Libra (weight)

The units of measurement of ancient Rome were generally consistent and well documented.

Length

The basic unit of Roman linear measurement was the ''pes'' (plural: ''pedes'') or Roman foot. Investigation of its relation to the English foot goes back at least to 1647, when John Greaves published his ''Discourse on the Romane foot''. Greaves visited Rome in 1639, and measured, among other things, the foot measure on the tomb of Titus Statilius Aper, that on the statue of Cossutius formerly in the gardens of Angelo Colocci, the congius of Vespasian previously measured by Villalpandus, a number of brass measuring-rods found in the ruins of Rome, the paving-stones of the Pantheon and many other ancient Roman buildings, and the distance between the milestones on the Appian Way. He concluded that the Cossutian foot was the "true" Roman foot, and reported these values compared to the iron standard of the English foot in the Guildhall in London

William Smith (1851) gives a value of 0.9708 English feet, or about 295.9 mm. An accepted modern value is 296 mm. That foot is also called the ''pes monetalis'' to distinguish it from the ''pes Drusianus'' (about 333 or 335 mm) sometimes used in some provinces, particularly Germania Inferior.

The Roman foot was sub-divided either like the Greek '' pous'' into 16 ''digiti'' or fingers; or into 12 ''unciae'' or inches. Frontinus writes in the 1st century AD that the ''digitus'' was used in Campania and most parts of Italy. The principal Roman units of length were:

Other units include the schoenus (from the Greek for " rush rope") used for the distances in Isidore of Charax's '' Parthian Stations'' (where it had a value around ) and in the name of the Nubian land of Triacontaschoenus between the First and Second Cataracts on the Nile (where it had a value closer to ).

Area

The ordinary units of measurement of area were:

Other units of area described by Columella in his ''De Re Rustica'' include the ''porca'' of 180 × 30 Roman feet (about ) used in Hispania Baetica and the Gallic ''candetum'' or ''cadetum'' of 100 feet in the city or 150 in the country. Columella also gives uncial divisions of the ''jugerum'', tabulated by the anonymous translator of the 1745 Millar edition as follows:

Volume

Both liquid and dry volume measurements were based on the . The sextarius was defined as of a cubic (Roman foot), known as an . Using the value for the Roman foot, an amphora quadrantal can be computed at approximately , so a sextarius (by the same method) would theoretically measure , which is about 95% of an imperial pint ().

Archaeologically, however, the evidence is not as precise. No two surviving vessels measure an identical volume, and scholarly opinion on the actual volume ranges between .

The core volume units are:
* '' amphora quadrantal'' (Roman jar) – one cubic ''pes'' (Roman foot)
* ''congius'' – a half-''pes'' cube (thus ''amphora quadrantal'')
* ''sextarius'' – literally of a ''congius''

Liquid measure

Dry measure

Weight

The units of weight or mass were mostly based on factors of 12. Several of the unit names were also the names of coins during the Roman Republic and had the same fractional value of a larger base unit: libra for weight and as for coin. Modern estimates of the libra range from with 5076 grains or an accepted figure. The ''as'' was reduced from 12 ounces to 2 after the First Punic War, to 1 during the Second Punic War, and to half an ounce by the 131 BC Lex Papiria...

The divisions of the libra were:

The subdivisions of the uncia were:

Time

Years

The complicated Roman calendar was replaced by the Julian calendar in 45 BC. In the Julian calendar, an ordinary year is 365 days long, and a leap year is 366 days long. Between 45 BC and AD 1, leap years occurred at irregular intervals. Starting in AD 4, leap years occurred regularly every four years. Year numbers were rarely used; rather, the year was specified by naming the Roman consuls for that year. (As consuls' terms latterly ran from January to December, this eventually caused January, rather than March, to be considered the start of the year.) When a year number was required, the Greek Olympiads were used, or the count of years since the founding of Rome, "ab urbe condita" in 753 BC. In the Middle Ages, the year numbering was changed to the Anno Domini count, based on the supposed birth year of Jesus.

The calendar used in most of the modern world, the Gregorian calendar, differs from the Julian calendar in that it skips three leap years every four centuries (i.e. 97 leap years in every 400) to more closely approximate the length of the tropical year.

Weeks

The Romans grouped days into an eight-day cycle called the , with every eighth day being a market day.

Independent of the , astrologers kept a seven-day cycle called a '' hebdomas'' where each day corresponded to one of the seven classical planets, with the first day of the week being Saturn-day, followed by Sun-day, Moon-day, Mars-day, Mercury-day, Jupiter-day, and lastly Venus-day. Each astrological day was reckoned to begin at sunrise. The Jews also used a seven-day week, which began Saturday evening. The seventh day of the week they called Sabbath; the other days they numbered rather than named, except for Friday, which could be called either the Parasceve or the sixth day. Each Jewish day begins at sunset. Christians followed the Jewish seven-day week, except that they commonly called the first day of the week the , or the Lord's day. In 321, Constantine the Great gave his subjects every Sunday off, thus cementing the seven-day week into Roman civil society.

Hours

The Romans divided the daytime into twelve ''horae'' or hours starting at sunrise and ending at sunset. The night was divided into four watches. The duration of these hours varied with seasons; in the winter, when the daylight period was shorter, its 12 hours were correspondingly shorter and its four watches were correspondingly longer.

Astrologers divided the solar day into 24 equal hours, and these astrological hours became the basis for medieval clocks and our modern 24-hour mean solar day.

Although the division of hours into minutes and seconds did not occur until the Middle Ages, Classical astrologers had a ''minuta'' equal to of a day (24 modern minutes), a ''secunda'' equal to of a day (24 modern seconds), and a ''tertia'' equal to of a day (0.4 modern seconds).

Unicode

A number of special symbols for Roman currency were added to the Unicode Standard version 5.1 (April 2008) as the Ancient Symbols block (U+10190–U+101CF, in the Supplementary Multilingual Plane ).

As mentioned above, the names for divisions of an coin (originally one libra of bronze) were also used for divisions of a libra, and the symbols U+10190–U+10195 are likewise also symbols for weights:
* U+10190 (𐆐): Sextans
* U+10191 (𐆑): Uncia
* U+10192 (𐆒): Semuncia
* U+10193 (𐆓): Sextula
* U+10194 (𐆔): Semisextula
* U+10195 (𐆕): Siliqua

See also

* Ancient Egyptian units
* Ancient Greek units
* Biblical and Talmudic units of measurement
* Byzantine units
* History of measurement

Notes

References

External links

Proposal to Add Ancient Roman Weights and Monetary Signs to UCS (Universal Character Set)

{{Systems of measurement