Submilliarcsecond Astrometry Over Large Sky Separations

We observed the pair of radio sources 1150+812 and 1803+784 in November 1993 with an intercontinental array of radio telescopes, simultaneously recording at 8.4 and 2.3 GHz, using the very-long-baseline interferometric technique. We determined the angular separation between the two sources with submilliarcsecond accuracy by using differential techniques. This result validates the feasibility of high precision differential astrometry for radio sources separated in the sky by almost 15°, and opens the avenue to its application to larger samples of radio sources. This revised version was published online in July 2006 with corrections to the Cover Date.