The main objective of stellar astrometry is to determine the positions of stars in space at some epoch and to describe their displacements in time. The instruments used for observations, and how the positions are determined, are given in Kovalevsky (2002). The reference frames have been discussed in Chapters 7–10 of the present book. The observational reduction procedures and corrections for apparent displacements were described in Chapter 6.

Star positions are not an objective per se. What is of interest are the motions and distances of celestial bodies. The first are the proper motions that describe the apparent displacements due to the actual motion of stars with respect to the barycenter of the Solar System. As we have already seen in Chapter 6, in order to transfer Earth-based observations to a barycentric position, one has to correct for annual parallax. However, the value of the parallax coefficient has a major importance in astronomy, because it is the basic source of distances in the Universe. This is why the determination of distances is discussed in detail in this chapter. By adding radial velocity to a combination of proper motion and distance, one obtains the space motion of a star. In addition, a section on magnitudes and spectra is given, not only because they provide important information to be used together with astrometric parameters, but also because they enter in the reduction of astrometric observations.