Astrometric observations are commonly measurements of angular positions of bodies on the celestial sphere in a given reference frame. However, the precision of such measurements is limited by the instruments and by the receptors used for these observations. Astronomers saw very early that celestial bodies were often involved in phenomena such as eclipses of the Sun or the Moon. Such phenomena correspond to specific positions of the involved bodies in space. At least, one may say that the topocentric positions of the Moon and the Sun are very close at the time of an eclipse. This is an astrometric observation. Since the possible phenomena are numerous in the solar system thanks to the velocity of the moving objects, astronomers made predictions of such events and made observations as precise as possible in order to deduce astrometric positions from these events. Of course, a model was necessary for that purpose but this was the main way to improve our knowledge of the solar system at the beginning of astronomical research.