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The parsec (parallax of one arcsecond; symbol: pc) is a unit of length, equal to just under 31 trillion (31×10^{12}) kilometres (about 19 trillion miles), 206265 AU, or about 3.26 lightyears. The parsec measurement unit is used in astronomy. It is defined as the length of the adjacent side of an imaginary right triangle in space. The two dimensions that specify this triangle are the parallax angle (defined as 1 arcsecond) and the opposite side (defined as 1 astronomical unit (AU), the distance from the Earth to the Sun). Given these two measurements, along with the rules of trigonometry, the length of the adjacent side (the parsec) can be found.
One of the oldest methods for astronomers to calculate the distance to a star was to record the difference in angle between two measurements of the position of the star in the sky. The first measurement was taken from the Earth on one side of the Sun, and the second was taken half a year later when the Earth was on the opposite side of the Sun. Thus, the distance between the two measurements was known to be twice the distance between the Earth and the Sun. The distance to the star could be calculated using trigonometry. Since the parsec is based on an angle and the distance between the Earth and the Sun, it is fundamentally derived from the degree and the astronomical unit. The length of a parsec is about 30.857 petametres, 3.26156 lightyears or 1.9174×1013 mi. The first documented use of the term parsec was in 1913,^{[1]} and is attributed to Herbert Hall Turner.^{[2]}
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