Cubit is the name for any one of many units of measure used by various ancient peoples. The natural cubit is based on the distance between thumb and an other finger to the elbow on an average person. It was employed consistently — to measure originally cords and textiles for example — also in Middle-Ages up to the Early Modern Times. This natural cubit measures 24 digits or 6 palms or 1½ foot. This is about 45 cm or 18 inches.
Early on, greater cubits (in a larger sense) featured on: 7 palms, ~52.5 cm or 21 inches, also cubits of 8 palms, ~60 cm or 24 inches, like cubits of 9 palms, ~67.5 cm or 27 inches. From the late Antiquity a Roman 16-palms-cubit (ulna ~1.20 cm) is as well attested. This is the measure from a men's hip to his fingers of the other side with a stretched arm; very practical for measuring quickly textiles or cords over one's ellbow. Even the English yard can be considered to be a 12-palms-cubit, ~90 cm or 36 inches. This is the measure from the middle of human's body to his fingers always with streched arm. (To conceive this measure as being the distance between the tip of his nose and the end of the thumb of Henry I of England is rather anecdotic or actually legendary.) The English Ell is essentially such a great cubit of 15 palms or 45 inches.
[image] The Egyptian Old Royal Cubit has been securely estabished from surviving architectural evidence that a standard measure was employed as early as c. 2750 BC (Dynasty III) at Saqqara. From the evidence this is widely accepted to have been 523.5 to 524 mm (20.61 to 20.63 in) in length, and was subdivided into 7 palms of 4 digits, giving a 28 part measure in total. This Old Royal Cubit is directly deduced from the then prevalent and widely-used Nippur Cubit.
Old Egyptian geometers could not calculate the radix of two but they needed the value of the hypotenuse. The well attested old Egyptian set square called "Construction Remen" use a good approximation: 20 radix 2 greater equal 28. The sides of this surveying instrument measured 20 digits of the Nippur cubit (about 518.5 mm) shared into 28 equal parts, that's about 20 x 18.5 = 370 mm, the remen measure. The digit of the sides is identical to the later called Roman digit. The hypotenuse of this square set give now the Old Royal Cubit with about 523.5 mm. (See schema image: From the Nippur Ell to the Old Royal Cubit)
So, the old Egyptian geometers found a very good approximation for land survey after floods of the Nil river. Therefore the Egyptian cubit is shared into 28 digits. It is also obvious: they knew that the two digit are not identical; but a difference of only one percent is satisfactory in praxis.
A shorter Egyptian rule of 6 palms may also have been employed, but based on the same 7 part standard. During Dynasty XII (1900 BC) the length of the Royal Cubit grew up to about 529.2 mm (+1% more than the former Old Royal Cubit), because, since, for the sides of the Construction Remen the digits of the Old Royal Cubits were used. Then the Egyptian length standard did not change any more. Since the ratio between the digits of the Royal Cubit and the later called Roman digits is 100 to 98 exactly one.
The Greek 40-digit-measure, called bema, corresponds to the latin gradus, the step or half-a-pace.
Many other cultures used cubits as well:
Other less diffused cubits well existed. For example the Pergamon cubit 520.9 mm or 75/64 Roman cubit and the Salamis cubit 484.0 mm or 98/90 Roman cubit and the Persian cubit of about 500.1 mm or 9/8 Roman cubit, which is also 9/10 Guard cubit. In Izapa, a Precolumbian Mesoamerican city, the measuring unit was also equivalent to about 495 mm. This must be a coincidence, because a diffusion of culture from Babylonia to Mesoamerica is not conclusive.
The different Jewish cubits (הָּמַא 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 (= 26/27 Roman cubit). During the Second Temple period, a cubit of about 444.5 mm (= 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). (Cf. Biblical Archaeology Review, March-April 1983, and Newsletter and Proceedings of the Society for Early Historical Archaeology, issue 159.)
* 17.5 inches, with 25.4 mm per inch gives 444.5 mm for the Roman cubit.
Some modern metrologists consider that the English inch should be redefined officially to be hence 25.4016 mm, because a inch of 72 x 52 x 34 x 28 nanometres better matches with the omnipresent simple primary factors of the ancient system of measure than the current 127 x 55 x 26 nanometres. Just 1.6 µm more per inch, that is 8/127,000 or about 0.0063 %. That is nothing even for high-level technical precision. In praxis nothing changes, but this would simplify all the theoretical values for all the ancient measures in a considerable manner.