ABSTRACT
Astrometry is probably the most ancient branch of astronomy, dating back to at least several centuries BCE and possibly to a couple of millennia BCE. Astrometry may be Absolute (sometimes called ‘fundamental’) when the position of a star is determined without knowing the positions of other stars, or Relative when the star’s position is found with respect to the positions of its neighbours. The measurement of a star’s position in the sky must be with respect to some coordinate system. There are several such systems in use by astronomers, but the commonest is that of right ascension and declination. Two more modern instruments that until recently have performed the same function as the transit telescope are the photographic zenith tube and the astrolabe. By operating instruments in space and so removing the effects of the atmosphere and of gravitational loading, absolute astrometry is expected to reach accuracies of a few microarcseconds in the next decade or so.
