete nonsense - in Earth terms, that corresponds to 477.075 earth days, and for it to complete an orbit at 1.6 AU in that time means that the star has to be 2.4 solar masses, which means it has to be an A3 - A4 V star which cannot possibly support advanced life (according to the mass table in CT Book 6).
Using CT Book 6's definition of the mass of an F8 V star (1.144 Sols), we find that the orbital distance required for a planet to have an orbital period of 477.075 earth days is actually very close to 1.25 AU (= 360.12 Vilani days). This coincidentally is not too dissimilar to the mass of a real F8 V star, so at least this part of the CT stellar mass table isn't too unreasonable. At this distance, and given the star's luminosity of 2.126 Sols, the planet's blackbody temperature would be 300 K (compared to Earth's 278 K), which means it'd be significantly warmer than Earth if it had an atmosphere exactly like our own (but it wouldn't be uninhabitable).
So assuming we shuffle the rest of the system around to accommodate this change, we now have Vland orbiting its 1.144 solar mass F8 V primary star in 360.12 of its (almost) 32-hour long days. Which (getting back to the point) is pretty much a perfect fit for the definition of a degree - ancient Vilani astronomers would have noted that their "fixed stars" rotated by a specific amount every day, and that amount would be 1/360th of a circle, which is the same as a Terran degree.
Therefore it is not unreasonable to assume that the Vilani and Solomani definitions of "degree" would have been the same. It is also not unreasonable that they would have come up with similar subdivisions of a degree - the minute of arc (1/60th degree) and the second of arc (1/60th of an arcminute, which is 1/3600th of a degree).
Distance
Now that we've established Vland's orbital distance from its star, it's a simple matter to determine the size of its "Astronomical Unit" - it'd just be the distance that the planet orbits its star, which is 1.25 AU.
The Vilani Lightyear (Vly)
Of course, the Vilani lightyear (Vly) is also different to the Terran one, because while the speed of light is the same, the year length is different. Vland's orbital period of 360.12 Vilani days is equal to 477.23 Terran days, which is 1.3066 Terran years. So one Vilani lightyear = 1.3066 Terran lightyears.
The Vilani Parsec (Vpc)
Now that we've established angle and distance, it's a simple matter to define the Vilani parsec (Vpc). The angle, by happy coincidence, is the same - 1/3600th of a degree is the same in Vilani as it is for Terrans. The "short side" distance of the triangle is Vland's orbital distance, which is 1.25 AU. So after all this, we see that the Vilani definition of the parsec is 1.25 times that of the Terran definition, which is 4.077 Terran lightyears (Tly) - this is equal to 3.1203 Vilani lightyears (Vly).
The point
Which brings me to the point of this exercise, which turned out to be more fiddly than originally anticipated. My original assumption was that a Vilani parsec would just be 1.6 times that of a Terran parsec, but that was thrown out of the window when I found that the Vilani orbital period was completely miscalculated in the DGP book. So now we're left with 1 Vpc being equal to 1.25 Terran parsecs, which isn't actually that much bigger than the latter. However, if J1 was actually equal to 1 Vpc, then that would mean that Alpha Centauri (at 4.3 Tly) would be just over J1 from Earth (1.055 Vpc) - reachable in a single jump and a little M-drive. Barnard's Star would be 1.46 Vpc - still problematic, but better than 1.83 Tpc.
Had a Vpc been equal to 1.6 Tpc, then Barnard's Star would have been 1.15 Vpc, which could have possibly solved the "problem" of empty hex jumps because that 0.15 Vpc would have been easier to traverse using M-drives, and you wouldn't need bases or substellar objects to do it. But alas, it wasn't to be.
Oh well. That's probably utterly useless to anyone, but make of that what you will...
ADDENDUM: The point of this post isn't to restart the EHJ trainwreck. As far as I'm concerned there isn't a "problem" with EHJs - I just wondered what the effect of using a Vpc as a base unit of jump distance would be and if it would circumvent that issue.
