The siriometer is an obsolete astronomical

unit of length A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary uni ...

, defined to be equal to one million

astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbi ...

s (au). One siriometer is approximately . The distance from Earth to the star

Sirius Sirius is the brightest star in the night sky. Its name is derived from the Greek word , or , meaning 'glowing' or 'scorching'. The star is designated α Canis Majoris, Latinized to Alpha Canis Majoris, and abbreviated Alpha CMa ...

is then approximately 0.54 siriometers. The unit was proposed in 1911 by

Carl V. L. Charlier Carl Vilhelm Ludwig Charlier (1 April 1862 – 4 November 1934) was a Swedish astronomer. His parents were Emmerich Emanuel and Aurora Kristina (née Hollstein) Charlier. Career Charlier was born in Östersund. He received his Ph.D. fro ...

who used the symbol 'sir.' Others have suggested denoting the unit with the symbol 'Sm'. The siriometer never gained widespread usage. The first General Assembly of the International Astronomical Union in 1922 adopted the

parsec 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 or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, a ...

as the standard unit of stellar distances, which simplified the definition of

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it we ...

. Use of the siriometer seems to have disappeared from the astronomical literature by . Professional astronomers use the parsec as their primary unit of distances larger than the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

References

Units of length {{astronomy-stub

Further reading

References