astrometrie

Astrometrie, or astrometry in English, is the branch of astronomy that concerns the precise measurements of the positions, motions, and distances of celestial objects. It provides the celestial reference frame for astronomical observations and underpins the cosmic distance scale. Core measurements include the angular position of objects on the sky, their proper motion across the celestial sphere, and annual parallax used to estimate distance.

Historically, astrometrie has early roots in naked-eye star catalogs and positional astronomy. The first reliable stellar parallax measurements were achieved in the 19th century (notably by Friedrich Bessel in 1838). The field advanced through improvements in telescopes, detectors, and, crucially, space-based observing, which removed atmospheric distortion and yielded much higher precision.

Techniques in astrometry rely on precise angular measurements tied to inertial reference frames, such as the International Celestial Reference System (ICRS). Major outputs include star catalogs containing positions and motions for hundreds of millions to billions of objects. Notable achievements come from space missions like Hipparcos (1989–1993) and Gaia (launched 2013), which have produced extensive, high-precision astrometric data. Ground-based methods include interferometry, such as very long baseline interferometry (VLBI), which achieves microarcsecond precision and anchors the celestial reference frame through distant radio sources.

Applications of astrometrie are diverse. They include determining stellar and galactic distances via parallax, studying the kinematics and structure of the Milky Way, refining orbits of Solar System bodies, and detecting exoplanets through astrometric wobbles of host stars. Astrometry also supports spacecraft navigation and improves the calibration of the cosmic distance ladder. Ongoing and future data releases from Gaia and complementary techniques continue to enhance the precision and scope of astrometric measurements.