Astrometry is the branch of astronomy concerned with the precise measurements of the positions and movements of stars and other celestial bodies.
Astrometry has been important in history for maritime navigation, since navigators used to calculate their position on Earth upon the observation of stars. (Yes, Jimmy, back in the olden days before GPS satellites!)
Today, astrometry is still important for keeping time. The international time standard is the Coordinated Universal Time (UTC), which is the atomic time synchronized to Earth's rotation by means of exact observations.
Astrometry dates back at least to the Greek astronomer Hipparcos in the 2nd century B.C. He compiled the first catalogue of stars and also invented the brightness scale (magnitude) we still use today.
From 1989 to 1993, the European Space Agency's Hipparcos satellite performed astrometric measurements resulting in a catalogue of positions accurate to 20-30 milliarcseconds for over a million stars.
Modern astrometry was founded by the German mathematician and astronomer, Friedrich Bessel. Bessel was the first to measure the distance to a star as a result of measuring its parallax. He gave the distance to 61 Cygni as 10.3 light years, which is very close to the currently accepted distance of 11.4 light years.
Although once thought of as an esoteric field with little useful application for the future, information obtained by astrometric measurements is now very important in today's research.
Today, astronomers use astrometric techniques for tracking near-Earth objects, and to detect extrasolar planets by measuring the slight wiggle in the parent star's motion on the sky. NASA's planned Space Interferometry Mission will utilize astrometric techniques to detect gas giants around other stars, and perhaps even terrestrial planets nearby.