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 all but the closest 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 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. Most of the stars visible to the unaided eye in the nighttime sky are within 500 parsecs of the Sun. The nearest star, Proxima Centauri, is about 1.3 parsecs from the Sun. A parsec is equal to about 3.26 light-years (31 trillion kilometres or 19 trillion miles) in length. One parsec is the distance at which one astronomical unit subtends an angle of one arcsecond. A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (the diagram is not to scale).Ī parsec (symbol: pc) is a unit of length used to measure large distances to objects outside the Solar System.
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