The International Astronomical Union, whose headquarters is at the Institute of Astrophysics in Paris, is a non-governmental organization created in 1919 that coordinates the work of astronomers. It is recognized worldwide to define the names of celestial bodies and has established since 1976 an astronomical system of units, including the length.
Given the large distances in space, a distinction exists between distances measured inside and outside the solar system. Within the solar system, distances between planets and between a planet and the Sun are measured in kilometers or in astronomical units (au or ua) which corresponds to the distance Earth-Sun. In 2012, the IAU precisely defined the astronomical unit at 149,597,870,700 meters, consistent with the theory of general relativity.
Mars, for example, is located at 1.52 AU from the Sun and Neptune at 30.11.Beyond the solar system, since distances are much greater, the astronomical unit becomes too small and the unit of measurement used to measure distances within galaxies or between galaxies or stars is based on the speed of light.
The light year (lh) is the distance traveled by light in a vacuum during one year. The Milky Way galaxy measures for example 75,000 al, the nearest galaxy to the Milky Way, the Andromeda galaxy, is 2.5 million al away from the Earth and the nearest star to the Sun, Proxima Centauri, is 4.22 al away.
When even larger distances between galaxies have to be measured, scientists use the parsec (pc). This unit of measurement was used as early as 1906, the term was proposed by a British astronomer, Herbert Hall Turner, by contracting the terms parallax and second and then the term was taken up by the IAU in 1919. Proxima Centauri is for example at about 1.316 parsecs from the Earth. Larger distances are measured in kiloparsec, megaparsec or gigaparsec.
Astronomical unit: 1 AU = about 150 million kilometers, 150x106Light-Year: 1 AL = about 9,460 billion kilometers, 9.46×1012, or 63,240 AUParsec: 1 pc = about 3.086×1013 km, or 3.261 AL, or 206,264 AUKiloparsec: 1 Kpc = 1,000 parsecsMegaparsec: 1 Mpc = 1 million parsecsGigaparsec: 1 Gpc = 1 billion parsecs