Comparing diameter to radius is a little misleading, the ring is actually closer than Voyager, at 120 AU. On the same scale, Pluto is 78 AU and Voyager is 320 AU.
> Comparing diameter to radius is a little misleading
Nah. It is comparing a distance with a distance.
Besides I assume more people reading my comment are familiar with the concepts of radius/diameter than the meaning of astronomical units. But now, thanks for your clarification, even the geometrically challenged will have the correct mental model. You never know.
> On the same scale, Pluto is 78 AU and Voyager is 320 AU.
Have to disagree. You take your pocket tape measure, carefully hook the end of the tape on the Sun and slowly walk to Pluto. You read what it says. That is a distance.
Then you walk to the disk, hook one end of the tape on one side (carefully, since it is made of dust) and walk to the other side. That is a distance.
Where do I hook my tape measure to measure 78 AU for Pluto? Orbits are not real. You cannot kick them, you cannot lick them, you cannot hook a tape measure on them.
“See where that planet is? Now imagine how far it will be in 124[1] years in a sun fixed coordinate system!” does not quite have the same impact on me as “See that blob? It is huuuge!”
Doubling the number with Voyager makes even less sense. You cannot even say that you are measuring the antipodal points of its orbit, since it is not orbiting the Sun. So the “diameter” of what are you even talking about there?
> Have to disagree. You take your pocket tape measure, carefully hook the end of the tape on the Sun and slowly walk to Pluto. You read what it says. That is a distance.
By that measure then, you carefully hook your tape measure to Fomalhaut and slowly walk out to the outer ring, whereupon your ancient eyes perceive the distance to be 120 AU, not 240 AU (since 240 AU is the diameter). A given rock in the outer ring is therefore less distant from Fomalhaut than Voyager is from the Sun.
That's all the parent is saying, and I agree with them - I, too, know the difference between radius and diameter but a quick read of your comment left me thinking that Fomalhaut's outer ring is orbiting further away from Fomalhaut than Voyager is from the Sun.
It's clearer to compare a radius to a radius, is all.
>It's clearer to compare a radius to a radius, is all.
The OP wasn't comparing a diameter to a radius, he was comparing one line to another line: the diameter of the outer ring (line going from one end to the other passing through the middle) and the distance between Pluto and the Sun (line between two points). The fact that Pluto orbits the Sun is just a coincidence for the purposes of this comparison.
The "Sun-Pluto distance" is colloquially understood as both of those things - a distance in a straight line right now, and the radius of its orbit on average. I think it's far more common for people to assume the latter as I did, but feel free to ask around and find out for yourself. Note that the instantaneous Sun-Pluto distance varies considerably due to the eccentricity of its orbit, and the distance OP quoted is the orbital semi-major axis (what one might refer to as the orbital radius for the sake of brevity), not the present distance.
OP was mathematically correct, just a bit unclear. We really don't need to spend so much time on this.
