Astrometry, also called positional astronomy, is the branch of astronomy that deals with determining the positions and motions
of
celestial bodies. This science is also concerned with measuring other quantities, such as the diameter and polar flattening of the Sun and planets, and determining the orbits of double-star components (see binary stars).
Astrometry is one of the oldest branches of astronomy. In ancient times the altitude of celestial bodies was determined by the gnomon, the Jacob's staff, the quadrant, and the armillary sphere. The accuracy obtained with the best of these instruments was within 2 minutes of arc. The invention in the 17th century of the micrometer, telescope, Transit, and pendulum clock improved the accuracy of these measurements immensely. After the discovery in the 18th century that, aside from parallax, the stars had their own motion, known as proper motion, the determination of their positions and the measurement of stellar parallaxes caused by the Earth's motion around the Sun became the major objects of astrometry.
The position of a celestial object can be given by two
coordinates, and either of two pairs of coordinates can be used. The first pair, the altitude and the azimuth, are related to the terrestrial horizon and are dependent on the time and place of observation. For this reason the second pair, right ascension and declination, are more frequently used (see celestial sphere; coordinate systems in astronomy).
Two methods for determining the position of stars are used in astrometry. In the absolute method, the coordinates of a star are measured independently of those of any other star, usually by reading the altitude on the transit circle and timing the transit of the star. The absolute system has many disadvantages. Only a small number of stars can be positioned, because the process is time-consuming and can only be used for stars visible in the transit instrument. In addition, measurements taken with a transit instrument are affected by parallax, the small wobbles of the Earth known as precession and nutation, and the proper motion of the Sun. The second method, that of relative determination, is called the differential method. In this method the coordinates of the star whose position is sought are determined by comparing its position to the positions of other stars, called fundamental stars. The coordinates of those stars, which are distributed over the whole celestial sphere, have been determined with high precision, corrected for precession, nutation, and the proper motion of the Sun, and tabulated in fundamental catalogs. A position micrometer or a heliometer is used to measure the angular distances between two stars within the field of a telescope.
Today, instead of visual observation, a photographic method is predominantly used. Stars being measured are photographed with fundamental stars, and the necessary measurements are made on the photographic plate itself. Two types of telescopes are used: telescopes with short focuses are used for faint stars; long-focus telescopes, developed for this purpose, are used for high-precision measurements of such phenomena as parallax and the orbits of double-star components. Long-focus astrometry can also detect disturbances in the proper motion of stars, for example, Barnard's Star, caused by invisible companions. This technique can exceed an accuracy of 0.01 second of arc.
Despite such achievements as these, the fact that all objects in the sky are in motion necessitates the periodic revision of fundamental catalogs. In addition, parallaxes at present can be established only out to a distance of about 3,000 light-years. Beyond that distance, the motions and distances of celestial objects are estimated only in terms of various astrophysical assumptions (see distance, astronomical). Radio and, increasingly, optical interferometry are used to establish the positions in the sky of very distant objects.