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Therefore, given a measurement of the parallax angle, along with the rules of trigonometry, the distance from the Sun to the star can be found. The length of the opposite side to the parallax angle is the distance from the Earth to the Sun (defined as one astronomical unit (au), and the length of the adjacent side gives the distance from the sun to the star. The star, the Sun and the Earth form the corners of an imaginary right triangle in space: the right angle is the corner at the Sun, and the corner at the star is the parallax angle. Equivalently, it is the subtended angle, from that star's perspective, of the semi-major axis of Earth's orbit. The parallax of a star is taken to be half of the angular distance that a star appears to move relative to the celestial sphere as Earth orbits the Sun. Stellar parallax motion from annual parallax The first successful published direct measurements of an object at interstellar distances were undertaken by German astronomer Friedrich Wilhelm Bessel in 1838, who used this approach to calculate the three and a half parsec distance of 61 Cygni. Then the distance to the star could be calculated using trigonometry. The difference in angle between the two measurements was known to be twice the parallax angle, which is formed by lines from the Sun and Earth to the star at the distant vertex. The distance between the two positions of the Earth when the two measurements were taken was known to be twice the distance between the Earth and the Sun. 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. 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. Applying the rules of trigonometry to these two values, the unit length of the other leg of the triangle (the parsec) can be derived. The two dimensions on which this triangle is based are its shorter leg, of length one astronomical unit (the average Earth-Sun distance), and the subtended angle of the vertex opposite that leg, measuring one arcsecond. The parsec is defined as being equal to the length of the longer leg of an extremely elongated imaginary right triangle in space. Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kiloparsecs for the more distant objects within and around the Milky Way, megaparsecs for the nearer of other galaxies, and gigaparsecs for many quasars and the most distant galaxies. Partly for this reason, it is still the unit preferred in astronomy and astrophysics, though the light year remains prominent in popular science texts and more everyday usage. Named from an abbreviation of the parallax of one arc second, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data. The parsec unit was likely first suggested in 1913 by British astronomer Herbert Hall Turner. Nevertheless, most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun. About 3.26 light-years (31 trillion kilometres or 19 trillion miles) in length, the parsec is shorter than the distance from our solar system to the nearest star, Proxima Centauri, which is 1.3 parsecs from the Sun. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. A parsec (symbol: pc) is a unit of length used to measure the astronomically large distances to objects outside the Solar System.