Astrometry in the solar system made more progress during the past ten years than it did during the seven or eight decades before. In fact, the development of large refractors with micrometers, and after the improvement of the photographic technique, allowed one to get very accurate positions at the end of the 19th century. After that, relatively little progress was made. Astronomers faced several difficulties: first, was the poor sensitivity of the photographic plates leading to long exposures and therefore to low accuracy for faint objects; second, was the poor quality of the catalogues of stars which depended upon a very small number of available astrometric reference stars.
New receptors appeared around the 1950, such as electronic cameras or T.V. tubes. But, most of these new receptors were not interesting for astrometry, mainly because of the field of view – which was too small – and of the difficulty of using such receptors. However, the development of a new technique based upon the CCD targets changed most of the astrometric observational programs. In fact, CCD receptors were made for small fields, but the CCDs were easy to use; the images were numerical, allowing processing by computers, and the sensitivity was much more important. Because of that, the accuracy increased and relative astrometry was developed. The recent development of new catalogues, such as the Guide Star Catalogue and Tycho, permits astronomers to reduce small fields, which was not possible earlier.