There are several branches of astronomy, one of which mainly analyses the movements and positions of celestial objects; this industry, called astrometry, also includes the measurement of other values, such as the diameter of planets, and the assessment of orbits of dual-star components. Four classes of astrometry are small-field astrometry, semi-global astrometry, ground astrometry and space astrometry. Astrometry is also called position astronomy because of its primary role in the study of the position of stars. It is considered to be one of the oldest industries in astronomy.
In ancient times, one of the values defined in astronomy was the height of celestial objects by means of instruments such as the Quadrant, the Jacob staff and the gnome. However, these instruments were not considered to be good enough to produce accurate measurements. With the invention in the 17th century of more sophisticated instruments such as telescopes, pendulum clocks and micrometer, measurements became more precise. In the 18th century, astronomers discovered that stars had their own motion, which was called their own motion. Since then, the position of stars and the measurement of star parallax, or the difference in the apparent position of a star in observation from two different locations caused by the motion of the Earth around the Sun, became two important tasks in astronomy.
The two coordinates can give the position of the celestial body as a slope and a direct climb. In addition, the position of the stars can be determined in two ways: by an absolute method and by a differential method. An absolute method can be implemented by reading the height of the star in the transit circle and determining the time of its passage, in order to measure the coordinates of the star, irrespective of the coordinates of other stars. However, the differential method can be achieved by comparing the position of the star with that of other stars, which are called fundamental stars. For differential observations, the most common method is a photographic method in which a star whose position is measured is photographed with the fundamental stars, while measurements are made on the photograph itself.
However remarkable these methods may be, the periodic revision of fundamental catalogues is important simply because all celestial objects are constantly in motion. Moreover, parallaxing can only be calculated to a distance of about 3,000 light years. Outside this distance, astronomers can only estimate the movements and distances of celestial bodies according to various astrophysical assumptions. In order to determine the position of very remote celestial objects, astronomers use instruments such as radio and interferometers. The use of astrometric satellites, such as the Hipparchos satellite launched in 1989, also plays an important role in the field of astrometry.