thumb|thumb|right|stellar parallax motion from annual parallax  stellar parallax is parallax on an interstellar scale: the apparent shift of position of any nearby star (or other object) against the background of distant objects. created by the different orbital positions of the earth, the extremely small observed shift is largest at time intervals of about six months, when the earth arrives at exactly opposite sides of the sun in its orbit, giving a baseline distance of about two astronomical units between observations. the parallax itself is considered to be half of this maximum, about equivalent to the observational shift that would occur due to the different positions of the earth and the sun, a baseline of one au. stellar parallax is so difficult to detect that its existence was the subject of much debate in astronomy for hundreds of years. it was only first proven in 1838 when friedrich bessel made the first successful parallax measurement ever, for the star 61&nbsp;cygni, using a fraunhofer heliometer at kÃ¶nigsberg observatory..alan w. hirshfeld - parallax: the race to measure the cosmos (2002) - page 259, google books 2010 once a star's parallax is known, its distance from earth can be computed trigonometrically. but the more distant an object is, the smaller its parallax. even with 21st-century techniques in astrometry, the limits of accurate measurement make distances farther away than about 100 parsecs (roughly 326 light years) too approximate to be useful when obtained by this technique. relatively close on a galactic scale, the applicability of stellar parallax leaves most astronomical distance measurements to be calculated by spectral red-shift or other methods. stellar parallax measures are given in the tiny units of arcseconds, or even in thousandths of arcseconds (milliarcseconds). the distance unit parsec is defined as the length of the leg of a right triangle adjacent to the angle of one arcsecond at one vertex, where the other leg is one au long. since stellar parallaxes and distances all involve such skinny right triangles, a convenient trigonometric approximation can be used to 
Astrometry Parallax