astrometry

Measurement of the precise positions of stars, planets, and other bodies in space. Such information is needed for practical purposes including accurate timekeeping, surveying and navigation, and calculating orbits and measuring distances in space. Astrometry is not concerned with the surface features or the physical nature of the body under study.

Before telescopes, astronomical observations were simple astrometry. Precise astrometry has shown that stars are not fixed in position but have a proper motion caused as they and the Sun orbit the Milky Way galaxy. The nearest stars also show parallax (apparent change in position), from which their distances can be calculated. Above the distorting effects of the atmosphere, satellites such as the European Space Agency's Hipparcos, which operated from 1989 to 1993, can make even more precise measurements than ground telescopes, so refining the distance scale of space.