ABSTRACT
Astrometry is probably the most ancient branch of astronomy, dating back to at least several centuries BC and possibly to a couple of millennia BC. Indeed, until the late eighteenth century, astrometry was the whole of astronomy. Yet although it is such an ancient sector of astronomy, it is still alive and well today and employing the most modern techniques, plus some that William Herschel would recognise. Astrometry is the science of measuring the positions in the sky of galaxies, stars, planets, comets, asteroids and recently, spacecra . From these positional measurements come determinations of distance via parallax, motions in space via proper motion, orbits and hence sizes and masses within binary systems and a reference framework that is used by the whole of the rest of astronomy and astrophysics as well as by space scientists to direct and navigate their spacecra . Astrometry also leads to the production of catalogues of the positions, and sometimes the nature, of objects that are then used for other astronomical purposes.
