secular parallaxThe apparent and continuously increasing displacement of stars as a result of the Sun's motion through space. This form of parallax can be used to determine the distance of nearby groups of stars. It is assumed that the stars are at approximately the same distance from the Sun and that their random individual proper motions tend to cancel out leaving only the parallactic motion. The baseline for this parallax measurement is the distance moved by the Sun relative to the local standard of rest and is hence increasing by the Sun's velocity of 19.5 km s–1, i.e. 4.11 AU per year (see apex). The secular parallax per annum, H (in arcsecs), is then given by:
sin H = 4.11 sin α/dwhere d is the mean distance to the group and α is the angle of the group to the solar apex (see illustration).
secular parallax[′sek·yə·lər ′par·ə‚laks]
An apparent angular displacement of a star, resulting from the sun's motion.