secular parallax

The angular displacement over time of a star's position caused by the Sun's motion through space relative to the local standard of rest. Measurement of the secular parallax provides a method of determining the distance to nearby star groups, on the assumption that the individual motions of the stars in such a group are random with an average of zero. The Sun's relative velocity, also needed in the calculation, is 19.5 km/s, which amounts to 4.11 AU per year.