What is Astrometry? - Hipparcos
What is Astrometry?
Astrometry has two main scientific objectives:
Fundamental though the stellar astrometric parameters (positions, parallaxes and proper motions) are, the angles involved are extremely small (typically parallaxes and annual proper motions are measured in milliarcsec), and the measurements required to quantify them must be extremely precise. Conditions experienced from ground-based astronomical observatories, such as the perturbing atmosphere, lack of all-sky visibility, and gravitational and thermal flexure of telescopes, complicate the measurements and the separability of the positional and time-dependent parameters. These factors tended to result in a limit beyond which progress proved technologically complex and observationally time-consuming.