  Accurate distances to stars and galaxies allow for significant tests of
stellar evolution, galaxy formation and evolution and cosmology. NASA's Space
Interferometry Mission (SIM) will obtain precision [1-30 microarcsec]
astrometry for 10-20k objects brighter than V=20 mag.
  In this paper we discuss a method to determine the distance of nearby spiral
galaxies using the technique of rotational parallaxes, and show that it is
possible to achieve distance errors of only a few percent. With such distances
at hand, it becomes possible to determine an accurate zero-point for the
Tully-Fisher relation, one of the tools to measure distances throughout the
universe.
  The rotational parallax method employs the common orbital motions of a of
stars within a galaxy to determine their distance. For stars on circular
orbits, the two proper motions and the radial velocity suffice to determine the
inclination of the orbit, the rotational velocity and the distance. Several
factors complicate the application of this simple technique to real galaxies:
1) the target galaxy may have substantial space-motion, 2) stars in real
galaxies are not on circular orbits (e.g. due to spiral-arm streaming motions
or induced by random motions), 3) stars in the galaxy are at significantly
different distances from the Sun (the near side of M 31 is ~5% closer than the
far side).
  Notwithstanding these complication, distances can be determined to within
several percent for the nearest spirals. (abridged)
