# Turn (angle)

(Redirected from Turn (geometry))

Turn
Counterclockwise rotations about the center point where a complete rotation corresponds to an angle of rotation of 1 turn.
General information
Unit ofPlane angle
Symboltr, pla, rev, cyc
Conversions
1 tr in ...... is equal to ...
degrees   360°

One turn (symbol tr or pla) is a unit of plane angle measurement equal to  radians, 360 degrees or 400 gradians. Thus it is the angular measure subtended by a complete circle at its center.

Subdivisions of a turn include half-turns and quarter-turns, spanning a semicircle and a right angle, respectively; metric prefixes can also be used as in, e.g., centiturns (ctr), milliturns (mtr), etc.

As an angular unit, one turn also corresponds to one cycle (symbol cyc or c)[1] or to one revolution (symbol rev or r).[2]

In the ISQ, an arbitrary "number of turns" (also known as "number of revolutions" or "number of cycles") is formalized as a dimensionless quantity called rotation, defined as the ratio of a given angle and the full turn. (See below for the formula.)

Common related units of frequency are cycles per second (cps) and revolutions per minute (rpm).[a]

## History

The word turn originates via Latin and French from the Greek word τόρνος (tórnos – a lathe).

In 1697, David Gregory used π/ρ (pi over rho) to denote the perimeter of a circle (i.e., the circumference) divided by its radius.[3][4] However, earlier in 1647, William Oughtred had used δ/π (delta over pi) for the ratio of the diameter to perimeter. The first use of the symbol π on its own with its present meaning (of perimeter divided by diameter) was in 1706 by the Welsh mathematician William Jones.[5] Euler adopted the symbol with that meaning in 1737, leading to its widespread use.

Percentage protractors have existed since 1922,[6] but the terms centiturns, milliturns and microturns were introduced much later by the British astronomer Fred Hoyle in 1962.[7][8] Some measurement devices for artillery and satellite watching carry milliturn scales.[9][10]

## Unit symbols

The German standard DIN 1315 (March 1974) proposed the unit symbol "pla" (from Latin: plenus angulus 'full angle') for turns.[11][12] Covered in DIN 1301-1 [de] (October 2010), the so-called Vollwinkel ('full angle') is not an SI unit. However, it is a legal unit of measurement in the EU[13][14] and Switzerland.[15]

The scientific calculators HP 39gII and HP Prime support the unit symbol "tr" for turns since 2011 and 2013, respectively. Support for "tr" was also added to newRPL for the HP 50g in 2016, and for the hp 39g+, HP 49g+, HP 39gs, and HP 40gs in 2017.[16][17] An angular mode TURN was suggested for the WP 43S as well,[18] but the calculator instead implements "MULπ" (multiples of π) as mode and unit since 2019.[19][20]

## Subdivisions

A turn can be divided in 100 centiturns or 1000 milliturns, with each milliturn corresponding to an angle of 0.36°, which can also be written as 21′ 36″.[7][8] A protractor divided in centiturns is normally called a "percentage protractor".

Binary fractions of a turn are also used. Sailors have traditionally divided a turn into 32 compass points, which implicitly have an angular separation of 1/32 turn. The binary degree, also known as the binary radian (or brad), is 1/256 turn.[21] The binary degree is used in computing so that an angle can be represented to the maximum possible precision in a single byte. Other measures of angle used in computing may be based on dividing one whole turn into 2n equal parts for other values of n.[22]

The notion of turn is commonly used for planar rotations.

SI multiples of turn (tr)
Submultiples Multiples
Value SI symbol Name Value SI symbol Name
10−1 tr dtr deciturn 101 tr datr decaturn
10−2 tr ctr centiturn 102 tr htr hectoturn
10−3 tr mtr milliturn 103 tr ktr kiloturn
10−6 tr μtr microturn 106 tr Mtr megaturn
10−9 tr ntr nanoturn 109 tr Gtr gigaturn
10−12 tr ptr picoturn 1012 tr Ttr teraturn
10−15 tr ftr femtoturn 1015 tr Ptr petaturn
10−18 tr atr attoturn 1018 tr Etr exaturn
10−21 tr ztr zeptoturn 1021 tr Ztr zettaturn
10−24 tr ytr yoctoturn 1024 tr Ytr yottaturn
10−27 tr rtr rontoturn 1027 tr Rtr ronnaturn
10−30 tr qtr quectoturn 1030 tr Qtr quettaturn

## Unit conversion

One turn is equal to 2π (≈ 6.283185307179586)[23] radians, 360 degrees, or 400 gradians.

Conversion of common angles
1/2π turn 1 rad c. 57.3° c. 63.7g

## Proposals for a single letter to represent 2π

In 1746, Leonhard Euler first used the Greek letter pi to represent the circumference divided by the radius of a circle (i.e., π = 6.28...).[24]

In 2001, Robert Palais proposed using the number of radians in a turn as the fundamental circle constant instead of π, which amounts to the number of radians in half a turn, in order to make mathematics simpler and more intuitive. His proposal used a "π with three legs" symbol to denote the constant (${\displaystyle \pi \!\;\!\!\!\pi =2\pi }$).[25]

In 2008, Thomas Colignatus proposed the uppercase Greek letter theta, Θ, to represent 2π.[26] The Greek letter theta derives from the Phoenician and Hebrew letter teth, 𐤈 or ט, and it has been observed that the older version of the symbol, which means wheel, resembles a wheel with four spokes.[27] It has also been proposed to use the wheel symbol, teth, to represent the value 2π, and more recently a connection has been made among other ancient cultures on the existence of a wheel, sun, circle, or disk symbol—i.e. other variations of teth—as representation for 2π.[28]

In 2010, Michael Hartl proposed to use the Greek letter tau to represent the circle constant: τ = 2π. He offered two reasons. First, τ is the number of radians in one turn, which allows fractions of a turn to be expressed more directly: for instance, a 3/4 turn would be represented as 3τ/4 rad instead of 3π/2 rad. Second, τ visually resembles π, whose association with the circle constant is unavoidable.[29] Hartl's Tau Manifesto[30] gives many examples of formulas that are asserted to be clearer where τ is used instead of π,[31][32][33] such as a tighter association with the geometry of Euler's identity using e = 1 instead of e = −1.

Initially, neither of these proposals received widespread acceptance by the mathematical and scientific communities.[34] However, the use of τ has become more widespread,[35] for example:

The following table shows how various identities appear if τ = 2π was used instead of π.[52][25] For a more complete list, see List of formulae involving π.

Formula Using π Using τ Notes
Circumference C of a circle of radius r C = 2πr C = τr
Area of a circle A = πr2 A = 1/2τr2 The area of a sector of angle θ is A = 1/2θr2.
Area of a regular n-gon with unit circumradius A = n/2 sin /n A = n/2 sin τ/n
n-ball and n-sphere volume recurrence relation Vn(r) = r/n Sn−1(r) Sn(r) = 2πr Vn−1(r) Vn(r) = r/n Sn−1(r) Sn(r) = τr Vn−1(r) V0(r) = 1
S0(r) = 2
Cauchy's integral formula ${\displaystyle f(a)={\frac {1}{{\color {orangered}2\pi }i}}\oint _{\gamma }{\frac {f(z)}{z-a}}\,dz}$ ${\displaystyle f(a)={\frac {1}{{\color {orangered}\tau }i}}\oint _{\gamma }{\frac {f(z)}{z-a}}\,dz}$
Standard normal distribution ${\displaystyle \varphi (x)={\frac {1}{\sqrt {\color {orangered}2\pi }}}e^{-{\frac {x^{2}}{2}}}}$ ${\displaystyle \varphi (x)={\frac {1}{\sqrt {\color {orangered}\tau }}}e^{-{\frac {x^{2}}{2}}}}$
Stirling's approximation ${\displaystyle n!\sim {\sqrt {{\color {orangered}2\pi }n}}\left({\frac {n}{e}}\right)^{n}}$ ${\displaystyle n!\sim {\sqrt {{\color {orangered}\tau }n}}\left({\frac {n}{e}}\right)^{n}}$
Euler's identity 0      eiπ = −1
eiπ + 1 = 0
0    eiτ = 1
eiτ - 1 = 0
For any integer k, eikτ = 1
nth roots of unity ${\displaystyle e^{{\color {orangered}2\pi }i{\frac {k}{n}}}=\cos {\frac {{\color {orangered}2}k{\color {orangered}\pi }}{n}}+i\sin {\frac {{\color {orangered}2}k{\color {orangered}\pi }}{n}}}$ ${\displaystyle e^{{\color {orangered}\tau }i{\frac {k}{n}}}=\cos {\frac {k{\color {orangered}\tau }}{n}}+i\sin {\frac {k{\color {orangered}\tau }}{n}}}$
Planck constant ${\displaystyle h={\color {orangered}2\pi }\hbar }$ ${\displaystyle h={\color {orangered}\tau }\hbar }$ ħ is the reduced Planck constant.
Angular frequency ${\displaystyle \omega ={\color {orangered}2\pi }f}$ ${\displaystyle \omega ={\color {orangered}\tau }f}$

## In the ISQ/SI

Rotation
Other names
number of revolutions, number of cycles, number of turns, number of rotations
Common symbols
N
SI unitUnitless
Dimension1

A concept related to the angular unit "turn" is the physical quantity rotation (symbol N) defined as number of revolutions:[53]

N is the number (not necessarily an integer) of revolutions, for example, of a rotating body about a given axis. Its value is given by:

where φ denotes the measure of rotational displacement.

The above definition is part of the International System of Quantities (ISQ), formalized in the international standard ISO 80000-3 (Space and time),[53] and adopted in the International System of Units (SI).[54][55]

Rotation count or number of revolutions is a quantity of dimension one, resulting from a ratio of angles. It can be negative and also greater than 1 in modulus. The relationship between quantity rotation, N, and unit turns, tr, can be expressed as:

N=φ/tr={φ}tr

where {φ}tr is the numerical value of the angle φ in units of turns (see Physical quantity#Components).

In the ISQ/SI, rotation is used to derive rotational frequency, n=dN/dt, with SI base unit of reciprocal seconds (s-1); common related units of frequency are hertz (Hz), cycles per second (cps), and revolutions per minute (rpm).

Revolution
Unit ofRotation
Symbolrev, r, cyc, c
Conversions
1 rev in ...... is equal to ...
Base units   1

The superseded version ISO 80000-3:2006 defined "revolution" as a special name for the dimensionless unit "one",[c] which also received other special names, such as the radian.[d] Despite their dimensional homogeneity, these two specially named dimensionless units are applicable for non-comparable kinds of quantity: rotation and angle, respectively.[57] "Cycle" is also mentioned in ISO 80000-3, in the definition of period.[e]

## Notes

1. ^ The angular unit terms "cycles" and "revolutions" are also used, ambiguously, as shorter versions of the related frequency units.[citation needed]
2. ^ In this table, 𝜏 denotes 2π.
3. ^ "The special name revolution, symbol r, for this unit [name 'one', symbol '1'] is widely used in specifications on rotating machines."[56]
4. ^ "Measurement units of quantities of dimension one are numbers. In some cases, these measurement units are given special names, e.g. radian..."[56]
5. ^ "3-14) period duration, period: duration (item 3‑9) of one cycle of a periodic event"[53]

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