# Turn (geometry)

Turn | |
---|---|

Unit of | Plane angle |

Symbol | tr or pla |

Unit conversions | |

1 tr in ... |
... is equal to ... |

radians | 6.283185307179586... rad |

radians | 2π rad |

degrees | 360° |

gradians | 400^{g} |

A **turn** is a unit of plane angle measurement equal to 2π radians, 360 degrees or 400 gradians. A turn is also referred to as a **revolution** or **complete rotation** or **full circle** or **cycle** or **rev** or **rot**.

A turn can be subdivided in many different ways: into half turns, quarter turns, centiturns, milliturns, binary angles, points etc.

## Contents

## Subdivision of turns[edit]

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″. 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, the *binary degree*, also known as the *binary radian* (or *brad*), is ^{1}⁄_{256} turn.^{[1]} 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 2^{n} equal parts for other values of *n*.^{[2]}

The notion of turn is commonly used for planar rotations. Two special rotations have acquired appellations of their own: a rotation through 180° is commonly referred to as a half-turn (π radians),^{[3]} a rotation through 90° is referred to as a quarter-turn.

## History[edit]

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 *p*erimeter of a circle (i.e., the circumference) divided by its *r*adius.^{[4]}^{[5]} However, earlier in 1647, William Oughtred had used δ/π (delta over pi) for the ratio of the *d*iameter to *p*erimeter. 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.^{[6]} Euler adopted the symbol with that meaning in 1737, leading to its widespread use.

Percentage protractors have existed since 1922,^{[7]} but the terms centiturns and milliturns were introduced much later by Sir Fred Hoyle.^{[8]}

The German standard DIN 1315 (1974-03) proposed the unit symbol *pla* (from Latin: *plenus angulus* "full angle") for turns.^{[9]}^{[10]} Since 2011, the HP 39gII and HP Prime support the unit symbol *tr* for turns. In 2016, support for turns was also added to newRPL for the HP 50g;^{[11]} in June 2017, for release 3.6, the Python programming language adopted the name *tau* to represent the number of radians in a turn.^{[12]}

## Unit conversion[edit]

One turn is equal to 2π (≈ 6.283185307179586)^{[13]} radians.

Turns | Radians | Degrees | Gradians (Gons) |
---|---|---|---|

0 | 0 | 0° | 0^{g} |

1/24 | π/12 | 15° | 16 2/3^{g} |

1/12 | π/6 | 30° | 33 1/3^{g} |

1/10 | π/5 | 36° | 40^{g} |

1/8 | π/4 | 45° | 50^{g} |

1/2π | 1 | c. 57.3° | c. 63.7^{g} |

1/6 | π/3 | 60° | 66 2/3^{g} |

1/5 | 2π/5 | 72° | 80^{g} |

1/4 | π/2 | 90° | 100^{g} |

1/3 | 2π/3 | 120° | 133 1/3^{g} |

2/5 | 4π/5 | 144° | 160^{g} |

1/2 | π | 180° | 200^{g} |

3/4 | 3π/2 | 270° | 300^{g} |

1 | 2π | 360° | 400^{g} |

## Tau proposal[edit]

In 1958, Albert Eagle proposed the Greek letter tau τ as a symbol for 1/2π, by analogy with the quarter period of elliptic functions, selecting the new symbol because π resembles two τ symbols conjoined (ττ).^{[14]}

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 "pi with three legs" symbol to denote the constant ( = 2π).^{[15]}

In 2010, Michael Hartl proposed to use tau to represent Palais' circle constant (not Eagle's): 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 3/4τ rad instead of 3/2π rad. Second, τ visually resembles π, whose association with the circle constant is unavoidable.^{[16]} Hartl's *Tau Manifesto* gives many examples of formulas that are intuitively clearer where tau is used instead of pi, particularly in cases where a factor of 2 is thus not canceled with another, unrelated factor.^{[17]}^{[18]}^{[19]}

## Examples of use[edit]

- As an angular unit, the turn or revolution is particularly useful for large angles, such as in connection with electromagnetic coils and rotating objects. See also winding number.
- The angular speed of rotating machinery, such as automobile engines, is commonly measured in revolutions per minute or RPM.
- Turn is used in complex dynamics for measure of external and internal angles. The sum of external angles of a polygon equals one turn. Angle doubling map is used.
- Pie charts illustrate proportions of a whole as fractions of a turn. Each one percent is shown as an angle of one centiturn.

## Kinematics of turns[edit]

In kinematics, a **turn** is a rotation less than a full revolution. A turn may be represented in a mathematical model that uses expressions of complex numbers or quaternions; in the complex plane every non-zero number has a polar coordinate expression *z* = *r* cis(*a*) = *r* cos(*a*) + *r*i sin(*a*) where *r* > 0 and *a* is in [0, 2π). A turn of the complex plane arises from multiplying *z* = *x* + i*y* by an element *u* = e^{bi} that lies on the unit circle:

*z*↦*uz*.

Frank Morley consistently referred to elements of the unit circle as *turns* in the book *Inversive Geometry*, (1933) which he coauthored with his son Frank Vigor Morley.^{[20]}

The Latin term for *turn* is versor, which is a quaternion that can be visualized as an arc of a great circle. The product of two versors can be compared to a spherical triangle where two sides add to the third, for the kinematics of rotation in three dimensions, see quaternions and spatial rotation.

## See also[edit]

- Angle of rotation
- Revolutions per minute
- Repeating circle
- Spat (unit)
- Unit interval
- Turn (rational trigonometry)
- Spread

## Notes and references[edit]

**^**"ooPIC Programmer's Guide". www.oopic.com.**^**Hargreaves, Shawn. "Angles, integers, and modulo arithmetic". blogs.msdn.com.**^**"Half Turn, Reflection in Point". cut-the-knot.org.**^**Beckmann, Petr (1989).*A History of Pi*. Barnes & Noble Publishing.**^**Schwartzman, Steven (1994).*The Words of Mathematics: An Etymological Dictionary of Mathematical Terms Used in English*. The Mathematical Association of America. p. 165.**^**"Pi through the ages".**^**Croxton, Frederick E. (1922). "A Percentage Protractor".*Journal of the American Statistical Association*.**18**: 108–109. doi:10.1080/01621459.1922.10502455.**^**Hoyle, Fred (1962).*Astronomy*. London: Macdonald.**^**German, Sigmar; Drath, Peter (2013-03-13) [1979].*Handbuch SI-Einheiten: Definition, Realisierung, Bewahrung und Weitergabe der SI-Einheiten, Grundlagen der Präzisionsmeßtechnik*(in German) (1 ed.). Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, reprint: Springer-Verlag. ISBN 3322836061. 978-3-528-08441-7, 9783322836069. Retrieved 2015-08-14.**^**Kurzweil, Peter (2013-03-09) [1999].*Das Vieweg Einheiten-Lexikon: Formeln und Begriffe aus Physik, Chemie und Technik*(in German) (1 ed.). Vieweg, reprint: Springer-Verlag. ISBN 3322929205. doi:10.1007/978-3-322-92920-4. 978-3-322-92921-1. Retrieved 2015-08-14.**^**http://www.hpmuseum.org/forum/thread-4783-post-55836.html#pid55836**^**https://www.python.org/dev/peps/pep-0628/**^**Sequence A019692**^**Eagle, Albert (1958).*The Elliptic Functions as They Should Be: An Account, with Applications, of the Functions in a New Canonical Form*. Cambridge, England: Galloway and Porter. pp. ix–x.**^**Palais, Robert (2001). "Pi is Wrong" (PDF).*The Mathematical Intelligencer*. New York, USA: Springer-Verlag.**23**(3): 7–8. doi:10.1007/bf03026846.**^**Hartl, Michael (2013-03-14). "The Tau Manifesto". Retrieved 2013-09-14.**^**Aron, Jacob (2011-01-08). "Interview: Michael Hartl: It's time to kill off pi".*New Scientist*.**209**(2794): 23. Bibcode:2011NewSc.209...23A. doi:10.1016/S0262-4079(11)60036-5.**^**Landau, Elizabeth (2011-03-14). "On Pi Day, is 'pi' under attack?".*cnn.com*.**^**"Why Tau Trumps Pi".*Scientific American*. 2014-06-25. Retrieved 2015-03-20.**^**Morley, Frank; Morley, Frank Vigor (2014) [1933].*Inversive Geometry*. Boston, USA; New York, USA: Ginn and Company, reprint: Courier Corporation, Dover Publications. ISBN 978-0-486-49339-8. 0-486-49339-3. Retrieved 2015-10-17.

## External links[edit]

- Palais, Robert (2001). "Pi is Wrong" (PDF).
*The Mathematical Intelligencer*. New York, USA: Springer-Verlag.**23**(3): 7–8. doi:10.1007/bf03026846.