thumb|One degree (shown in red) and
eighty nine degrees (shown in blue)
A degree (in full, a degree of arc, arc degree, or arcdegree), usually denoted by ° (the degree symbol
is a measurement of a plane angle
in which one full rotation
is 360 degrees.
It is not an SI unit
—the SI unit of angular measure is the radian
—but it is mentioned in the SI brochure
as an accepted unit
. Because a full rotation equals 2 radians, one degree is equivalent to radians.
The original motivation for choosing the degree as a unit of rotations and angles is unknown. One theory states that it is related to the fact that 360 is approximately the number of days in a year.
noticed that the sun, which follows through the ecliptic
path over the course of the year, seems to advance in its path by approximately one degree each day. Some ancient calendar
s, such as the Persian calendar
and the Babylonian calendar
, used 360 days for a year. The use of a calendar with 360 days may be related to the use of sexagesimal
Another theory is that the Babylonians subdivided the circle using the angle of an equilateral triangle
as the basic unit, and further subdivided the latter into 60 parts following their sexagesimal
numeric system. The earliest trigonometry
, used by the Babylonian astronomers
and their Greek
successors, was based on chords
of a circle. A chord of length equal to the radius made a natural base quantity. One sixtieth of this, using their standard sexagesimal
divisions, was a degree.
Aristarchus of Samos
seem to have been among the first Greek scientists
to exploit Babylonian astronomical knowledge and techniques systematically. Timocharis
, Aristarchus, Aristillus
, and Hipparchus were the first Greeks known to divide the circle in 360 degrees of 60 arc minute
used a simpler sexagesimal
system dividing a circle into 60 parts.
The division of the circle into 360 parts also occurred in ancient India
, as evidenced in the Rigveda
Another motivation for choosing the number 360 may have been that it is readily divisible
: 360 has 24 divisor
[The divisors of 360 are 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 18, 20, 24, 30, 36, 40, 45, 60, 72, 90, 120, 180, and 360.]
making it one of only 7 numbers such that no number less than twice as much has more divisors .
Furthermore, it is divisible by every number from 1 to 10 except 7.
[Contrast this with the relatively unwieldy 2520, which is the least common multiple for every number from 1 to 10.]
This property has many useful applications, such as dividing the world into 24 time zone
s, each of which is nominally 15° of longitude
, to correlate with the established 24-hour day
Finally, it may be the case that more than one of these factors has come into play. According to that theory, the number is approximately 365 because of the apparent movement of the sun against the celestial sphere, and that it was rounded to 360 for some of the mathematical reasons cited above.
For many practical purposes, a degree is a small enough angle that whole degrees provide sufficient precision. When this is not the case, as in astronomy
or for geographic coordinates
), degree measurements may be written using decimal degrees
, with the degree symbol behind the decimals; for example, 40.1875°.
Alternatively, the traditional sexagesimal unit
subdivisions can be used. One degree is divided into 60 ''minutes (of arc)'', and one minute into 60 ''seconds (of arc)''. Use of degrees-minutes-seconds is also called DMS notation. These subdivisions, also called the ''arcminute
'' and ''arcsecond
'', are respectively represented by a single prime (′) and a double prime
For example, , or, using quotation mark
characters, . Additional precision can be provided using decimals for the arcseconds component.
Maritime charts are marked in degrees and decimal minutes to facilitate measurement; 1 minute of latitude is 1 nautical mile
. The example above would be given as 40° 11.25′ (commonly written as 11′25 or 11′.25).
The older system of thirds, fourths, etc., which continues the sexagesimal unit subdivision, was used by al-Kashi
and other ancient astronomers, but is rarely used today. These subdivisions were denoted by writing the Roman numeral
for the number of sixtieths in superscript: 1I
for a "prime
" (minute of arc), 1II
for a second
for a third
for a fourth
Hence, the modern symbols for the minute and second of arc, and the word "second" also refer to this system.
In most mathematical
work beyond practical geometry, angles are typically measured in radian
s rather than degrees. This is for a variety of reasons; for example, the trigonometric function
s have simpler and more "natural" properties when their arguments are expressed in radians. These considerations outweigh the convenient divisibility of the number 360. One complete turn
(360°) is equal to 2''
'' radians, so 180° is equal to radians, or equivalently, the degree is a mathematical constant
: 1° = .
(or revolution, full circle, full rotation, cycle) is used in technology
. One turn is equal to 360°.
With the invention of the metric system
, based on powers of ten, there was an attempt to replace degrees by decimal "degrees"
[These new and decimal "degrees" must not be confused with decimal degrees.]
'' or ''gon'', where the number in a right angle is equal to 100 gon with 400 gon in a full circle (1° = gon). Although that idea was abandoned by Napoleon, grades continued to be used in several fields and many scientific calculator
s support them. Decigrades () were used with French artillery sights in World War I.
An angular mil
, which is most used in military applications, has at least three specific variants, ranging from to . It is approximately equal to one milliradian
( ). A mil measuring of a revolution originated in the imperial Russian army
, where an equilateral chord was divided into tenths to give a circle of 600 units. This may be seen on a lining plane (an early device for aiming indirect fire
artillery) dating from about 1900 in the St. Petersburg
Museum of Artillery.
* Degree of curvature
* Geographic coordinate system
* Meridian arc
* Square degree
* Square minute
* Square second
* , with interactive animation
Category:Units of plane angle
Category:Customary units of measurement in the United States