It seems that the value has only 1 1/2 significant digits. Why are values for the measurement of this radius not more precise? What prevents a greater precision? I have recently been trying to learn about the sun's orbit in the Milky Way (MW) in order to calculate estimates re DM for another thread.. I have found that different sources give values for the radius of he sun's orbit that vary more than I expected. Here are examples: 1. 25+ kly =7.7+ kpc https://en.wikipedia.org/wiki/Galactic_year https://en.wikipedia.org/wiki/File:Milky_Way_Arms_ssc2008-10.svg2. 26 kly = 7.97 kpc http://www.universetoday.com/18028/sun-orbit/3. 27.2 ± 1.1 kly = 8.33 +/- 0.35 kpc (= +/- 4%) https://en.wikipedia.org/wiki/Milky_Way http://arxiv.org/abs/0810.46744. 27.7 kly = 8.5 kpc http://web.mit.edu/redingtn/www/netadv/specr/012/node7.html I am using (3) in my calculations since that at least provides an error range.