<P> 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 . This limits the applicability of parallax as a measurement of distance to objects that are relatively close on a galactic scale . Other techniques, such as spectral red - shift, are required to measure the distance of more remote objects . </P> <P> 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 1 AU long . Because stellar parallaxes and distances all involve such skinny right triangles, a convenient trigonometric approximation can be used to convert parallaxes (in arcseconds) to distance (in parsecs). The distance is simply the reciprocal of the parallax: d (p c) = 1 / p (a r c s e c). (\ displaystyle d (\ mathrm (pc)) = 1 / p (\ mathrm (arcsec)).) For example, Proxima Centauri (the nearest star to Earth other than the Sun), whose parallax is 0.7687, is 1 / 0.7687 = 1.3009 parsecs (4.243 ly) distant . </P> <P> Stellar parallax is so small (as to be unobservable until the 19th century) that its apparent absence was used as a scientific argument against heliocentrism during the early modern age . It is clear from Euclid's geometry that the effect would be undetectable if the stars were far enough away, but for various reasons such gigantic distances involved seemed entirely implausible: it was one of Tycho Brahe's principal objections to Copernican heliocentrism that in order for it to be compatible with the lack of observable stellar parallax, there would have to be an enormous and unlikely void between the orbit of Saturn and the eighth sphere (the fixed stars). </P> <P> James Bradley first tried to measure stellar parallaxes in 1729 . The stellar movement proved too insignificant for his telescope, but he instead discovered the aberration of light and the nutation of Earth's axis, and catalogued 3222 stars . </P>

For much of history the inability to detect stellar parallax was interpreted to mean that
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