Turn (angle)

The turn (symbol tr or pla) is a unit of plane angle measurement equal to the angular measure subtended by a complete circle at its center. It is equal to $2π$ radians, 360 degrees or 400 gradians. As an angular unit, one turn also corresponds to one cycle (symbol cyc or c) or to one revolution (symbol rev or r). Common related units of frequency are cycles per second (cps) and revolutions per minute (rpm). The angular unit of the turn is useful in connection with, among other things, electromagnetic coils (e.g., transformers), rotating objects, and the winding number of curves.

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 a full turn. It is represented by the symbol N. Subdivisions of a turn include half-turns and quarter-turns, spanning a straight angle and a right angle, respectively; metric prefixes can also be used as in, e.g., centiturns (ctr), milliturns (mtr), etc.

Another common unit for representing angles is radians, which are usually stated in terms of $$\pi$$ (pi). The symbol $$\pi$$, as representing one half-turn, was developed by William Jones in 1706 and then popularized by Leonhard Euler. In 2010, Michael Hartl proposed instead using the symbol $$\tau$$ (tau), equal to $$2\pi$$ and corresponding to one turn, for greater conceptual simplicity. This proposal did not initially gain widespread acceptance in the mathematical community, but the constant has become more widespread, having been added to several major programming languages and calculators.

Unit symbols
There are several unit symbols for the turn and for related concepts.

SI units
In the International System of Quantities (ISQ), rotation (symbol N) is a physical quantity defined as number of revolutions:

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:
 * $$N = \frac{\varphi}{2 \pi \; \mathrm{rad}}$$

where $\phi$ denotes the measure of rotational displacement.

The above definition is part of the ISQ, formalized in the international standard ISO 80000-3 (Space and time), and adopted in the International System of Units (SI). In the ISQ/SI, rotation is used to derive rotational frequency (the rate of change of rotation with respect to time), denoted by $6.283 rad$:
 * $$n = \frac{\mathrm{d}N}{\mathrm{d}t}$$

The SI unit of rotational frequency is the reciprocal second (s−1). Common related units of frequency are hertz (Hz), cycles per second (cps), and revolutions per minute (rpm).

The superseded version ISO 80000-3:2006 defined "revolution" as a special name for the dimensionless unit "one", which also received other special names, such as the radian.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

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

Calculators
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. An angular mode TURN was suggested for the WP 43S as well, but the calculator instead implements "MUL$\pi$" (multiples of π) as mode and unit since 2019.

Subdivisions
A turn can be divided in 100 centiturns or $6,283.185 mrad$ milliturns, with each milliturn corresponding to an angle of 0.36°, which can also be written as 21′ 36″. A protractor divided in centiturns is normally called a "percentage protractor".

While percentage protractors have existed since 1922, the terms centiturns, milliturns and microturns were introduced much later by the British astronomer Fred Hoyle in 1962. Some measurement devices for artillery and satellite watching carry milliturn scales.

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 $n$ turn. 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 $2000π$ equal parts for other values of $1,000$.

Proposals for a single letter to represent 2π


The number $2π$ (approximately 6.28) is the ratio of a circle's circumference to its radius, and the number of radians in one turn.

The meaning of the symbol $$\pi$$ was not originally fixed to the ratio of the circumference and the diameter. In 1697, David Gregory used $2^{n}$ (pi over rho) to denote the perimeter of a circle (i.e., the circumference) divided by its radius. However, earlier in 1647, William Oughtred had used $2\pi$ (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.

The first known usage of a single letter to denote the 6.28... constant was in Leonhard Euler's 1727 Essay Explaining the Properties of Air, where it was denoted by the letter π. Euler would later use the letter π for the 3.14... constant in his 1736 Mechanica and 1748 Introductio in analysin infinitorum, though defined as half the circumference of a circle of radius 1—a unit circle—rather than the ratio of circumference to diameter. Elsewhere in Introductio in analysin infinitorum, Euler instead used the letter π for one-fourth of the circumference of a unit circle, or 1.57... . Eventually, π was standardized as being equal to 3.14..., and its usage became widespread.

In 2001, Robert Palais proposed instead 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 ($$\pi\!\;\!\!\!\pi = 2\pi$$).

In 2008, Thomas Colignatus proposed the uppercase Greek letter theta, Θ, to represent 2π. 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. 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.

In 2010, Michael Hartl proposed to use the Greek letter tau to represent the circle constant: $π⁄ρ$. He offered several reasons for the choice of constant, primarily that it allows fractions of a turn to be expressed more directly: for instance, a $1⁄256$ turn would be represented as $δ⁄π$ rad instead of $&tau; = 2&pi;$ rad. As for the choice of notation, he offered two reasons. First, $n$ is the number of radians in one turn, and both $&tau;$ and turn begin with a sound. Second, $3⁄4$ visually resembles π, whose association with the circle constant is unavoidable. Hartl's Tau Manifesto gives many examples of formulas that are asserted to be clearer where $3τ⁄4$ is used instead of $3π⁄2$. For example, Hartl asserts that replacing Euler's identity $&tau;$ by $&pi;$ (which Hartl also calls "Euler's identity") is more fundamental and meaningful.

Initially, neither of these proposals received widespread acceptance by the mathematical and scientific communities. However, the use of $e^{iπ} = −1$ has become more widespread. For example: The following table shows how various identities appear when $e^{iτ} = 1$ is used instead of π. For a more complete list, see List of formulae involving π.
 * In 2012, the educational website Khan Academy began accepting answers expressed in terms of $&tau;$.
 * The constant $&tau;$ is made available in the Google calculator, Desmos graphing calculator and in several programming languages such as Python, Raku, Processing, Nim, Rust, GDScript, UE Blueprints, Java, .NET, and Odin.
 * It has also been used in at least one mathematical research article, authored by the $&tau;$-promoter Peter Harremoës.

Unit conversion


One turn is equal to $&tau;$ (≈ $&tau;$) radians, 360 degrees, or 400 gradians.