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 farsang or Sogdian fasukh), the term also appears in Greek as Παρασάγγης parasanges, in Latin as parasanga, in Armenian as hrasakh, in Georgian as parsakhi, in Syriac as prsha, in Arabic as farsakh (فرسخ) and in Turkish fersah. The present-day New Persian word is also farsakh, and should not be confused with the present-day farsang (فرسنگ), which is now a metric unit of 6 km.[a] or 4 miles
The parasang may have originally been some fraction of the distance an infantryman could march in some predefined period of time. Herodotus (v.53) speaks of [an army] traveling the equivalent of five parasangs per day.
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. This comparison is also made 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—note that in his time the Pérsai considered the parasang to have only 21 stadia. Strabo (xi) 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. On the authority of older sources, the 14th-century Qazvinian historiographer Hamdullah Mostofi records 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 Greek version of 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). More recently, "[empirical tests] reckoning ten stades to the English mile (1.609 km), and three miles to the parasang (4.827 km) have given excellent results in practice. Whatever the basis of calculation, theoretical values for the stade and the parasang must be sought which do not greatly exceed [those] estimates."
See also 
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- Bivar, A. D. H. (1985), "Achaemenid Coins, Weights and Measures", in Gershevich, Ilya, The Cambridge history of Iran: The Median and Achamenian Periods 2, Cambridge: Cambridge University Press, pp. 610–639, ISBN 0-521-20091-1.
- Henning, Walter Bruno (1942), "Mani's Last Journey", Bulletin of the School of Oriental and African Studies 10 (4): 941–953, doi:10.1017/S0041977X00090133.
- Herodot (1859), in Murray, John, trans., The History of Herodotus: A New English Version, Oxford University Press.
- Mason, Kenneth (1920), "Notes on the Canal System and Ancient Sites of Babylonia in the Time of Xenophon", The Geographical Journal (The Geographical Journal, Vol. 56, No. 6) 56 (6): 468–481, doi:10.2307/1780469, JSTOR 1780469.
- Houtum-Schindler, Albert (1888), "On the Length of the Persian Farsakh", Proceedings of the Royal Geographical Society and Monthly Record of Geography, New Monthly Series 10 (9): 584–588.
- Smith, William, ed. (1870), "Parasanga", Dictionary of Greek and Roman antiquities, C. Little, and J. Brown, pp. 866–867.