
Fig7_11 A star moves
Fig. 7.11 . The space velocity, V, of a star relative to an observer can be resolved into two mutually perpendicular components, the radial velocity, Vr, directed along the line of sight, and the tangential velocity, V^, which is perpendicular or transverse to the line of sight. From the Pythagorean theorem V2 = Vr2 + V^2. Over a given interval of time, shown here as one year, the star will move through a proper motion angle m, which depends on V^ and the star’s distance, D, from the observer. In this figure, the proper motion m = V^/D is greatly exaggerated, by more than 10,000 for even the closest star. At a distance of only 6 lightyears, Barnard’s star has the largest known proper motion of 10.3 seconds of arc per year.
Copyright 2010, Professor Kenneth R. Lang, Tufts University

