astrometry

Astrometry is the branch of astronomy that measures the precise positions and motions of celestial objects. It uses angular coordinates on the sky, such as right ascension and declination, and tracks motion across the sky known as proper motion. A central parallax measurement—the apparent shift of a nearby object against distant background objects as the observer’s vantage point changes—yields distance, with one parsec corresponding to a parallax of one arcsecond. Radial velocity is a complementary quantity along the line of sight.

Historically, accurate positional measurements emerged with Tycho Brahe, who laid the groundwork for modern celestial mechanics. In the space era, Hipparcos (1989–1993) produced a precise catalog of about 118,000 stars; Gaia, launched in 2013, is mapping over a billion stars with microarcsecond precision and provides positions, parallaxes, proper motions, and photometry.

Astrometry is anchored to the International Celestial Reference System (ICRS). Coordinates are usually given for a standard epoch such as J2000.0, with transformations that account for aberration, precession, and nutation. Modern data sets also deliver orbital parameters for astrometric binaries and exoplanet hosts.

Applications include measuring stellar distances and motions to map the structure and dynamics of the Milky Way, calibrating the cosmic distance ladder, and aiding spacecraft navigation. Ground-based work faces atmospheric limitations, while space-based astrometry reduces these errors through precise instrumentation and calibration.