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The parasang may have originally been some fraction of the distance an infantryman could march in some predefined period of time. Mid-5th-century BCE Herodotus (v.53) speaks of [an army] traveling the equivalent of five parasangs per day.
In antiquity, the term was used throughout much of the Middle East, and the Old Iranian language from which it derives can no longer be determined (only two—of what must have been dozens—of Old Iranian languages are attested). There is no consensus with respect to its etymology or literal meaning. In addition to its appearance in various forms in later Iranian languages (e.g. Middle Persian frasang or Sogdian fasukh), the term also appears in Greek as parasangēs (παρασάγγης), in Latin as parasanga, in Hebrew as parasa (פרסה), in Armenian as hrasakh (հրասախ), in Georgian as parsakhi, in Syriac as parsḥā (ܦܪܣܚܐ), in Turkish as fersah, and in Arabic as farsakh (فرسخ). The present-day New Persian word is also farsakh (فرسخ), and should not be confused with the present-day farsang (فرسنگ), which is a metric unit.[n 1]
The earliest surviving mention of the parasang comes from the mid-5th-century BCE Herodotus (Histories ii.6, v.53, vi.42), who defines the measure to be equivalent to 30 stadia, or half a schoenus. A length of 30 stadia is also given by several later Greek and Roman writers (10th-century Suidas and Hesychius, 5th/4th-century BCE Xenophon Anab. ii.2.6). The 6th-century Agathias (ii.21) however—while referring to Herodotus and Xenophon—notes that in his time the Pérsai considered the parasang to have only 21 stadia. Strabo (xi.xi.5) also notes that some writers considered it to be 60, others 40, and yet others 30. In his 1st-century Parthian stations, Isidore of Charax "evidently [used for schoenus] the same measure as the Arabic parasang (while in Persia proper 4 sch[onii] equal 3 par[asang])."
The 1st-century Pliny (Natural History vi.26) noted that the Iranians themselves assigned different lengths to it. The Bundahishn (GBd XXII), a 9th/10th-century text of Zoroastrian tradition, glosses Avestan language hathra as equivalent to a "parasang of 1000 paces" (a Roman mile), and then defines the parasang as the distance at which a man with good eyesight could determine whether a beast of burden was black or white. On the authority of older sources, the 14th-century Qazvinian historiographer Hamdullah Mostofi recorded that in the 10th century the north-eastern parasang was 15,000 paces, the north-western one was 18,000 paces, and the one of the south-west was merely 6,000 paces (but the "true" parasang, so Mostofi, was 9,000 paces). Recalling local legend, Mostofi states the unit was defined by the mythological Kai Kobad to be equal to 12,000 cubits.
Following the 30-stadia definition of Herodotus and Xenophon, the parasang would be equal to either 5.7 km (Olympic measure) or 5.3 km (Attic measure). But in 1920, Kenneth Mason of the Royal Geographical Society adduced that the parasang used in Xenophon's Babylonian travel accounts was equal to only 2.4 miles (3.9 km). A mid-1960s search for the Parthian city of Hekatompylos based on distances given in mid-4th-century BCE chronologies of Alexander's conquests generated empirical estimates of ten stades to the English mile (1.609 km), and three miles to the parasang (4.827 km). "Whatever the basis of calculation, theoretical values for the stade and the parasang must be sought which do not greatly exceed [those] estimates." A 1985 suggestion proposes that the parasang and Attic stade were defined in terms of the Babylonian beru, an astromically-derived sexagesimal unit of time and linear distance. At 1 beru = 60 stadia = 2 parasang, the parasang could then "be expressed as 10,800 'common' [i.e. trade] Babylonian cubits, or 18,000 Attic feet, both figures exactly." A 2010 study of the term parasang in Xenophon's account of Cyrus the Younger's late-5th-century BCE campaign against Artaxerxes II demonstrated that the length of Xenophon's parasang varied with weather and the terrain across which the army travelled. The parasangs were longer when the road was flat and dry, but shorter when travel was slower.
The term has survived in Modern Greek in the stereotypical expression "απέχει παρασάγγας", i.e., "it is parasangs away", meaning that something is very far away from something else, particularly in terms of quality. As Hebrew 'parsah' (pl. parsaoth), the parasang also finds use in the Babylonian Talmud, in several uses, for instance in a description of the biblical ladder to heaven, the width of which is given as 8,000 parsaoth (Chullin 91b). In the commentary of Pesachim 9, the 4th-century Rabbah bar bar Hana, on the authority of the 3rd-century Rabbi Johanan, gives ten parsaoth as the distance that a man can walk in a day. The farsang was also used as Ethiopian unit for length.