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 ...
, the polar coordinate system is a
two-dimensional coordinate system in which each
point on a
plane is determined by a
distance
Distance is a numerical or occasionally qualitative measurement of how far apart objects or points are. In physics or everyday usage, distance may refer to a physical length or an estimation based on other criteria (e.g. "two counties over"). ...
from a reference point and an
angle
In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle.
Angles formed by two rays lie in the plane that contains the rays. Angles ...
from a reference direction. The reference point (analogous to the origin of a
Cartesian coordinate system) is called the ''pole'', and the
ray
Ray may refer to:
Fish
* Ray (fish), any cartilaginous fish of the superorder Batoidea
* Ray (fish fin anatomy), a bony or horny spine on a fin
Science and mathematics
* Ray (geometry), half of a line proceeding from an initial point
* Ray (gr ...
from the pole in the reference direction is the ''polar axis''. The distance from the pole is called the ''radial coordinate'', ''radial distance'' or simply ''radius'', and the angle is called the ''angular coordinate'', ''polar angle'', or ''
azimuth
An azimuth (; from ar, اَلسُّمُوت, as-sumūt, the directions) is an angular measurement in a spherical coordinate system. More specifically, it is the horizontal angle from a cardinal direction, most commonly north.
Mathematical ...
''.
Angles in polar notation are generally expressed in either
degrees or
radian
The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. The unit was formerly an SI supplementary unit (before that ...
s (2
rad being equal to 360°).
Grégoire de Saint-Vincent and
Bonaventura Cavalieri independently introduced the concepts in the mid-17th century, though the actual term "polar coordinates" has been attributed to
Gregorio Fontana in the 18th century. The initial motivation for the introduction of the polar system was the study of
circular and
orbital motion.
Polar coordinates are most appropriate in any context where the phenomenon being considered is inherently tied to direction and length from a center point in a plane, such as
spirals. Planar physical systems with bodies moving around a central point, or phenomena originating from a central point, are often simpler and more intuitive to model using polar coordinates.
The polar coordinate system is extended to three dimensions in two ways: the
cylindrical and
spherical coordinate systems.
History
The concepts of angle and radius were already used by ancient peoples of the first millennium
BC. The
Greek astronomer and
astrologer Hipparchus
Hipparchus (; el, Ἵππαρχος, ''Hipparkhos''; BC) was a Greek astronomer, geographer, and mathematician. He is considered the founder of trigonometry, but is most famous for his incidental discovery of the precession of the e ...
(190–120 BC) created a table of
chord functions giving the length of the chord for each angle, and there are references to his using polar coordinates in establishing stellar positions. In ''
On Spirals'',
Archimedes describes the
Archimedean spiral, a function whose radius depends on the angle. The Greek work, however, did not extend to a full coordinate system.
From the 8th century AD onward, astronomers developed methods for approximating and calculating the direction to
Mecca
Mecca (; officially Makkah al-Mukarramah, commonly shortened to Makkah ()) is a city and administrative center of the Mecca Province of Saudi Arabia, and the holiest city in Islam. It is inland from Jeddah on the Red Sea, in a narrow v ...
(
qibla)—and its distance—from any location on the Earth. From the 9th century onward they were using
spherical trigonometry and
map projection
In cartography, map projection is the term used to describe a broad set of transformations employed to represent the two-dimensional curved surface of a globe on a plane. In a map projection, coordinates, often expressed as latitude and l ...
methods to determine these quantities accurately. The calculation is essentially the conversion of the
equatorial polar coordinates of Mecca (i.e. its
longitude
Longitude (, ) is a geographic coordinate that specifies the east– west position of a point on the surface of the Earth, or another celestial body. It is an angular measurement, usually expressed in degrees and denoted by the Greek let ...
and
latitude
In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north ...
) to its polar coordinates (i.e. its qibla and distance) relative to a system whose reference meridian is the
great circle through the given location and the Earth's poles and whose polar axis is the line through the location and its
antipodal point.
There are various accounts of the introduction of polar coordinates as part of a formal coordinate system. The full history of the subject is described in
Harvard professor
Julian Lowell Coolidge's ''Origin of Polar Coordinates.''
Grégoire de Saint-Vincent and Bonaventura Cavalieri independently introduced the concepts in the mid-seventeenth century. Saint-Vincent wrote about them privately in 1625 and published his work in 1647, while Cavalieri published his in 1635 with a corrected version appearing in 1653. Cavalieri first used polar coordinates to solve a problem relating to the area within an
Archimedean spiral.
Blaise Pascal subsequently used polar coordinates to calculate the length of
parabolic arcs.
In ''
Method of Fluxions'' (written 1671, published 1736), Sir
Isaac Newton
Sir Isaac Newton (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, alchemist, Theology, theologian, and author (described in his time as a "natural philosophy, natural philosopher"), widely ...
examined the transformations between polar coordinates, which he referred to as the "Seventh Manner; For Spirals", and nine other coordinate systems. In the journal ''
Acta Eruditorum'' (1691),
Jacob Bernoulli used a system with a point on a line, called the ''pole'' and ''polar axis'' respectively. Coordinates were specified by the distance from the pole and the angle from the ''polar axis''. Bernoulli's work extended to finding the
radius of curvature of curves expressed in these coordinates.
The actual term ''polar coordinates'' has been attributed to
Gregorio Fontana and was used by 18th-century Italian writers. The term appeared in
English
English usually refers to:
* English language
* English people
English may also refer to:
Peoples, culture, and language
* ''English'', an adjective for something of, from, or related to England
** English national ...
in
George Peacock's 1816 translation of
Lacroix La Croix primarily refers to:
* ''La Croix'' (newspaper), a French Catholic newspaper
* La Croix Sparkling Water, a beverage distributed by the National Beverage Corporation
La Croix or Lacroix may also refer to:
Places
* Lacroix-Barrez, a muni ...
's ''Differential and Integral Calculus''.
Alexis Clairaut was the first to think of polar coordinates in three dimensions, and
Leonhard Euler
Leonhard Euler ( , ; 15 April 170718 September 1783) was a Swiss mathematician, physicist, astronomer, geographer, logician and engineer who founded the studies of graph theory and topology and made pioneering and influential discoveries ...
was the first to actually develop them.
Conventions
The radial coordinate is often denoted by ''r'' or
''ρ'', and the angular coordinate by
''φ'',
''θ'', or ''t''. The angular coordinate is specified as ''φ'' by
ISO standard
31-11. However, in mathematical literature the angle is often denoted by θ instead.
Angles in polar notation are generally expressed in either
degrees or
radian
The radian, denoted by the symbol rad, is the unit of angle in the International System of Units (SI) and is the standard unit of angular measure used in many areas of mathematics. The unit was formerly an SI supplementary unit (before that ...
s (2
rad being equal to 360°). Degrees are traditionally used in
navigation
Navigation is a field of study that focuses on the process of monitoring and controlling the movement of a craft or vehicle from one place to another.Bowditch, 2003:799. The field of navigation includes four general categories: land navigation ...
,
surveying
Surveying or land surveying is the technique, profession, art, and science of determining the terrestrial two-dimensional or three-dimensional positions of points and the distances and angles between them. A land surveying professional is ...
, and many applied disciplines, while radians are more common in mathematics and mathematical
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
.
The angle ''φ'' is defined to start at 0° from a ''reference direction'', and to increase for rotations in either
clockwise (cw) or counterclockwise (ccw) orientation. For example, in mathematics, the reference direction is usually drawn as a
ray
Ray may refer to:
Fish
* Ray (fish), any cartilaginous fish of the superorder Batoidea
* Ray (fish fin anatomy), a bony or horny spine on a fin
Science and mathematics
* Ray (geometry), half of a line proceeding from an initial point
* Ray (gr ...
from the pole horizontally to the right, and the polar angle increases to positive angles for ccw rotations, whereas in navigation (
bearing,
heading) the 0°-heading is drawn vertically upwards and the angle increases for cw rotations. The polar angles decrease towards negative values for rotations in the respectively opposite orientations.
Uniqueness of polar coordinates
Adding any number of full
turns (360°) to the angular coordinate does not change the corresponding direction. Similarly, any polar coordinate is identical to the coordinate with the negative radial component and the opposite direction (adding 180° to the polar angle). Therefore, the same point (''r'', ''φ'') can be expressed with an infinite number of different polar coordinates and , where ''n'' is an arbitrary
integer
An integer is the number zero (), a positive natural number (, , , etc.) or a negative integer with a minus sign ( −1, −2, −3, etc.). The negative numbers are the additive inverses of the corresponding positive numbers. In the languag ...
. Moreover, the pole itself can be expressed as (0, ''φ'') for any angle ''φ''.
Where a unique representation is needed for any point besides the pole, it is usual to limit ''r'' to positive numbers () and ''φ'' to either the
interval or the interval , which in radians are or . Another convention, in reference to the usual codomain of the
arctan function, is to allow for arbitrary nonzero real values of the radial component and restrict the polar angle to . In all cases a unique azimuth for the pole (''r'' = 0) must be chosen, e.g., ''φ'' = 0.
Converting between polar and Cartesian coordinates
The polar coordinates ''r'' and ''φ'' can be converted to the Cartesian coordinates ''x'' and ''y'' by using the
trigonometric function
In mathematics, the trigonometric functions (also called circular functions, angle functions or goniometric functions) are real functions which relate an angle of a right-angled triangle to ratios of two side lengths. They are widely used in a ...
s sine and cosine:
The Cartesian coordinates ''x'' and ''y'' can be converted to polar coordinates ''r'' and ''φ'' with ''r'' ≥ 0 and ''φ'' in the interval (−, ] by:
where hypot is the
Pythagorean addition, Pythagorean sum and
atan2
In computing and mathematics, the function atan2 is the 2-argument arctangent. By definition, \theta = \operatorname(y, x) is the angle measure (in radians, with -\pi < \theta \leq \pi) between the positive is a common variation on the
arctangent
In mathematics, the inverse trigonometric functions (occasionally also called arcus functions, antitrigonometric functions or cyclometric functions) are the inverse functions of the trigonometric functions (with suitably restricted domains). Spe ...
function defined as
If ''r'' is calculated first as above, then this formula for ''φ'' may be stated more simply using the
arccosine
In mathematics, the inverse trigonometric functions (occasionally also called arcus functions, antitrigonometric functions or cyclometric functions) are the inverse functions of the trigonometric functions (with suitably restricted domains). S ...
function:
Complex numbers
Every
complex number
In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the fo ...
can be represented as a point in the
complex plane, and can therefore be expressed by specifying either the point's Cartesian coordinates (called rectangular or Cartesian form) or the point's polar coordinates (called polar form). The complex number ''z'' can be represented in rectangular form as
where ''i'' is the
imaginary unit
The imaginary unit or unit imaginary number () is a solution to the quadratic equation x^2+1=0. Although there is no real number with this property, can be used to extend the real numbers to what are called complex numbers, using addition an ...
, or can alternatively be written in polar form as
and from there, by
Euler's formula
Euler's formula, named after Leonhard Euler, is a mathematical formula in complex analysis that establishes the fundamental relationship between the trigonometric functions and the complex exponential function. Euler's formula states that ...
, as
where ''e'' is
Euler's number, and ''φ'', expressed in radians, is the
principal value of the complex number function
arg applied to ''x'' + ''iy''. To convert between the rectangular and polar forms of a complex number, the conversion formulae given above can be used. Equivalent are the and
angle notations:
For the operations of
multiplication,
division,
exponentiation
Exponentiation is a mathematical operation, written as , involving two numbers, the '' base'' and the ''exponent'' or ''power'' , and pronounced as " (raised) to the (power of) ". When is a positive integer, exponentiation corresponds to ...
, and
root extraction
In mathematics, a radicand, also known as an nth root, of a number ''x'' is a number ''r'' which, when raised to the power ''n'', yields ''x'':
:r^n = x,
where ''n'' is a positive integer, sometimes called the ''degree'' of the root. A root ...
of complex numbers, it is generally much simpler to work with complex numbers expressed in polar form rather than rectangular form. From the laws of exponentiation:
; Multiplication:
; Division:
; Exponentiation (
De Moivre's formula):
; Root Extraction (Principal root):
Polar equation of a curve
The equation defining an
algebraic curve expressed in polar coordinates is known as a ''polar equation''. In many cases, such an equation can simply be specified by defining ''r'' as a
function of ''φ''. The resulting curve then consists of points of the form (''r''(''φ''), ''φ'') and can be regarded as the
graph of the polar function ''r''. Note that, in contrast to Cartesian coordinates, the independent variable ''φ'' is the ''second'' entry in the ordered pair.
Different forms of
symmetry
Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definiti ...
can be deduced from the equation of a polar function ''r'':
* If the curve will be symmetrical about the horizontal (0°/180°) ray;
* If it will be symmetric about the vertical (90°/270°) ray:
* If it will be
rotationally symmetric by α clockwise and counterclockwise about the pole.
Because of the circular nature of the polar coordinate system, many curves can be described by a rather simple polar equation, whereas their Cartesian form is much more intricate. Among the best known of these curves are the
polar rose,
Archimedean spiral,
lemniscate
In algebraic geometry, a lemniscate is any of several figure-eight or -shaped curves. The word comes from the Latin "''lēmniscātus''" meaning "decorated with ribbons", from the Greek λημνίσκος meaning "ribbons",. or which alternative ...
,
limaçon, and
cardioid.
For the circle, line, and polar rose below, it is understood that there are no restrictions on the domain and range of the curve.
Circle
The general equation for a circle with a center at
and radius ''a'' is
This can be simplified in various ways, to conform to more specific cases, such as the equation
for a circle with a center at the pole and radius ''a''.
When or the origin lies on the circle, the equation becomes
In the general case, the equation can be solved for , giving
The solution with a minus sign in front of the square root gives the same curve.
Line
''Radial'' lines (those running through the pole) are represented by the equation
where
is the angle of elevation of the line; that is,
, where
is the
slope
In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is use ...
of the line in the Cartesian coordinate system. The non-radial line that crosses the radial line
perpendicular
In elementary geometry, two geometric objects are perpendicular if they intersect at a right angle (90 degrees or π/2 radians). The condition of perpendicularity may be represented graphically using the '' perpendicular symbol'', ⟂. It c ...
ly at the point
has the equation
Otherwise stated
is the point in which the tangent intersects the imaginary circle of radius
Polar rose
A
polar rose is a mathematical curve that looks like a petaled flower, and that can be expressed as a simple polar equation,
for any constant γ
0 (including 0). If ''k'' is an integer, these equations will produce a ''k''-petaled rose if ''k'' is
odd
Odd means unpaired, occasional, strange or unusual, or a person who is viewed as eccentric.
Odd may also refer to:
Acronym
* ODD (Text Encoding Initiative) ("One Document Does it all"), an abstracted literate-programming format for describing X ...
, or a 2''k''-petaled rose if ''k'' is even. If ''k'' is rational, but not an integer, a rose-like shape may form but with overlapping petals. Note that these equations never define a rose with 2, 6, 10, 14, etc. petals. The
variable ''a'' directly represents the length or amplitude of the petals of the rose, while ''k'' relates to their spatial frequency. The constant γ
0 can be regarded as a phase angle.
Archimedean spiral
The
Archimedean spiral is a spiral discovered by
Archimedes which can also be expressed as a simple polar equation. It is represented by the equation
Changing the parameter ''a'' will turn the spiral, while ''b'' controls the distance between the arms, which for a given spiral is always constant. The Archimedean spiral has two arms, one for and one for . The two arms are smoothly connected at the pole. If , taking the mirror image of one arm across the 90°/270° line will yield the other arm. This curve is notable as one of the first curves, after the
conic section
In mathematics, a conic section, quadratic curve or conic is a curve obtained as the intersection of the surface of a cone with a plane. The three types of conic section are the hyperbola, the parabola, and the ellipse; the circle is a ...
s, to be described in a mathematical treatise, and as a prime example of a curve best defined by a polar equation.
Conic sections
A
conic section
In mathematics, a conic section, quadratic curve or conic is a curve obtained as the intersection of the surface of a cone with a plane. The three types of conic section are the hyperbola, the parabola, and the ellipse; the circle is a ...
with one focus on the pole and the other somewhere on the 0° ray (so that the conic's
major axis
In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the ...
lies along the polar axis) is given by:
where ''e'' is the
eccentricity and
is the
semi-latus rectum
In mathematics, a conic section, quadratic curve or conic is a curve obtained as the intersection of the surface of a cone with a plane. The three types of conic section are the hyperbola, the parabola, and the ellipse; the circle is a sp ...
(the perpendicular distance at a focus from the major axis to the curve). If , this equation defines a
hyperbola
In mathematics, a hyperbola (; pl. hyperbolas or hyperbolae ; adj. hyperbolic ) is a type of smooth curve lying in a plane, defined by its geometric properties or by equations for which it is the solution set. A hyperbola has two pieces, ca ...
; if , it defines a
parabola
In mathematics, a parabola is a plane curve which is mirror-symmetrical and is approximately U-shaped. It fits several superficially different mathematical descriptions, which can all be proved to define exactly the same curves.
One descri ...
; and if , it defines an
ellipse. The special case of the latter results in a circle of the radius
.
Intersection of two polar curves
The graphs of two polar functions
and
have possible intersections of three types:
# In the origin, if the equations
and
have at least one solution each.
# All the points