|Unit of||Plane angle|
|Symbol||tr or pla|
|1 tr in ...||... is equal to ...|
A turn can be subdivided in many different ways: into half turns, quarter turns, centiturns, milliturns, binary angles, points etc.
Subdivision of turns
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. 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.
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), a rotation through 90° is referred to as a quarter-turn.
The word turn originates via Latin and French from the Greek word τόρνος (tórnos – a lathe).
In 1697, David Gregory used π/ (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 δ/ (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. Euler adopted the symbol with that meaning in 1737, leading to its widespread use.
The German standard DIN 1315 (1974-03) proposed the unit symbol pla (from Latin: plenus angulus "full angle") for turns. 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.
|1/||1||c. 57.3°||c. 63.7g|
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).
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/ turn would be represented as 3/τ rad instead of 3/π rad. Second, τ visually resembles π, whose association with the circle constant is unavoidable. Hartl's Tau Manifesto gives many examples of formulas that are simpler if tau is used instead of pi. However, a rebuttal was given in The Pi Manifesto, stating a variety of reasons tau should not supplant pi. A more recent version of the Tau Manifesto rebutted the rebuttal, and extended the analysis of the surface area and volume of a n-dimensional sphere to show "the geometric significance of π is the result of a mathematical pun". Namely:
- , where and with . 
Examples of use
- 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
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) + ri sin(a) where r > 0 and a is in [0, 2π). A turn of the complex plane arises from multiplying z = x + iy by an element u = ebi that lies on the unit circle:
- z ↦ uz.
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.
- Angle of rotation
- Revolutions per minute
- Repeating circle
- Spat (unit)
- Unit interval
- Turn (rational trigonometry)
Notes and references
- "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) . 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) . Das Vieweg Einheiten-Lexikon: Formeln und Begriffe aus Physik, Chemie und Technik (in German) (1 ed.). Vieweg, reprint: Springer-Verlag. doi:10.1007/978-3-322-92920-4. ISBN 3322929205. 978-3-322-92921-1. Retrieved 2015-08-14.
- 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.
- 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.
- MSC (4 July 2011). "The Pi Manifesto". Retrieved 13 January 2017.
- Hartl, Michael (2015-06-28). "The Tau Manifesto". Retrieved 2017-01-14.
- Morley, Frank; Morley, Frank Vigor (2014) . 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.