The astrometry It is the specialization of astronomy that studies the position of the stars in the sky, in order to establish the celestial coordinates and their variations in time and reconstruct the movements of the stars.

Among the applications of astrometry is the control of Earth's movements. Astrometry, also called position astronomy, clearly differs from another astronomical specialization, Astrophysics, since while the former deals with the position of the celestial bodies, the latter studies their physical nature.

The ancient astronomical observations were exclusively astrometric in nature: they served to obtain, from the study of the movement of the stars, the first and gross calendars and the criteria of subdivision of time. Classical astrometry was born with the compilation of stellar catalogs

The instruments used in antiquity to determine the stellar positions were the Astrolabe, the Armillary Sphere, the Quadrant, etc. A Renaissance astronomer who dedicated most of his life to astrometry was Tycho de Brahe (1546-1601), who built large and very precise instruments, although lacking the optical part, since he lived before the invention of the eyeglass .

Modern astrometry uses refined instruments that, combined with sidereal clocks, serve to determine with great precision the passage of a star through the meridian and, therefore, its coordinates. The measurements, repeated over time, establish the variations attributable either to the movements of the Earth, such as precession, or to the Movement proper to the stars.

To determine the coordinates of the weakest objects, astrometry uses photographic methods and instruments that allow to measure the stellar positions directly on the film. The measurement of the distances of objects very close to each other and weak, to the point that the current observation instruments are not enough to distinguish them, is performed with optical and radio interfering techniques.

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