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The official SI unit of astronomical distance; the name is a contraction of "parallax-second". The measure was introduced by UK astronomer Herbert Hall Towner (1861-1930). As the Earth travels from one side of the Sun to the other in half a year, parallax makes the position of any comparatively nearby star apparently shift. Using simple trigonometry, from the observed angular displacement of the star's measured position and knowledge of the distance between Earth and Sun the distance of the star can be calculated. One parsec is defined (essentially) as the distance at which a star would show a parallax displacement of one second of arc, a distance which proves to be approximately 3.258 light years.

The term "parsec" is a common item of sf Terminology, either correctly as a unit of distance or, depressingly often – especially in Pulp-magazine, juvenile and cinematic sf – mistakenly as a unit of velocity ("We're moving at 17 parsecs!" the hero of Space: 1999 might cry) or of Time: Harrison Ford boasts in Star Wars: Episode IV – A New Hope (1977) that his spaceship "made the Kessel run in less than four parsecs." [PN]

see also: Scientific Errors.

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