ABSTRACT
The movements of the stars with respect to the observer reveal themselves in two different ways: as radial velocities Vr along the line of sight and as angular tangential motions on the celestial sphere. Radial velocities could be measured only toward the end of the 19th century, after Christian Doppler discovered the effect that bears his name and spectroscopy entered the astronomical field. The radial velocity is measured through the Doppler effect, namely, through the variation of the wavelength λ of the radiation, caused by the relative motion Vr along the line of sight. The methods of secular and statistical parallaxes have been applied to stars as distant as 500 pc, but with great uncertainties. The astrometric satellites Hipparcos and GAIA have extended the trigonometric parallaxes to a point where those two statistical indicators are mostly of historical significance, but worth mentioning to appreciate the developments in the field.
