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 recognize. Astrometry is the science of measuring the positions in the sky of galaxies, stars, planets, comets, asteroids, and recently, spacecraft. 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 spacecraft. Astrometry also leads to the production of catalogs of the positions, and sometimes the natures, of objects that are then used for other astronomical purposes.
