| Abstract | Using as an example astrometric observations of the quasars 3C 345 and NRAO 512 which are separated by ≡0°.5 at the sky, it is shown that under certain conditions, annual parallaxes of galactic radio sources can be determined with VLBI with an accuracy of 50 μas (standard error). The, so far, most direct method of determining distances to extragalactic sources combines VLBI measurements of a supernova's angular expansion velocity with optical spectroscopic measurements of the supernova's radial expansion velocity. The author has estimated the distance to SN 1979C in M100 in the Virgo cluster to be 20(+7,-6) Mpc, a lower bound on the distance to SN 1980K in NGC 6946 to be 5 Mpc, and H0 to be 65(+25,-20) km s-1Mpc-1. |
