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The parsec (abbreviated pc) is a unit of length used in astronomy. It stands for "parallax of one arc second". It is based on the method of trigonometric parallax, the most ancient and standard method of determining stellar distances. The angle subtended at a star by the mean radius of the Earth's orbit, around the Sun, is called the parallax. The parsec is defined to be the distance from the Earth of a star that has a parallax of 1 arcsecond. Alternatively, the parsec is the distance at which two objects, separated by 1 astronomical unitThe astronomical unit AU is a unit of distance, approximately equal to the mean distance between Earth and Sun. The currently accepted value of the AU is 149,597,870,691+-30 metres (about 150 million kilometres or 93 million miles). Earth's orbit is not a, appear to be separated by an angle of 1 arcsecond. It is therefore approximately 360×60×60/2 πOr pi with a lower-case p . minuscule, or lower-case, pi The mathematical constant (written as pi when the Greek letter is not available) is ubiquitous in mathematics and physics. In Euclidean plane geometry, π may be defined as either the ratio of a c AUThe astronomical unit AU is a unit of distance, approximately equal to the mean distance between Earth and Sun. The currently accepted value of the AU is 149,597,870,691+-30 metres (about 150 million kilometres or 93 million miles). Earth's orbit is not a = 206,265 AUThe astronomical unit AU is a unit of distance, approximately equal to the mean distance between Earth and Sun. The currently accepted value of the AU is 149,597,870,691+-30 metres (about 150 million kilometres or 93 million miles). Earth's orbit is not a = 3.08568×10¹⁶ mFor other uses of "metre" and "meter", see Metre (disambiguation). The metre is the basic unit of length in the International System of Units (SI: Systeme International d'Unites). It is defined as the length of the path travelled by light in absolute vacu = 30.8568 Pm ( petametreA petametre ( American spelling: petameter (symbol: Pm is a unit of length equal to 1015 metres. Petametres could be used for measuring interstellar distances, but astronomers have been long accustomed to using light-years (about 9. 5 petametres) and pars) = 3.2616 light-years. See 1 E16 mTo help compare different distances this page lists lengths starting at 1016 m (67,000 AU, 1. 1 light years). See also lengths of other orders of magnitude. Distances shorter than 1016 m 100,000 AU 1. 6 light years Outer radius of Oort cloud 210,000 AU 3. for a list of comparable lengths and scientific notation for an explanation of the notation.

Astronomers usually express distances to astronomical objects in units of parsecs, instead of light-years. This is both for historical reasons, and because it avoids including conversion factors such as the length of an astronomical unit which can complicate the calculation. The first direct measurements of an object at interstellar distances (of the star 61 Cygni, by Friedrich Wilhelm Bessel in 1838) were done using trigonometry using the width of the Earth's orbit as a baseline. The parsec follows naturally from this method, since the distance (in parsecs) is simply the reciprocal of the parallax angle (in arcseconds).

Though it had probably been used before, the term parsec was first mentioned in an astronomical publication in 1913, when Frank Watson Dyson expressed his concern for the need of a name for that unit of distance. He himself proposed the name astron, while Carl Charlier had suggested siriometer. However, Herbert Hall Turner's suggestion, parsec, was eventually adopted.

There is no star whose parallax is 1 arcsecond. The greater the parallax of the star the closer it is to the Earth, and the smaller its distance in parsecs. Therefore the closest star to the Earth will have the largest measured parallax. This belongs to the star Proxima Centauri, with a parallax of 0.772 arcseconds, and thus lying approximately 1.29 parsecs, or 4.22 light-years, away from us.

The measurement of distances of celestial bodies from the Earth in parsecs is a key aspect of astrometry, the science of making positional measurements of celestial bodies.

Because of the extremely small scale of parallactic shifts, ground-based parallax methods provide reliable measurements of stellar distances of no more than 325 light-years, or about 100 parsecs, corresponding to parallaxes of no less than 1/100 of 1 arcsecond, or 10 mas (1 mas or milliarcsecond = 1/1000 arcsecond).

Between 1989 and 1993 the Hipparcos satellite, launched by the European Space Agency (ESA) in 1989, measured parallaxes for about 100,000 stars, with a precision of about 0.97 mas, and obtained accurate measurements for stellar distances of around 1000 pc.

However, the mission's funding was withdrawn by NASA in January 2002.

The ESA's GAIA satellite, due to be launched in mid-2012, will be of sufficiently high astrometric precision to measure stellar distances to within 10% accuracy as far as the galactic centre about 8 kpc away in the Sagittarius constellation.