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Positions on the sky

Astrometry is the art of obtaining the highest possible precision positional information for objects on the sky. The farthest known sources in the Universe, quasars, do not show measurable motions on the sky. Therefore, by measuring their positions with extreme accuracy, one can use them to define a fixed reference frame of coordinates on the sky.

Most surveys are conducted from the ground, but there has also been a dedicated satellite mission, producing the most accurate coordinates for a large number of objects so far, Hipparcos.

Once such a reference frame is established, large-scale surveys of high-precision positional measurements of objects (including moving ones) can rely on it for very precise relative position offset determinations. Such knowledge of the positions (and motions) of celestial objects can lead directly to very interesting scientific results, e.g. in studies of stellar clusters.

High-accuracy positional information is used more indirectly, e.g. to ensure that telescopes with a long focal length (and thus small field of view), such as e.g. the Hubble Space Telescope (HST) point in the right direction and don't miss their target by accident.

Positions on Earth

Astrometry serves other fields of science as well as astronomy. By having established a highly accurate reference frame of positions on the sky, one can also turn around the argument and say: "Well, if I observe a well-measured position on the sky at a precise moment in time and then measure exactly which direction I am looking at, why then I can also determine exactly where I am on the surface on the Earth." This way, plate tectonic measurements on the surface of the Earth have reached unprecedented levels of accuracy. This is the reason why geodesists work hand-in-hand with astronomers.

And many other fruitful ways of making use of this information have been found; prominent amongst these is the Global Positioning Satellite (GPS) network, which provides us with high-precision data about our position on the Earth's surface. It helps us navigate in unknown terrain, on land, in the air and at sea.

Military application

Unfortunately, exactly this applicability to another area (the only one, by the way) is also a weak point of astrometry (at least I personally perceive it as a weakness), because one can abuse the positional data for other, less peaceful purposes, namely to obtain high-precision target coordinates and thereby trajectories from one's location to the target...