siriometer
Noun
A unit of distance equal to one million astronomical units or 15.813 light-years.
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The siriometer dates its pedigree from the founder of stellar astronomy, William Herschel. As is well known, the distance unit used by Herschel is the mean distance of the first magnitude stars, which distance was called by him the distance of Sirius.
He had used the star Sirius as his standard, so he decided to call the distance to Sirius, whatever it might turn out to be in miles or kilmetres, one 'siriometer'. Herschel calculated that the grindstone measured 1,000 siriometers across and was 100 siriometers thick.
Word forms
siriometers