The parsec is defined as the length of the adjacent side of an imaginary right triangle in space. The two dimensions that this triangle is based on are the angle (which is defined as 1 arcsecond), and the opposite side (which is defined as 1 Astronomical Unit, which is the distance from the Earth to the sun). Using these two measurements, along with the rules of trigonometry, the length of the adjacent side (the parsec) can be found.
One of the oldest methods for astronomers to calculate the distance to a particular 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 from the Earth when it was on the other side of the sun. Thus, the distance between the two measurements was known to be twice the distance between the Earth and the sun. The distance to the star could be found using calculations of trigonometric parallax. Since it is based on an angle and the distance between the Earth and the sun, it is fundamentally derived from the degree and the AU. The length of a parsec is about 30.857 petametres, 3.26156 light-years or .
The first documented use of the term "parsec" was in an astronomical publication in 1913, and attributed to Herbert Hall Turner.
Though it had probably been used before, the term parsec was first mentioned in an astronomical publication in 1913, when Astronomer Royal Frank Watson Dyson expressed his concern for the need of a name for that unit of distance: he proposes the name astron, but mentions that Carl Charlier had suggested siriometer, and Herbert Hall Turner had suggested parsec (par-allax sec-ond).
Other than the Sun, which has a parallax of 90 degrees, there is no known star whose parallax is more than one arcsecond (that is, there is no known star whose distance from Earth is less than one parsec). The next closest star is Proxima Centauri with a parallax of 0.77233 arcseconds; it is thus 1.295 pc (4.225 ly) away from the Earth.
Refraction caused by the atmosphere, also known as astronomical seeing, limits ground-based telescopes to parallax angle measurement accuracies of less than approximately 0.01 arcsec, so reliable measurements, those with errors of 10% or less, can only be achieved at stellar distances of no more than about 100 pc, or 326 ly. Space-based telescopes are not limited by this effect and can accurately measure distances to objects beyond the limit of ground-based observations.
Between 1989 and 1993, the Hipparcos satellite, launched by the European Space Agency (ESA), measured parallaxes for about 100,000 stars with an astrometric precision of about 0.97 milliarcseconds, and obtained accurate measurements for stellar distances of stars up to 1,000 pc away. NASA's FAME satellite was due to be launched in 2004, to measure parallaxes for about 40 million stars with sufficient precision to measure stellar distances of up to 2,000 pc. However, the mission's funding was withdrawn by NASA in January 2002. ESA's Gaia satellite, due to be launched in December 2011, is intended to measure one billion stellar distances to within 20 microarcseconds, producing errors of 10% in measurements as far as the Galactic Center, about 8,000 pc away in the constellation of Sagittarius.
Distances measured in fractions of a parsec usually involve objects within a single star system. So, for example:
Galactic distances are sometimes given in units of Mpc/h (as in "50/h Mpc"). h is a parameter in the range [0.5,0.75] reflecting the uncertainty in the value of the Hubble constant for the rate of expansion of the universe (H = 100h km/s/Mpc). The Hubble constant becomes relevant when converting an observed redshift z into a distance using the formula d ≈ (c / H) × z (where c is the speed of light).
One gigaparsec (Gpc) is one billion parsecs — one of the largest distance measures commonly used. One gigaparsec is about 3.262 billion light-years, or roughly one fourteenth of the distance to the horizon of the observable universe (dictated by the cosmic background radiation). Astronomers typically use gigaparsecs to measure large-scale structures such as the size of, and distance to, the Great Wall; the distances between clusters of galaxies; and the distance to quasars.
In order to determine the number of stars in the Milky Way Galaxy volumes in cubic kiloparsecs (kpc3) are selected in various directions. All the stars in these volumes are counted and the total number of stars is statistically determined. The number of globular clusters, dust clouds and interstellar gas is determined in a similar fashion.
In order to determine the number of galaxies in superclusters volumes in cubic megaparsecs (Mpc3) are selected. All the galaxies in these volumes are classified and tallied. The total number of galaxies can then be determined statistically. The huge void in Bootes is measured in cubic megaparsecs.
In Cosmology volumes of cubic gigaparsecs (Gpc3) are selected to determine the distribution of matter in the visible universe and to determine the number of galaxies and quasars.
The Sun is alone in its cubic parsec (pc3) but in globular clusters the stellar density per cubic parsec could be from 100 to 1,000.
In the diagram above (not to scale), S represents the Sun, and E the Earth at one point in its orbit. D is a point in space at a distance of one parsec from the Sun. By definition, the angle SDE is one arcsecond (1/3600 of a degree), and the distance ES is one astronomical unit (AU). By trigonometry, the distance SD is
One AU = 149,597,870,700 m, so 1 parsec ≈ 3.085 678×1016 metres ≈ 3.261 564 light-years.