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 deduced 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