Fundamental Astrometry

the branch of astrometry that deals with the establishment of the most precisely defined fundamental system of celestial coordinates. Such a fundamental coordinate system is implemented as an equatorial coordinate system and is required for the study of the positions and motions of celestial bodies and of artificial objects in space, as well as for geodetic determinations. A fundamental coordinate system is fixed by the data contained in a fundamental catalog, which provides the positions and proper motions of a certain number of stars, as derived from observations, in the fundamental coordinate system. To construct such a coordinate system, positional observations of stars, galaxies, and objects in the solar system are made; the theory and practice of such observations are the subjects of fundamental astrometry.

Because of the proper motions of the stars serving as reference points that fix the fundamental coordinate system, changes in the celestial coordinates of such stars are determined from observations made at different epochs. The orientation of the fundamental coordinate system on the celestial sphere is refined on the basis of observations of objects in the solar system, including the sun, the moon, the major planets, and asteroids. The proper motions of the stars are refined with respect to galaxies, which are, in practice, fixed bodies on the celestial sphere. Positional observations of stars must be made at regular intervals, owing both to the reduction of the accuracy of the fundamental coordinate system with time because of the accumulation of errors in the proper motions and to the necessity of extending the fundamental coordinate system to a larger number of stars in order to solve problems of photographic astrometry.

In fundamental astrometry, observations may be made by either visual or photographic methods. The coordinates of stars, the sun, Mercury, and Venus are determined visually by means of meridian circles, transit instruments, and vertical circles. The positions of faint stars, galaxies, the major planets, and asteroids are obtained photographically from observations made with as-trographs. Experimental positional observations of celestial radio sources with radio interferometers have been undertaken.

The solution of the problems of fundamental astrometry lies in and is related to the study of the regularities of the rotational and orbital motion of the earth, since all observations made from the earth’s surface should be free from effects caused by the motion of the earth. The fundamental coordinate system for some fixed epoch is taken as an approximation of the inertial frame of reference for the study of the motions of celestial bodies.