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 3.5-parsec distance of 61 Cygni. Then the distance to the star could be calculated using trigonometry. The difference in angle between the two measurements is 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 is twice the distance between the Earth and the Sun. The first measurement is taken from the Earth on one side of the Sun, and the second is taken approximately half a year later, when the Earth is on the opposite side of the Sun. One of the oldest methods used by astronomers to calculate the distance to a star is 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 adjacent leg (opposite leg being 1 AU) of an extremely elongated imaginary right triangle in space. Please help improve it by merging similar text or removing repeated statements. This section may contain content that is repetitive or redundant of text elsewhere in the article. This corresponds to the small-angle definition of the parsec found in many astronomical references.

In August 2015, the International Astronomical Union (IAU) passed Resolution B2 which, as part of the definition of a standardized absolute and apparent bolometric magnitude scale, mentioned an existing explicit definition of the parsec as exactly 648 000 / π au, or approximately 30.856 775 814 913 673 ×10 15 metres (based on the IAU 2012 definition of the astronomical unit). 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 (kpc) for the more distant objects within and around the Milky Way, megaparsecs (Mpc) for mid-distance galaxies, and gigaparsecs (Gpc) for many quasars and the most distant galaxies.

Partly for this reason, it is the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and common usage. The word parsec is a portmanteau of 'parallax of one second' and was coined by the British astronomer Herbert Hall Turner in 1913 to make calculations of astronomical distances from only raw observational data easy for astronomers. Most stars visible to the naked eye are within a few hundred parsecs of the Sun, with the most distant at a few thousand. The nearest star, Proxima Centauri, is about 1.3 parsecs (4.2 light-years) from the Sun. The parsec unit is obtained by the use of parallax and trigonometry, and is defined as the distance at which 1 AU subtends an angle of one arcsecond ( 1 / 3600 of a degree). 30.9 trillion kilometres (19.2 trillion miles). The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to 3.26 light-years or 206,265 astronomical units (AU), i.e. A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (not to scale)

Astronomy parsec definition