The siriometer is an obsolete astronomical unit of length, defined to be equal to one million astronomical units (au).[ One siriometer is approximately . The distance from Earth to the star Sirius is then approximately 0.54 siriometers.][
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The unit was proposed in 1911 by Carl V. L. Charlier,[ who worked on stellar statistics.][ Charlier originally used the symbol 'sir']
The siriometer never gained widespread usage. Frank Dyson (the Astronomer Royal) objected to the name siriometer, because "it suggests a machine for measuring".[ The first General Assembly of the International Astronomical Union in 1922 adopted the parsec as the standard unit of stellar distances,][ which simplified the definition of absolute magnitude.][ Use of the siriometer seems to have disappeared from the astronomical literature by .][ Modern professional astronomers use the parsec as their primary unit for distances larger than the Solar System.
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