W3(OH): accurate relative positions of water-vapor emission features

Very-long-baseline interferometric observations made on 1971 March 28 were used to determine the relative angular positions of water-vapor emission features in the radio source W3 (OH) to within 0'!0003 through combined use of fringe-rate and fringe-phase information. Observations on the longest available Earth-based interferometer baselines should yield more than an order of magnitude greater accuracy and would be sensitive, within a year, to transverse (proper) motions of about 1 km s ' between features at the 2.5 kpc distance of W3(OH). Subject headings: infrared sources - molecules, interstellar