Around AD 830, Caliph
Al-Ma'mun commissioned a group of
Muslim astronomers and
Muslim geographers to perform an
arc measurement. The distance between two pillars whose latitudes differed by 1 degree in a north–south direction was measured using sighting pegs along a flat desert plane. One was in the
Sinjar plain and the other near
Kufa.
Firuzabadi in his dictionary says that a mile equals 3000 old dhira` (ie 3000 old cubits). Another determination by his astronomers was (or ) Arabian miles per degree, which corresponds to a circumference of 20,400 Arabian miles. Using the above-mentioned lengths based on the legal cubit or the surveying cubit, this is very close to the currently known values of per degree and circumference, respectively. ==See also==