Astrometry is a branch of science which develops methods for the quantitative descriptions of places and time instants of astronomical events on the basis of observations of celestial bodies. For this purpose a theoretical coordinate system is introduced (e.g. equatorial a, α, δ). The aim of astrometry is to apply this system to the observed reference objects (stars, planets etc.) so that their coordinates α(t), δ(t) can be calculated according to the relations α(t) =f1(Pk, t-to) and δ(t) =f2(Pk, t-to) where Pk are parameters, to is the conventional time instant and t is the current time. In order to understand the term inertial coordinate system assume that the coordinates α(t), δ(ti), i=1,2,…n are used for plotting the coordinate origins. If these coincide then the system is conventional-fixed and therefore inertial. Thus, the inertial coordinate system in astrometry is a conventional-fixed reference frame reproduced with the use of celestial bodies whose law of motion is known with sufficient accuracy.