Marianna Safronova and colleagues from the Univ. of Delaware and from the Kurchatov Institute in St. Petersburg have performed new atomic structure calculations for Th+and Th2+that are used to relate measured spectroscopic isotope shifts to differences of the nuclear charge radii. Such calculations are notoriously difficult for these thorium ions because of strong configuration mixing of the electrons. Combined with experimental data on isotope shifts in Th+from 227Th to 232Th, measured at KfK Karlsruhe in 1989, and the recent results on isotope shifts of Th2+obtained by PTB and LMU within nuclock, they provide a more reliable picture of the thorium nuclear charge radii, including an improved value for the radius change between 229Th and the229mTh isomer: Excitation to the isomeric state increases the charge radius by less than 0.02%. The work is published in Physical Review Letters.