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A parsec (symbol: pc) is a unit of length used to measure large distances to objects outside the Solar System. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond.〔(【引用サイトリンク】url=https://heasarc.gsfc.nasa.gov/docs/cosmic/milkyway_info.html )〕 A parsec is equal to about 3.26 light-years (31 trillion kilometres or 19 trillion miles) in length. The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun. Most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun. The parsec unit was likely first suggested in 1913 by British astronomer Herbert Hall Turner.〔Dyson, F. W., ''Stars, Distribution and drift of, The distribution in space of the stars in Carrington's Circumpolar Catalogue''. In: Monthly Notices of the Royal Astronomical Society, Vol. 73, p. 334–342. March 1913. () "There is a need for a name for this unit of distance. Mr. Charlier has suggested Siriometer ... Professor Turner suggests PARSEC, which may be taken as an abbreviated form of 'a distance corresponding to a parallax of one second.'"〕 Named from an abbreviation of the ''par''allax of one arc''sec''ond, it was defined so as to make calculations of astronomical distances quick and easy for astronomers from only their raw observational data. 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 everyday usage. 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 kilo In August 2015, the IAU passed Resolution B2, which as part of the definition of a standardized absolute and apparent bolometric magnitude scale, included an explicit definition of the parsec as exactly astronomical units, or approximately metres (based on the IAU 2012 exact SI definition of the astronomical unit). This corresponds to the small-angle definition of the parsec found in many contemporary astronomical references.〔(Cox, Arthur N. 2000, Allen's Astrophysical Quantities, 4th ed. Publisher: New York: AIP Press; Springer, 2000. Edited by Arthur N. Cox. ISBN 0387987460 )〕〔(Binney, James, and Tremaine, Scott 2008, Galactic Dynamics: Second Edition. ISBN 978-0-691-13026-2 (HB). Published by Princeton University Press, Princeton, NJ USA, 2008. )〕 == History and derivation == The parsec is defined as being equal to the length of the longer leg of an extremely elongated imaginary right triangle in space. 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. Applying the rules of trigonometry to these two values, the unit length of the other leg of the triangle (the parsec) can be derived. 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. 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 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. Then the distance to the star could be calculated using trigonometry. 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.〔Bessel, FW, "(Bestimmung der Entfernung des 61sten Sterns des Schwans )" (1838) ''Astronomische Nachrichten'', vol. 16, pp. 65–96.〕 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. Equivalently, it is the subtended angle, from that star's perspective, of the semi-major axis of Earth's orbit. 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. 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. 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. A parsec is defined as the length of the side adjacent to the vertex occupied by a star whose parallax angle is one arcsecond. The use of the parsec as a unit of distance follows naturally from Bessel's method, because the distance in parsecs can be computed simply as the reciprocal of the parallax angle in arcseconds (''i. e.'', if the parallax angle is 1 arcsecond, the object is 1 pc from the Sun; If the parallax angle is 0.5 arcsecond, the object is 2 pc away; ''etc.''). No trigonometric functions are required in this relationship because the very small angles involved mean that the approximate solution of the skinny triangle can be applied. Though it may have been used before, the term ''parsec'' was first mentioned in an astronomical publication in 1913. Astronomer Royal Frank Watson Dyson expressed his concern for the need of a name for that unit of distance. He proposed the name ''astron'', but mentioned that Carl Charlier had suggested ''siriometer'' and Herbert Hall Turner had proposed ''parsec''.〔Dyson, F. W., "(The distribution in space of the stars in Carrington's Circumpolar Catalogue )" (1913) ''Monthly Notices of the Royal Astronomical Society'', vol. 73, pp. 334–42, (p. 342 fn. ).〕 It was Turner's proposal that stuck. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「parsec」の詳細全文を読む スポンサード リンク
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