- For the multi-touch interface see CUBIT (multi-touch). For the unit of information, see qubit. For the bone, see ulna.
A cubit is any one of many units of measure used by various ancient peoples and is among the first recorded units of length.
The cubit is based on measuring by comparing – especially cords and textiles, but also for timbers and stones – to one's forearm length. The Egyptian hieroglyph for the unit shows this symbol. It was employed consistently through Antiquity, the Middle-Ages up to the Early Modern Times.
The distance between thumb and another finger to the elbow on an average person measures about 24 digits or 6 palms or 1½ feet. This is about 45 cm or 18 inches. This so-called "natural cubit" of 1½ feet is used in the Roman system of measures and in different Greek systems.
Over time, units similar in type to the cubit have measured:
- 6 palms = 24 digits, i.e. ~45.0 cm or 18 inches (1.50 ft)
- 7 palms = 28 digits, i.e. ~52.5 cm or 21 inches (1.75 ft)
- 8 palms = 32 digits, i.e. ~60.0 cm or 24 inches (2.00 ft)
- 9 palms = 36 digits, i.e. ~67.5 cm or 27 inches (2.25 ft)
From late Antiquity, the Roman ulna, a four-foot cubit (about 120 cm) is also attested. This length is the measure from a man's hip to the fingers of the outstretched opposite arm.
The English yard could be considered to be a type of cubit, measuring 12 palms, ~90 cm, or 36 inches (3.00 ft). This is the measure from the middle of a man's body to his fingers, always with outstretched arm. The English ell is essentially a kind of great cubit of 15 palms, 114 cm, or 45 inches (3.75 ft).
History of the different cubits
The Egyptian Royal Cubit and Sumerian Nippur cubit
The cubit is among the first recorded units of length used by an ancient people.
The earliest attested standard measure is from Egypt and was called the Royal Cubit (Mahe) and was 523.5 to 524 mm (20.61 to 20.63 inches) in length, and was subdivided into 7 palms of 4 digits, giving a 28 part measure in total. Secure evidence for this unit is known from architecture, from at least as early as the construction of the Step Pyramid of Djoser from around 2,700 B.C.
In 1916, during the last years of Ottoman Empire and in the middle of WWI the German assyriologist Eckhard Unger found a copper-alloy bar during excavation at Nippur from c. 2650 BC. which he claimed was a measurement standard. This irregularly formed and irregularly marked graduated rule supposedly defined the Sumerian cubit as about 518.5 mm or 20.4 inches, although this does not agree with more secure evidence from the statues of Guduea from the same region. A 30-digit-cubit known as a kus was nevertheless known from the 2nd millennium B.C., with a digit-length of about 17.28 mm (more than 0.68 inch).
Old Egyptian geometers could calculate the square root of two from the value of the hypotenuse of a Cubit. This well-attested old Egyptian unit was known as the "construction remen" and used a good approximation: 2×20/28 ≥ root 2.
Other important cubits
- The Roman cubitus is a six-palm cubit of about 444.5 mm. Twenty-four Roman cubits ≈ thirty-five English feet, so the Roman cubit is about 17.5 inches or 444.5 mm.
- The Greek pechus (πῆχυς) was also a 24-digit cubit. So, the Greek kyrenaika cubit measured about 463.1 mm and the Greek metrios cubit about 474.2 mm; respectively roughly 25/24 and 16/15 Roman cubits. Other Greek cubits based on different digit measures of other city-states are less important. The Greek 40-digit-measure, called bema, corresponds to the Latin gradus, the step or half-a-pace.
- The Arabic Hashimi cubit of about 650.2 mm (25.6 inches) is considered to measure two French feet. Since the established ratio between the French and English foot is about 16 to 15, one can give following equation: 5 Hashimi cubits ≈ 10 French feet ≈ 128 English inches. Also the length of 256 Roman cubits and the length of 175 Hashimi cubits are nearly equivalent.
- The guard cubit (Arabic: ammatu rabitu) measured about 555.6 mm; 5/4 of the Roman cubit. Therefore: 96 guard cubits ≈ 120 Roman cubits ≈ 175 English feet.
- The Arabic nil cubit (or black cubit) measured about 540.2 mm. This means 28 (later called) Greek digits of the "pous of kyrenaika" ≈ 25/24 of a Roman foot or just 308.7 mm. Thus 175 Roman cubits ≈ 144 black cubits.
- The Mesopotamian cubit measured about 533.4 mm, 6/5 Roman cubit. Thus, 20 Mesopotamian cubits ≈ 24 Roman cubits ≈ 35 English feet.
- The Babylonian cubit (or cubit of Lagash) measured about 496.1 mm. Also a Babylonian trade cubit existed, nine-tenths of the normal cubit, i.e. 446.5 mm. The Babylonian Cubit is fifteen-sixteenths of the royal cubit. 160 Babylonian trade cubits ≈ 144 Babylonian cubits ≈ 135 Egyptian royal cubits. (The royal cubit ≈ 529.2 mm. See above.)
- The Pergamon cubit 520.9 mm or 75/64 of the Roman cubit.
- The Salamis cubit 484.0 mm or 98/90 of the Roman cubit.
- The Persian cubit of about 500.1 mm or 9/8 of the Roman cubit, which is also 9/10 of the guard cubit.
- In Izapa, a pre-Columbian Mesoamerican city, the measuring unit was equivalent to about 495 mm, very close to the Lagash cubit. This is probably a coincidence, since a diffusion of culture from Mesopotamia to Mesoamerica has not been conclusively demonstrated.
- The different Jewish cubits (אַמָּה ama) are generally borrowed either from Babylonians or Greeks or Romans. In ancient Israel during the First Temple period, the cubit was 428.1 mm (16.85 in.) (≈ 26/27 Roman cubit). During the Second Temple period, a cubit of about 444.5 mm (17.5 in.) (≈ Roman cubit) was in general use, but in the sacred areas of the temple a special cubit of 437.6 mm seems to have been used instead (≈ 63/64 Roman cubit).
- Vormetrische Längeneinheiten by Rolf C. A. Rottländer, Rottenburg / Köln (also see Search-Engine).
- Recovery of the Ancient System of Length Units by Dieter Lelgemann, former Director of the Institute for Geodesy and Geo-Information Technology, TU Berlin
- On the Ancient Determination of Meridian Arc Length by Eratosthenes of Kyrene Dieter Lelgemann, WS – History of Surveying and Measurement, Athens, Greece, May 22-27, 2004.
- Knobloch, Eberhard, Dieter Lelgemann und Andreas Fuls: "Zur hellenistischen Methode der Bestimmung des Erdumfangs und zur Asienkarte des Klaudios Ptolemaios." published in zfv (Zeitschrift für Geodäsie, Geoinformation und Landmanagment) 128. Jahrgang, Heft 3/2003, S. 211-217.