In
mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, a spiral is a
curve
In mathematics, a curve (also called a curved line in older texts) is an object similar to a line (geometry), line, but that does not have to be Linearity, straight.
Intuitively, a curve may be thought of as the trace left by a moving point (ge ...
which emanates from a point, moving farther away as it revolves around the point.
Helices
Two major definitions of "spiral" in the
American Heritage Dictionary
American(s) may refer to:
* American, something of, from, or related to the United States of America, commonly known as the "United States" or "America"
** Americans, citizens and nationals of the United States of America
** American ancestry, pe ...
are:
[Spiral]
''American Heritage Dictionary of the English Language'', Houghton Mifflin Company, Fourth Edition, 2009.
# a curve on a plane that winds around a fixed center point at a continuously increasing or decreasing distance from the point.
# a three-dimensional curve that turns around an axis at a constant or continuously varying distance while moving parallel to the axis; a
helix
A helix () is a shape like a corkscrew or spiral staircase. It is a type of smooth space curve with tangent lines at a constant angle to a fixed axis. Helices are important in biology, as the DNA molecule is formed as two intertwined helices, ...
.
The first definition describes a
planar
Planar is an adjective meaning "relating to a plane (geometry)".
Planar may also refer to:
Science and technology
* Planar (computer graphics), computer graphics pixel information from several bitplanes
* Planar (transmission line technologies), ...
curve, that extends in both of the perpendicular directions within its plane; the groove on one side of a
record closely approximates a plane spiral (and it is by the finite width and depth of the groove, but ''not'' by the wider spacing between than within tracks, that it falls short of being a perfect example); note that successive loops ''differ'' in diameter. In another example, the "center lines" of the arms of a
spiral galaxy
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''[logarithmic spiral
A logarithmic spiral, equiangular spiral, or growth spiral is a self-similar spiral curve that often appears in nature. The first to describe a logarithmic spiral was Albrecht Dürer (1525) who called it an "eternal line" ("ewige Linie"). Mor ...]
s.
The second definition includes two kinds of 3-dimensional relatives of spirals:
# a conical or
volute spring
A volute spring, also known as a conical spring, is a compression spring in the form of a cone (somewhat like the classical volute decorative architectural ornament). Under compression, the coils slide past each other, thus enabling the spring to ...
(including the spring used to hold and make contact with the negative terminals of AA or AAA batteries in a
battery box), and the
vortex
In fluid dynamics, a vortex ( : vortices or vortexes) is a region in a fluid in which the flow revolves around an axis line, which may be straight or curved. Vortices form in stirred fluids, and may be observed in smoke rings, whirlpools in th ...
that is created when water is draining in a sink is often described as a spiral, or as a conical helix.
# quite explicitly, definition 2 also includes a cylindrical coil spring and a strand of
DNA, both of which are quite helical, so that "helix" is a more ''useful'' description than "spiral" for each of them; in general, "spiral" is seldom applied if successive "loops" of a curve have the same diameter.
[
In the side picture, the black curve at the bottom is an ]Archimedean spiral
The Archimedean spiral (also known as the arithmetic spiral) is a spiral named after the 3rd-century BC Greek mathematician Archimedes. It is the locus corresponding to the locations over time of a point moving away from a fixed point with a con ...
, while the green curve is a helix. The curve shown in red is a conic helix.
Two-dimensional
A two-dimensional
In mathematics, a plane is a Euclidean (flat), two-dimensional surface that extends indefinitely. A plane is the two-dimensional analogue of a point (zero dimensions), a line (one dimension) and three-dimensional space. Planes can arise as s ...
, or plane, spiral may be described most easily using polar coordinates
In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (analogous to the or ...
, where the radius
In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...
is a monotonic
In mathematics, a monotonic function (or monotone function) is a function between ordered sets that preserves or reverses the given order. This concept first arose in calculus, and was later generalized to the more abstract setting of order ...
continuous function
In mathematics, a continuous function is a function such that a continuous variation (that is a change without jump) of the argument induces a continuous variation of the value of the function. This means that there are no abrupt changes in value ...
of angle :
*
The circle would be regarded as a degenerate
Degeneracy, degenerate, or degeneration may refer to:
Arts and entertainment
* Degenerate (album), ''Degenerate'' (album), a 2010 album by the British band Trigger the Bloodshed
* Degenerate art, a term adopted in the 1920s by the Nazi Party i ...
case (the function
Function or functionality may refer to:
Computing
* Function key, a type of key on computer keyboards
* Function model, a structured representation of processes in a system
* Function object or functor or functionoid, a concept of object-oriente ...
not being strictly monotonic, but rather constant).
In ''--coordinates'' the curve has the parametric representation:
*
Examples
Some of the most important sorts of two-dimensional spirals include:
* The Archimedean spiral
The Archimedean spiral (also known as the arithmetic spiral) is a spiral named after the 3rd-century BC Greek mathematician Archimedes. It is the locus corresponding to the locations over time of a point moving away from a fixed point with a con ...
:
* The hyperbolic spiral
A hyperbolic spiral is a plane curve, which can be described in polar coordinates by the equation
:r=\frac
of a hyperbola. Because it can be generated by a circle inversion of an Archimedean spiral, it is called Reciprocal spiral, too..
Pier ...
:
* Fermat's spiral
A Fermat's spiral or parabolic spiral is a plane curve with the property that the area between any two consecutive full turns around the spiral is invariant. As a result, the distance between turns grows in inverse proportion to their distance ...
:
* The lituus
The word ''lituus'' originally meant a curved augural staff, or a curved war-trumpet in the ancient Latin language. This Latin word continued in use through the 18th century as an alternative to the vernacular names of various musical instruments ...
:
* The logarithmic spiral
A logarithmic spiral, equiangular spiral, or growth spiral is a self-similar spiral curve that often appears in nature. The first to describe a logarithmic spiral was Albrecht Dürer (1525) who called it an "eternal line" ("ewige Linie"). Mor ...
:
* The Cornu spiral
An Euler spiral is a curve whose curvature changes linearly with its curve length (the curvature of a circular curve is equal to the reciprocal of the radius). Euler spirals are also commonly referred to as spiros, clothoids, or Cornu spirals.
Eu ...
or ''clothoid''
* The Fibonacci spiral
Fibonacci (; also , ; – ), also known as Leonardo Bonacci, Leonardo of Pisa, or Leonardo Bigollo Pisano ('Leonardo the Traveller from Pisa'), was an Italian mathematician from the Republic of Pisa, considered to be "the most talented Western ...
and golden spiral
In geometry, a golden spiral is a logarithmic spiral whose growth factor is , the golden ratio. That is, a golden spiral gets wider (or further from its origin) by a factor of for every quarter turn it makes.
Approximations of the golden spira ...
* The Spiral of Theodorus
In geometry, the spiral of Theodorus (also called ''square root spiral'', ''Einstein spiral'', ''Pythagorean spiral'', or ''Pythagoras's snail'') is a spiral composed of right triangles, placed edge-to-edge. It was named after Theodorus of Cyre ...
: an approximation of the Archimedean spiral composed of contiguous right triangles
* The involute
In mathematics, an involute (also known as an evolvent) is a particular type of curve that is dependent on another shape or curve. An involute of a curve is the locus of a point on a piece of taut string as the string is either unwrapped from or ...
of a circle, used twice on each tooth of almost every modern gear
A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called ''cogs''), which mesh with another (compatible) toothed part to transmit (convert) torque and speed. The basic pr ...
Image:Archimedean spiral.svg, Archimedean spiral
Image:Hyperspiral.svg, hyperbolic spiral
Image:Fermat's spiral.svg, Fermat's spiral
Image:Lituus.svg, lituus
Image:Logarithmic Spiral Pylab.svg, logarithmic spiral
Image:Cornu Spiral.svg, Cornu spiral
Image:Spiral of Theodorus.svg, spiral of Theodorus
Image:Fibonacci_spiral.svg, Fibonacci Spiral (golden spiral)
Image:Archimedean-involute-circle-spirals-comparison.svg, The involute of a circle (black) is not identical to the Archimedean spiral (red).
An ''Archimedean spiral'' is, for example, generated while coiling a carpet.
A ''hyperbolic spiral'' appears as image of a helix with a special central projection (see diagram). A hyperbolic spiral is some times called ''reciproke'' spiral, because it is the image of an Archimedean spiral with a circle-inversion (see below).
The name ''logarithmic spiral'' is due to the equation . Approximations of this are found in nature.
Spirals which do not fit into this scheme of the first 5 examples:
A ''Cornu spiral'' has two asymptotic points.
The ''spiral of Theodorus'' is a polygon.
The ''Fibonacci Spiral'' consists of a sequence of circle arcs.
The ''involute of a circle'' looks like an Archimedean, but is not: see Involute#Examples.
Geometric properties
The following considerations are dealing with spirals, which can be described by a polar equation , especially for the cases (Archimedean, hyperbolic, Fermat's, lituus spirals) and the logarithmic spiral .
;Polar slope angle
The angle between the spiral tangent and the corresponding polar circle (see diagram) is called ''angle of the polar slope and the ''polar slope''.
From vector calculus in polar coordinates one gets the formula
:
Hence the slope of the spiral is
*
In case of an ''Archimedean spiral'' () the polar slope is
The ''logarithmic spiral'' is a special case, because of ''constant'' !
;curvature
The curvature of a curve with polar equation is
:
For a spiral with one gets
*
In case of ''(Archimedean spiral)''
.
Only for