The word parsec is a portmanteau of "parallax of one second" and was coined by the British astronomer Herbert Hall Turner in 1913 to make calculations of astronomical distances from only raw observational data easy for astronomers. It is defined as the distance at which an arc of the length of 1 astronomical unit subtends an angle of 1 second of arc. When this difference is n-arcseconds (1/3600 of a degree), the distance between the sun and the star is 1/n parsec. Parsec (parallactic second): It is the largest practical unit of distance used in astronomy. 1 astronomical unit 1 AU 1.496 × 10 11 m. The parsec (which equals 3.26 light-years) is defined as the distance at which a star will show an annual parallax of one arcsecond. Most stars visible to the naked eye are within a few hundred parsecs of the Sun, with the most distant at a few thousand. It is used in astronomy to measure the distance of planets. The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun. This corresponds to 648000/π astronomical units, i.e. A parsec is a unit of distance that is often used by astronomers as an alternative to the light-year, just as kilometers can be used as an alternative to miles. Alight-year (ly) is the distance that A light-year is the distance light (speed 2.998 times 108 m/s) travels in one year. Parsec is obtained by the use of parallax and trigonometry, and is defined as the distance at which 1 au subtends an angle of one arcsecond. A parsec (pc) is the distance at which a length of 1 AU would subtend an angle of exactly 1 second of arc. This measure of length is used to describe and measure the large distances between two astronomical objects. The parsec is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,000 astronomical units, i.e. A Parsec is a mathematical unit of length.
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