The degree symbol or degree sign, , is a glyph or symbol that is used, among other things, to represent degrees of arc (e.g. in geographic coordinate systems), hours (in the medical field), degrees of temperature or alcohol proof. The symbol consists of a small superscript circle.
History
The word degree is equivalent to Latin gradus which, since the medieval period, could refer to any stage in a graded system of ranks or steps. The number of the rank in question was indicated by ordinal numbers, in abbreviation with the ordinal indicator (a superscript o).
Use of "degree" specifically for the degrees of arc, used in conjunction with Arabic numerals, became common in the 16th century, but this was without the use of an ordinal marker or degree symbol. Similarly, the introduction of the temperature scales with degrees in the 18th century was at first without such symbols, but with the word "gradus" spelled out. Use of the degree symbol was introduced for temperature in the later 18th century and became widespread in the early 19th century. Antoine Lavoisier in his "Opuscles physiques et chymiques" (1774) used the ordinal indicator with Arabic numerals – for example, when he wrote in the introduction: (p. vi) (... a series of experiments [...] firstly, on the existence of that same elastic fluid [...]) The is to be read as meaning "in the first place", followed by ("in the second place"), etc. In the same work, when Lavoisier gives a temperature, he spells out the word "degree" explicitly, for example (p. 194): ("a temperature of 16 to 17 degrees of the thermometer").
An early use of the degree symbol proper is that by Henry Cavendish in 1776 for degrees of the Fahrenheit scale. The degree symbol for degrees of temperature appears to have been transferred to the use for degrees of arc early in the 19th century. An early textbook using this notation is Charles Hutton, "A Course of Mathematics" vol. 1 (1836), page 383. An earlier convention is found in Conrad Malte-Brun, "Universal Geography" vol. 1 (1827), where degrees of arc are abbreviated with a superscript "d" (alongside a superscript "m" for minutes of arc).
Typography
In the case of degrees of angular arc, the degree symbol follows the number without any intervening space, e.g. . The addition of minute and second of arc follows the degree units, with intervening spaces (optionally, non-breaking space) between the sexagesimal degree subdivisions but no spaces between the numbers and units, for example .
In the case of degrees of temperature, three scientific and engineering standards bodies (the International Bureau of Weights and Measures, the International Organization for Standardization and the U.S. Government Printing Office) prescribe printing temperatures with a space between the number and the degree symbol, e.g. . However, in many works with professional typesetting, including scientific works published by the University of Chicago Press or Oxford University Press, the degree symbol is printed with no spaces between the number, the symbol, and the Latin letters "C" or "F" representing Celsius or Fahrenheit, respectively, e.g. . This is also the practice of the University Corporation for Atmospheric Research, which operates the National Center for Atmospheric Research. Both ASTM International and NIST, the official US entities related to the standardization of the use of units, require a space between the numerical value and the unit designator, except when the degree symbol alone is used to denote an angular value.
Use of the degree symbol to refer to temperatures measured in kelvins (symbol: K) was abolished in 1967 by the 13th General Conference on Weights and Measures (CGPM). Therefore, the triple point of water, for instance, is written simply as 273.16 K. The name of the SI unit of temperature is now "kelvin", in lower case, and no longer "degrees Kelvin".
In photography, the symbol is used to denote logarithmic film speed grades. In this usage, it follows the number without spacing as in 21° DIN, 5° ASA or ISO 100/21°.
Encoding
The degree symbol is included in Unicode as .
For use with wide character fonts, there are also code points for and .
The degree sign was not included in the basic 7-bit ASCII set of 1963. In 1987, the ISO/IEC 8859 standard introduced it at position 0xB0 (176 decimal) in all variants except Part 5 (Cyrillic), 6 (Arabic), 7 (Greek) and 11 (Thai). In 1991, the Unicode standard incorporated all of the ISO/IEC 8859 code points and thus included the degree sign (at U+00B0)..