Ancient units of measurement

The Greeks had a coherent system for short distances which was inherited from the Egyptians and transmitted to the Romans:

• one Greek finger, daktylos: 19.25 mm, a Roman finger is 18.50 mm and an Egyptian finger  is 18.75 mm
• one Greek palm, palaiste: 77 mm, a Roman palm is 74 mm and an Egyptian palm is 75 mm (4 fingers)
• one Greek foot, pous: 0.308 m, a Roman foot is 0.296 m and an Egyptian foot is 0.300 m (16 fingers)
• one Greek cubit, pechos: 0.462 m, a Roman cubit is 0.444 m (24 fingers, 1.5 feet), but an Egyptian royal cubit is 0.525 m (28 fingers, 7/4 of a foot)
• one Greek step, bema: 0.77 m and a Roman step is 0.74 m (2.5 feet)
• one Greek pace: 1.54 m and a Roman pace, passus is 1.48 m (5 feet)
• one Greek fathom, orguia, orgye: 1.85 m (6 feet, 4 cubits)
• one Greek pleather, plethron: 30.8 m (100 feet, 40 steps), but also an area of 100 x 100 pleather: ca. 950 m²

The most commonly used unit for sailing distances was the stadium, but before the Romans put some order into it, there was much confusion on the length of an ancient stadium:

• one Athens stadium is 240 Greek steps, 600 Greek feet; this unit was used for the Roman stadium: 185 m, note this value is also equal to 1/10 nautical mile (or 1/10 of a minute of latitude[1]); it is still in use today as a ‘cable’. This unit was used by Pliny and by Strabo.
• one Delphi stadium: 177.7 m (used by Strabo and by Polybius)
• one Olympia stadium: 192.3 m (also used by Strabo)
• one Egyptian stadium: 157.5 m (used by Eratosthenes and by Arrian)

A Roman mile is 1480 m (8 Roman stadia, or 1000 Roman paces, or 2000 Roman steps, or 5000 Roman feet).

In times without any charts, the measure of distances at sea was focussed on the time needed to sail a given distance and sailors used the ‘day of navigation’ of a sailing cargo ship. They nevertheless had a few averaged benchmarks:

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Wind directions are defined according to their origin: to sail eastward, you are pushed by a western wind (“westerlies”), typically like the Zephyr which leads to Alexandria (where they call it the “etesian winds” or “summer winds”) if you leave from … Zephyrion Acra (modern Capo Bruzzano, in Calabria). A stable wind direction could therefore be used as a guide, but a rose with more than 8 directions has little practical value for an ancient sailor.

“If a man does not know to which port he is steering,
no wind is favorable to him”
(Seneca, Epistolae, 71, 3… but one might not agree…).

Other important units are:

• one Roman talent : mass of ca. 33 kg (60 Roman minae, or 100 Roman libra) but a Greek talent is only 26 kg. It is also a currency: a Greek talent of silver is 6000 Greek drachmas, or 36 000 Greek obols, or 24 000 Roman sesterces; but an Egyptian talent is only 6000 Roman sesterces.
• One sesterce: 10 €, based on the fairly low annual salary of a 1^st century soldier or worker of 1000 sesterces/year[5], compared to the French lowest revenue (RSA) of 6 720 €/year and to the netto minimum legal wage (SMIC) of 14 500 €/year in 2019 for a single man.
  Similarly, one ‘denarius communis’ from Diocletian’s Price Edict is worth around 1.5 €.
• amphora quadrantal: as a volume for liquids, one amphora is one Roman cubic foot (nearly one modern cubic foot) = 2 modii castrensis = 3 Italic modii = 8 congii = 48 sextarii, or around 26 litres. A full Dressel 1B amphora of the 1^st c. BC weights around 50 kg (olive oil, wine, fish brine), out of which around half is tare. However, an empty 1^st c. AD Gauloise 4 amphora from Narbonensis weights only ca. 10 kg and holds 30 to 38 litres because its wall is thinner. One Greek metretes was around 39 litres (1.5 amphorae).

• For dry bulk like grain, in Egypt, the Greeks used a larger unit of 52 liters (2 amphorae) called Ptolemaic medimnos weighting 40 kg when filled with wheat (dry wheat weights 780 kg/m³). It may be noted that Egyptian wheat was transported in sacks of one Ptolemaic artaba (ca. 39 litres) with a unit weight of ca. 30 kg. The Romans also commonly used the modius (1/3 of an amphora, or ca. 8.6 litres, or ca. 6.7 kg of wheat), hence one artaba is 4.5 modii, and 150 modii of wheat is one ton[6].
• Note also that wooden oak barrels (500 to 1000 litres) took over from amphorae (and dolia) for storage of wine during the Roman Empire.