Digit (unit)

For other uses, see Digit.
Detail of the Ancient Egyptian cubit rod in the Museo Egizio of Turin, showing digit, palm, hand and fist lengths

The digit or finger is an ancient and obsolete non-SI unit of measurement of length. It was originally based on the breadth of a human finger.[1] It was a fundamental unit of length in the Ancient Egyptian, Mesopotamian, Hebrew, Ancient Greek and Roman systems of measurement.

In astronomy a digit is one twelfth of the diameter of the sun or the moon.[2]

History

Ancient Egypt

The digit, also called a finger or fingerbreadth, is a unit of measurement originally based on the breadth of a human finger. In Ancient Egypt it was the basic unit of subdivision of the cubit.[1]

On surviving Ancient Egyptian cubit-rods, the royal cubit is divided into seven palms of four digits or fingers each.[3] The royal cubit measured approximately 525 mm,[4] so the length of the ancient Egyptian digit was about 19 mm.

Ancient Egyptian units of length[5]
NameEgyptian nameEquivalent Egyptian valuesMetric equivalent
Royal cubit
M23 t
n		 D42
meh niswt 		7 palms or 28 digits 525 mm     
Fist 6 digits 108 mm     
Hand 5 digits 94 mm     
Palm
D48
shesep 		4 digits 75 mm     
Digit
D50
djeba 		1/4 palm 19 mm     

Mesopotamia

In the classical Akkadian Empire system instituted in about 2150 BC during the reign of Naram-Sin, the finger was one-thirtieth of a cubit length. The cubit was equivalent to approximately 497 mm, so the finger was equal to about 17 mm. Basic length was used in architecture and field division.

Mesopotamian units of length
Unit Ratio  Metric
equivalent  		 Sumerian   Akkadian   Cuneiform 
 grain   1/180   2.8 mm    še  uţţatu  𒊺
 finger   1/30 17 mm   šu-si  ubānu 𒋗𒋛
 foot 2/3 331 mm    šu-du3-a  šīzu  𒋗𒆕𒀀
 cubit 1 497 mm    kuš3  ammatu  𒌑

Ancient Hebrew system

Ancient Greece

Ancient Rome

Britain

Main article: English units

A digit (lat. digitus, "finger"), when used as a unit of length, is usually a sixteenth of a foot or 3/4" (1.905 cm for the international inch).[6] The width of an adult human male finger tip is indeed about 2 centimetres. In English this unit has mostly fallen out of use, as do others based on the human arm: finger (7/6 digit), palm (4 digits), hand (16/3 digits), shaftment (8 digits), span (12 digits), cubit (24 digits) and ell (60 digits).

It is in general equal to the foot-nail, although the term nail can also be used as 1/16 of yard and other units.

Astronomy

In astronomy a digit is, or was until recently, one twelfth of the diameter of the sun or the moon.[2][7] This is found in the Moralia of Plutarch, XII:23,[8] but the definition as exactly one twelfth of the diameter may be due to Ptolemy. Sosigenes of Alexandria had observed in the 1st century AD that on a dioptra, a disc with a diameter of 11 or 12 digits (of length) was needed to cover the moon.[9]

The unit was used in Arab or Islamic astronomical works such as those of Ṣadr al‐Sharīʿa al‐Thānī (d.1346/7),[10] where it is called Arabic: إصبعا‎‎ iṣba' , digit or finger.[11]

The astronomical digit was in use in Britain for centuries. Heath, writing in 1760, explains that 12 digits are equal to the diameter in eclipse of the sun, but that 23 may be needed for that of the moon, those over 12 representing the extent to which the Earth's shadow is larger than the Moon.[12] The unit is apparently not in current use, but is found in recent dictionaries.[7]

Alcoholic Beverages

A 'finger' of an alcoholic beverage is colloquially referred to as a 'digit'.

See also

References

  1. 1 2 Hosch, William L. (ed.) (2010) The Britannica Guide to Numbers and Measurement New York, NY: Britannica Educational Publications, 1st edition. ISBN 978-1-61530-108-9, p.203
  2. 1 2 Chisholm, Hugh (ed.) (1910–1911) Encyclopædia Britannica (11th edition) Cambridge: University Press, "digit"
  3. ↑ Selin, Helaine, ed. (1997). Encyclopaedia of the History of Science, Technology and Medicine in non-Western Cultures. Dordrecht: Kluwer. ISBN 978-0-7923-4066-9.
  4. ↑ Lepsius, Richard (1865). Die altaegyptische Elle und ihre Eintheilung (in German). Berlin: Dümmler.
  5. ↑ Clagett, Marshall (1999). Ancient Egyptian Science, A Source Book. Volume 3: Ancient Egyptian Mathematics. Philadelphia: American Philosophical Society. ISBN 978-0-87169-232-0.
  6. ↑ Ronald Edward Zupko (1985). A dictionary of weights and measures for the British Isles: the Middle Ages to the twentieth century. American Philosophical Society. pp. 109–10. ISBN 978-0-87169-168-2. Retrieved 15 January 2012.
  7. 1 2 Macdonald, A.M. (ed.) (1972) Chambers Twentieth Century Dictionary Edinburgh: W. & R. Chambers ISBN 0-550-10206-X, "digit"
  8. ↑ Plutarchus Chaeronensis, Frank Cole Babbitt (trans.) (1957) Plutarch's Moralia: In fifteen volumes London: William Heinemann, Cambridge, Mass.: Harvard University Press, Volume XII p.144
  9. ↑ Neugebauer, Otto (1975) A History of Ancient Mathematical Astronomy Berlin: Springer, ISBN 978-0-387-06995-1 Volume 2, p.658
  10. ↑ Hockey, Thomas et al. (eds.) (2007) The Biographical Encyclopedia of Astronomers, Springer Reference New York: Springer pp. 1002–1003
  11. ↑ 'Ubayd Allāh ibn Mas'ūd Ṣadr al-S̆arīaẗ al-Aṣġar al-Maḥbūbī, Ahmad S. Dallal (1995) An Islamic response to Greek astronomy: kitāb Ta'dīl hay'at al-aflāk of Ṣadr al-Sharī'a (in Arabic and English) Leiden, New York: E.J. Brill, ISBN 978-90-04-09968-5 p.212
  12. ↑ Heath, Robert (1760) Astronomia accurata; or the royal astronomer and navigator: Containing new improvements in astronomy, chronology, and navigation. Particularly New and correct Solar and Lunar Tables; With Precepts and Examples of their Use, according to Old or New Style. The Elements of the Radical Mean Places and Motions of the Planets and Satellites. A New and Correct Catalogue of Stars. With their Right Ascensions, Declinations, and Annual Variations, (supplying the Place of Uranographia Britannica.) And Answers to Fifty-Three Astronomical Questions. Chronological Rules and Tables. Reduction of the Dates of Aeras. With Universal and Perpetual Time-Tables; serving for any Year, past, present, or to come. And Answers to many Chronological Questions. The Seaman's Ready Computer, Or New and Easy Navigation: Shewing how to keep an exact Ship's Reckoning; with Examples, according to all the Methods of Sailing in Practice, by only summing up a few Figures, taken out of Tables. Tabular Answers to all the Cases in Plain and Spherical Triangles; and to all the Cases of Sailing. An alphabetical Tide-Table: An universal Sea-Coast Table, of the Latitudes and Longitudes from Greenwich, and also the Island of Ferro, of the most remarkable Sea-Coast Places, Ports, and Harbours, lying contiguous round the World. To which are added An explanatory Comment on the Moon's Theory: An easy Method, with Examples, of computing the Lunations, and of Solar and Lunar Eclipses. And how to determine the Difference of Longitude, from the Royal Observatory, at Greenwich, by observing the Moon's Place, under any distant Meridian. With some useful Remarks and Improvements on the Motion and Theory of Comets. The whole interspersed with a Variety of necessary Tables, and Articles, subservient to the three principal Subjects. By Robert Heath, a military officer London: the author p.ix
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