Modern telescopes allow astronomers to use the annual parallax to measure the distance to nearby stars, using triangulation. The astronomer carefully measures the position of the star on two dates, spaced six months apart. The nearer the star is to the Sun, the larger the apparent shift in its position will be between the two dates.
Over the six-month period, the Earth has moved through half its orbit around the Sun; in this time its position has changed by 2 Astronomical Units (abbreviated AU; 1 AU is the distance from the Earth to the Sun, or about 150 million kilometers). This sounds like a really long distance, but even the nearest star to the Sun (alpha Centauri) is about 40 trillion kilometers away. Therefore, the annual parallax is very small, typically smaller than one arcsecond, which is only 1/3600 of one degree. A convenient distance unit for nearby stars is the parsec, which is short for "parallax arcsecond". One parsec is the distance a star would have if its observed parallax angle was one arcsecond. It is equal to 3.26 light-years, or 31 trillion kilometers. --
https://docs.kde.org/trunk5/en/kdeedu/kstars/ai-parallax.html
The nearest star, α Centauri, has a parallax angle of 0.76″. Therefore its distance is d = 1/0.76″ = 1.3 pc (4 ly). The ground‐based limit of parallax measurement accuracy is approximately 0.02 arc second, limiting determination of accurate distances to stars within 50 pc (160 ly). --
http://www.cliffsnotes.com/study-gu...rties-of-stars/stellar-parallax-and-distances
The nearest star exhibits a parallax of 0.762 arcsec, and therefore is 1.31 parsecs away. Some well-known examples of distance measurement by parallax are 61 Cygni at 1/3 of an arcsec, distance 3 parsecs, and Barnard's Star at 1.8 parsecs = 5.9 light years. Barnard's Star also exhibits a large proper motion. --
http://hyperphysics.phy-astr.gsu.edu/hbase/astro/para.html