Astrometric measurements not only determine the position of objects on the celestial sphere (sky), but also can be used to measure the distances to stars. By measuring the change in a star's position as the Earth revolves around the Sun, you can determine the distance to that star. This change in position is know as a star's parallax. The concept of parallax was discovered by the ancient Greeks, who learned that the stars are very far away because they were not able to measure the parallax of any stars. It wasn't until 1838 that German Mathematician and Astronomer Friedrich Bessel was able to measure the distance to the star 61 Cygnus.
Distances to astronomical objects are very difficult to determine. The astronomical distance scale is determined by a series of methods that work at different distances, with the parallax method being its foundation. Small errors in the measured distances of stars can therefore lead to large errors in the distances to galaxies. Accurate astrometric measurement are required for improving our understanding of the Universe.
Astrometry is the foundation on which almost all of astronomy is based. Not only is it the oldest branch of astronomy, but astrometric measurements are the bedrock of methods for determining distances to astronomical objects. Modifications and improvements to this distance scale impact our understanding of astrophysical phenomenon both nearby and at cosmological distance scales.