astrometry

Astrometry is the branch of astronomy for measuring the position of celestial bodies such as stars. I've also seen it defined specifically as the precise measurement of bodies on the celestial sphere. Angular resolution of telescopes used for astrometry plays a role in the resulting precision, but through analysis of multiple observations, positions of stars can be refined as much as down to 1/10 the "official" angular resolution of the instrument.

I've seen the phrases absolute astrometry and relative astrometry, which I interpret to mean determining an exact position in the celestial sphere, versus determining a position relative to other objects in the celestial sphere. The most precise astrometry requires analysis of the errors, i.e., pushing the precision to the maximum that can be provided by the observation(s) and statistical analysis of the noise (i.e., the measurement-uncertainty). The term global astrometry refers to efforts to do this using data collected throughout the celestial sphere, and adjusting the frame of reference and each coordinate so as to be consistent with the most likely noise distribution. There have been a number of surveys for the purposes of astrometry, which are followed up with analysis (using numerical methods) of all the data on the hundreds of thousands of individual objects observed. Modern high-precision astrometry of stars is dominated by special-purpose telescopes, not by any means the largest, but on space missions that naturally avoid the seeing problems of Earth atmosphere: Hipparcos circa 1990 and Gaia more recently have measured positions of over a billion stars to unprecedented precision.

Precise measurement of the apparent motion of a star in the sky can reveal its distance through parallax, or can reflect actual movement, showing something of its motion vector, or can indicate an astrometric binary or the presence of an extra-solar planet. Astrometry is also of interest in star catalogs used for navigation and for aiming telescopes (i.e., using guide stars).

The terms positional astronomy and spherical astronomy refer to the general science of determining the position of objects within the celestial sphere at various times, including not only stars, but also solar system bodies, such as the Sun, Moon, and planets. The term astrometry is sometimes used to mean the measurement-portion of this science.