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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 ...
, an ellipse is a plane curve surrounding two focal points, such that for all points on the curve, the sum of the two distances to the focal points is a constant. It generalizes a
circle A circle is a shape consisting of all points in a plane that are at a given distance from a given point, the centre. Equivalently, it is the curve traced out by a point that moves in a plane so that its distance from a given point is cons ...
, which is the special type of ellipse in which the two focal points are the same. The elongation of an ellipse is measured by its eccentricity e, a number ranging from e = 0 (the limiting case of a circle) to e = 1 (the limiting case of infinite elongation, no longer an ellipse but 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 ...
). An ellipse has a simple
algebra Algebra () is one of the broad areas of mathematics. Roughly speaking, algebra is the study of mathematical symbols and the rules for manipulating these symbols in formulas; it is a unifying thread of almost all of mathematics. Elementary ...
ic solution for its area, but only approximations for its perimeter (also known as
circumference In geometry, the circumference (from Latin ''circumferens'', meaning "carrying around") is the perimeter of a circle or ellipse. That is, the circumference would be the arc length of the circle, as if it were opened up and straightened out ...
), for which integration is required to obtain an exact solution. Analytically, the equation of a standard ellipse centered at the origin with width 2a and height 2b is: : \frac+\frac = 1 . Assuming a \ge b, the foci are (\pm c, 0) for c = \sqrt. The standard parametric equation is: : (x,y) = (a\cos(t),b\sin(t)) \quad \text \quad 0\leq t\leq 2\pi. Ellipses are the
closed Closed may refer to: Mathematics * Closure (mathematics), a set, along with operations, for which applying those operations on members always results in a member of the set * Closed set, a set which contains all its limit points * Closed interval, ...
type of
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 ...
: a plane curve tracing the intersection of a cone with a plane (see figure). Ellipses have many similarities with the other two forms of conic sections, parabolas and
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 ...
s, both of which are
open Open or OPEN may refer to: Music * Open (band), Australian pop/rock band * The Open (band), English indie rock band * Open (Blues Image album), ''Open'' (Blues Image album), 1969 * Open (Gotthard album), ''Open'' (Gotthard album), 1999 * Open (C ...
and unbounded. An angled cross section of a
cylinder A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an ...
is also an ellipse. An ellipse may also be defined in terms of one focal point and a line outside the ellipse called the directrix: for all points on the ellipse, the ratio between the distance to the focus and the distance to the directrix is a constant. This constant ratio is the above-mentioned eccentricity: : e = \frac = \sqrt. Ellipses are common in
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 ...
,
astronomy Astronomy () is a natural science that studies celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and evolution. Objects of interest include planets, moons, stars, nebulae, g ...
and
engineering Engineering is the use of scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad range of more speciali ...
. For example, the
orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...
of each planet in the
Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
is approximately an ellipse with the Sun at one focus point (more precisely, the focus is the barycenter of the Sunplanet pair). The same is true for moons orbiting planets and all other systems of two astronomical bodies. The shapes of planets and stars are often well described by
ellipsoid An ellipsoid is a surface that may be obtained from a sphere by deforming it by means of directional scalings, or more generally, of an affine transformation. An ellipsoid is a quadric surface;  that is, a surface that may be defined as th ...
s. A circle viewed from a side angle looks like an ellipse: that is, the ellipse is the image of a circle under
parallel Parallel is a geometric term of location which may refer to: Computing * Parallel algorithm * Parallel computing * Parallel metaheuristic * Parallel (software), a UNIX utility for running programs in parallel * Parallel Sysplex, a cluster o ...
or perspective projection. The ellipse is also the simplest Lissajous figure formed when the horizontal and vertical motions are sinusoids with the same frequency: a similar effect leads to elliptical polarization of light in
optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultrav ...
. The name, (, "omission"), was given by
Apollonius of Perga Apollonius of Perga ( grc-gre, Ἀπολλώνιος ὁ Περγαῖος, Apollṓnios ho Pergaîos; la, Apollonius Pergaeus; ) was an Ancient Greek geometer and astronomer known for his work on conic sections. Beginning from the contributio ...
in his ''Conics''.


Definition as locus of points

An ellipse can be defined geometrically as a set or locus of points in the Euclidean plane: : Given two fixed points F_1, F_2 called the foci and a distance 2a which is greater than the distance between the foci, the ellipse is the set of points P such that the sum of the distances , PF_1, ,\ , PF_2, is equal to 2a:E = \left\\ . The midpoint C of the line segment joining the foci is called the ''center'' of the ellipse. The line through the foci is called the ''major axis'', and the line perpendicular to it through the center is the ''minor axis''. The major axis intersects the ellipse at two '' vertices'' V_1,V_2, which have distance a to the center. The distance c of the foci to the center is called the ''focal distance'' or linear eccentricity. The quotient e=\tfrac is the ''eccentricity''. The case F_1=F_2 yields a circle and is included as a special type of ellipse. The equation , PF_2, + , PF_1 , = 2a can be viewed in a different way (see figure): : If c_2 is the circle with center F_2 and radius 2a, then the distance of a point P to the circle c_2 equals the distance to the focus F_1: :: , PF_1, =, Pc_2, . c_2 is called the ''circular directrix'' (related to focus F_2) of the ellipse. This property should not be confused with the definition of an ellipse using a directrix line below. Using
Dandelin spheres In geometry, the Dandelin spheres are one or two spheres that are tangent both to a plane and to a cone that intersects the plane. The intersection of the cone and the plane is a conic section, and the point at which either sphere touches the plan ...
, one can prove that any section of a cone with a plane is an ellipse, assuming the plane does not contain the apex and has slope less than that of the lines on the cone.


In Cartesian coordinates


Standard equation

The standard form of an ellipse in Cartesian coordinates assumes that the origin is the center of the ellipse, the ''x''-axis is the major axis, and: : the foci are the points F_1 = (c,\, 0),\ F_2=(-c,\, 0), : the vertices are V_1 = (a,\, 0),\ V_2 = (-a,\, 0). For an arbitrary point (x,y) the distance to the focus (c,0) is \sqrt and to the other focus \sqrt. Hence the point (x,\, y) is on the ellipse whenever: :\sqrt + \sqrt = 2a\ . Removing the
radicals Radical may refer to: Politics and ideology Politics *Radical politics, the political intent of fundamental societal change *Radicalism (historical), the Radical Movement that began in late 18th century Britain and spread to continental Europe and ...
by suitable squarings and using b^2 = a^2-c^2 (see diagram) produces the standard equation of the ellipse: :\frac + \frac = 1, or, solved for ''y:'' :y = \pm\frac\sqrt = \pm \sqrt. The width and height parameters a,\; b are called the semi-major and semi-minor axes. The top and bottom points V_3 = (0,\, b),\; V_4 = (0,\, -b) are the ''co-vertices''. The distances from a point (x,\, y) on the ellipse to the left and right foci are a + ex and a - ex. It follows from the equation that the ellipse is ''symmetric'' with respect to the coordinate axes and hence with respect to the origin.


Parameters


Principal axes

Throughout this article, the semi-major and semi-minor axes are denoted a and b, respectively, i.e. a \ge b > 0 \ . In principle, the canonical ellipse equation \tfrac + \tfrac = 1 may have a < b (and hence the ellipse would be taller than it is wide). This form can be converted to the standard form by transposing the variable names x and y and the parameter names a and b.


Linear eccentricity

This is the distance from the center to a focus: c = \sqrt.


Eccentricity

The eccentricity can be expressed as: : e = \frac = \sqrt, assuming a > b. An ellipse with equal axes (a = b) has zero eccentricity, and is a circle.


Semi-latus rectum

The length of the chord through one focus, perpendicular to the major axis, is called the ''latus rectum''. One half of it is the ''semi-latus rectum'' \ell. A calculation shows: : \ell = \fraca = a \left(1 - e^2\right). The semi-latus rectum \ell is equal to the radius of curvature at the vertices (see section
curvature In mathematics, curvature is any of several strongly related concepts in geometry. Intuitively, the curvature is the amount by which a curve deviates from being a straight line, or a surface deviates from being a plane. For curves, the can ...
).


Tangent

An arbitrary line g intersects an ellipse at 0, 1, or 2 points, respectively called an ''exterior line'', ''tangent'' and ''secant''. Through any point of an ellipse there is a unique tangent. The tangent at a point (x_1,\, y_1) of the ellipse \tfrac + \tfrac = 1 has the coordinate equation: :\fracx + \fracy = 1. A vector parametric equation of the tangent is: : \vec x = \beginx_1 \\ y_1\end + s\begin \;\! -y_1 a^2 \\ \;\ \ \ x_1 b^2 \end\ with \ s \in \mathbb\ . Proof: Let (x_1,\, y_1) be a point on an ellipse and \vec = \beginx_1 \\ y_1\end + s\beginu \\ v\end be the equation of any line g containing (x_1,\, y_1). Inserting the line's equation into the ellipse equation and respecting \frac + \frac = 1 yields: : \frac + \frac = 1\ \quad\Longrightarrow\quad 2s\left(\frac + \frac\right) + s^2\left(\frac + \frac\right) = 0\ . There are then cases: # \fracu + \fracv = 0. Then line g and the ellipse have only point (x_1,\, y_1) in common, and g is a tangent. The tangent direction has perpendicular vector \begin\frac & \frac\end, so the tangent line has equation \fracx + \tfracy = k for some k. Because (x_1,\, y_1) is on the tangent and the ellipse, one obtains k = 1. # \fracu + \fracv \ne 0. Then line g has a second point in common with the ellipse, and is a secant. Using (1) one finds that \begin -y_1a^2 & x_1b^2 \end is a tangent vector at point (x_1,\, y_1), which proves the vector equation. If (x_1, y_1) and (u, v) are two points of the ellipse such that \frac + \tfrac = 0, then the points lie on two ''conjugate diameters'' (see
below Below may refer to: *Earth * Ground (disambiguation) *Soil *Floor * Bottom (disambiguation) *Less than *Temperatures below freezing *Hell or underworld People with the surname *Ernst von Below (1863–1955), German World War I general *Fred Below ...
). (If a = b, the ellipse is a circle and "conjugate" means "orthogonal".)


Shifted ellipse

If the standard ellipse is shifted to have center \left(x_\circ,\, y_\circ\right), its equation is : \frac + \frac = 1 \ . The axes are still parallel to the x- and y-axes.


General ellipse

In
analytic geometry In classical mathematics, analytic geometry, also known as coordinate geometry or Cartesian geometry, is the study of geometry using a coordinate system. This contrasts with synthetic geometry. Analytic geometry is used in physics and enginee ...
, the ellipse is defined as a quadric: the set of points (X,\, Y) of the Cartesian plane that, in non-degenerate cases, satisfy the
implicit Implicit may refer to: Mathematics * Implicit function * Implicit function theorem * Implicit curve * Implicit surface * Implicit differential equation Other uses * Implicit assumption, in logic * Implicit-association test, in social psycholog ...
equation : AX^2 + B X Y + C Y^2 + D X + E Y + F = 0 provided B^2 - 4AC < 0. To distinguish the degenerate cases from the non-degenerate case, let ''∆'' be the
determinant In mathematics, the determinant is a scalar value that is a function of the entries of a square matrix. It characterizes some properties of the matrix and the linear map represented by the matrix. In particular, the determinant is nonzero if a ...
:\Delta = \begin A & \fracB & \fracD \\ \fracB & C & \fracE \\ \fracD & \fracE & F \end = \left(AC - \frac\right) F + \frac - \frac - \frac. Then the ellipse is a non-degenerate real ellipse if and only if ''C∆'' < 0. If ''C∆'' > 0, we have an imaginary ellipse, and if ''∆'' = 0, we have a point ellipse.Lawrence, J. Dennis, ''A Catalog of Special Plane Curves'', Dover Publ., 1972. The general equation's coefficients can be obtained from known semi-major axis a, semi-minor axis b, center coordinates \left(x_\circ,\, y_\circ\right), and rotation angle \theta (the angle from the positive horizontal axis to the ellipse's major axis) using the formulae: :\begin A &= a^2 \sin^2\theta + b^2 \cos^2\theta \\ B &= 2\left(b^2 - a^2\right) \sin\theta \cos\theta \\ C &= a^2 \cos^2\theta + b^2 \sin^2\theta \\ D &= -2A x_\circ - B y_\circ \\ E &= - B x_\circ - 2C y_\circ \\ F &= A x_\circ^2 + B x_\circ y_\circ + C y_\circ^2 - a^2 b^2. \end These expressions can be derived from the canonical equation \tfrac + \tfrac = 1 by an affine transformation of the coordinates (x,\, y): :\begin x &= \left(X - x_\circ\right) \cos\theta + \left(Y - y_\circ\right) \sin\theta \\ y &= -\left(X - x_\circ\right) \sin\theta + \left(Y - y_\circ\right) \cos\theta. \end Conversely, the canonical form parameters can be obtained from the general form coefficients by the equations: :\begin a, b &= \frac \\ x_\circ &= \frac \\ pt y_\circ &= \frac \\ pt \theta &= \begin \arctan\left(\frac\left(C - A - \sqrt\right)\right) & \text B \ne 0 \\ 0 & \text B = 0,\ A < C \\ 90^\circ & \text B = 0,\ A > C \\ \end \end


Parametric representation


Standard parametric representation

Using
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, a parametric representation of the standard ellipse \tfrac+\tfrac = 1 is: : (x,\, y) = (a \cos t,\, b \sin t),\ 0 \le t < 2\pi\ . The parameter ''t'' (called the '' eccentric anomaly'' in astronomy) is not the angle of (x(t),y(t)) with the ''x''-axis, but has a geometric meaning due to Philippe de La Hire (see '' Drawing ellipses'' below).


Rational representation

With the substitution u = \tan\left(\frac\right) and trigonometric formulae one obtains :\cos t = \frac\ ,\quad \sin t = \frac and the ''rational'' parametric equation of an ellipse : \begin x(u) &= a\frac \\ 0mu y(u) &= b\frac \end\;,\quad -\infty < u < \infty\;, which covers any point of the ellipse \tfrac + \tfrac = 1 except the left vertex (-a,\, 0). For u \in ,\, 1 this formula represents the right upper quarter of the ellipse moving counter-clockwise with increasing u. The left vertex is the limit \lim_ (x(u),\, y(u)) = (-a,\, 0)\;. Alternately, if the parameter :v/math> is considered to be a point on the
real projective line In geometry, a real projective line is a projective line over the real numbers. It is an extension of the usual concept of a line that has been historically introduced to solve a problem set by visual perspective: two parallel lines do not int ...
\mathbf(\mathbf), then the corresponding rational parametrization is : :v\mapsto \left(a\frac, b\frac \right). Then :0\mapsto (-a,\, 0). Rational representations of conic sections are commonly used in
computer-aided design Computer-aided design (CAD) is the use of computers (or ) to aid in the creation, modification, analysis, or optimization of a design. This software is used to increase the productivity of the designer, improve the quality of design, improve co ...
(see Bezier curve).


Tangent slope as parameter

A parametric representation, which uses the slope m of the tangent at a point of the ellipse can be obtained from the derivative of the standard representation \vec x(t) = (a \cos t,\, b \sin t)^\mathsf: :\vec x'(t) = (-a\sin t,\, b\cos t)^\mathsf \quad \rightarrow \quad m = -\frac\cot t\quad \rightarrow \quad \cot t = -\frac. With help of trigonometric formulae one obtains: :\cos t = \frac = \frac\ ,\quad\quad \sin t = \frac = \frac. Replacing \cos t and \sin t of the standard representation yields: : \vec c_\pm(m) = \left(-\frac,\;\frac\right),\, m \in \R. Here m is the slope of the tangent at the corresponding ellipse point, \vec c_+ is the upper and \vec c_- the lower half of the ellipse. The vertices(\pm a,\, 0), having vertical tangents, are not covered by the representation. The equation of the tangent at point \vec c_\pm(m) has the form y = mx + n. The still unknown n can be determined by inserting the coordinates of the corresponding ellipse point \vec c_\pm(m): : y = mx \pm\sqrt\; . This description of the tangents of an ellipse is an essential tool for the determination of the orthoptic of an ellipse. The orthoptic article contains another proof, without differential calculus and trigonometric formulae.


General ellipse

Another definition of an ellipse uses
affine transformation In Euclidean geometry, an affine transformation or affinity (from the Latin, ''affinis'', "connected with") is a geometric transformation that preserves lines and parallelism, but not necessarily Euclidean distances and angles. More generall ...
s: : Any ''ellipse'' is an affine image of the unit circle with equation x^2 + y^2 = 1. ;Parametric representation An affine transformation of the Euclidean plane has the form \vec x \mapsto \vec f\!_0 + A\vec x, where A is a regular
matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** '' The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...
(with non-zero
determinant In mathematics, the determinant is a scalar value that is a function of the entries of a square matrix. It characterizes some properties of the matrix and the linear map represented by the matrix. In particular, the determinant is nonzero if a ...
) and \vec f\!_0 is an arbitrary vector. If \vec f\!_1, \vec f\!_2 are the column vectors of the matrix A, the unit circle (\cos(t), \sin(t)), 0 \leq t \leq 2\pi, is mapped onto the ellipse: : \vec x = \vec p(t) = \vec f\!_0 + \vec f\!_1 \cos t + \vec f\!_2 \sin t \ . Here \vec f\!_0 is the center and \vec f\!_1,\; \vec f\!_2 are the directions of two
conjugate diameter In geometry, two diameters of a conic section are said to be conjugate if each chord (geometry), chord parallel (geometry), parallel to one diameter is bisection, bisected by the other diameter. For example, two diameters of a circle are conjugate ...
s, in general not perpendicular. ;Vertices The four vertices of the ellipse are \vec p(t_0),\;\vec p\left(t_0 \pm \tfrac\right),\; \vec p\left(t_0 + \pi\right), for a parameter t = t_0 defined by: : \cot (2t_0) = \frac. (If \vec f\!_1 \cdot \vec f\!_2 = 0, then t_0 = 0.) This is derived as follows. The tangent vector at point \vec p(t) is: : \vec p\,'(t) = -\vec f\!_1\sin t + \vec f\!_2\cos t \ . At a vertex parameter t = t_0, the tangent is perpendicular to the major/minor axes, so: : 0 = \vec p'(t) \cdot \left(\vec p(t) -\vec f\!_0\right) = \left(-\vec f\!_1\sin t + \vec f\!_2\cos t\right) \cdot \left(\vec f\!_1 \cos t + \vec f\!_2 \sin t\right). Expanding and applying the identities \; \cos^2 t -\sin^2 t=\cos 2t,\ \ 2\sin t \cos t = \sin 2t\; gives the equation for t = t_0\; . ;Area From Apollonios theorem (see below) one obtains:
The area of an ellipse \;\vec x = \vec f_0 +\vec f_1 \cos t +\vec f_2 \sin t\; is :A=\pi, \det(\vec f_1, \vec f_2), \ . ;Semiaxes With the abbreviations \; M=\vec f_1^2+\vec f_2^2, \ N = \left, \det(\vec f_1,\vec f_2)\ the statements of Apollonios's theorem can be written as: :a^2+b^2=M, \quad ab=N \ . Solving this nonlinear system for a,b yields the semiaxes: :a=\frac(\sqrt+\sqrt) :b=\frac(\sqrt-\sqrt)\ . ;Implicit representation Solving the parametric representation for \; \cos t,\sin t\; by
Cramer's rule In linear algebra, Cramer's rule is an explicit formula for the solution of a system of linear equations with as many equations as unknowns, valid whenever the system has a unique solution. It expresses the solution in terms of the determinants o ...
and using \;\cos^2t+\sin^2t -1=0\; , one obtains the implicit representation :\det(\vec x\!-\!\vec f\!_0,\vec f\!_2)^2+\det(\vec f\!_1,\vec x\!-\!\vec f\!_0)^2-\det(\vec f\!_1,\vec f\!_2)^2=0. Conversely: If the equation :x^2+2cxy+d^2y^2-e^2=0\ , with \; d^2-c^2 >0 \; , of an ellipse centered at the origin is given, then the two vectors :\vec f_1=,\quad \vec f_2=\frac\ point to two conjugate points and the tools developed above are applicable. ''Example'': For the ellipse with equation \;x^2+2xy+3y^2-1=0\; the vectors are :\vec f_1=,\quad \vec f_2=\frac . ;Rotated Standard ellipse For \vec f_0= ,\;\vec f_1= a ,\;\vec f_2= b one obtains a parametric representation of the standard ellipse rotated by angle \theta: :x=x_\theta(t)=a\cos\theta\cos t-b\sin\theta\sin t\ , :y=y_\theta(t)=a\sin\theta\cos t+b\cos\theta\sin t\ . ;Ellipse in space The definition of an ellipse in this section gives a parametric representation of an arbitrary ellipse, even in space, if one allows \vec f\!_0, \vec f\!_1, \vec f\!_2 to be vectors in space.


Polar forms


Polar form relative to center

In
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 th ...
, with the origin at the center of the ellipse and with the angular coordinate \theta measured from the major axis, the ellipse's equation is : r(\theta) = \frac=\frac where e is the eccentricity, not Euler's number


Polar form relative to focus

If instead we use polar coordinates with the origin at one focus, with the angular coordinate \theta = 0 still measured from the major axis, the ellipse's equation is : r(\theta)=\frac where the sign in the denominator is negative if the reference direction \theta = 0 points towards the center (as illustrated on the right), and positive if that direction points away from the center. In the slightly more general case of an ellipse with one focus at the origin and the other focus at angular coordinate \phi, the polar form is :r(\theta)=\frac. The angle \theta in these formulas is called the true anomaly of the point. The numerator of these formulas 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 ...
\ell=a (1-e^2).


Eccentricity and the directrix property

Each of the two lines parallel to the minor axis, and at a distance of d = \frac = \frac from it, is called a ''directrix'' of the ellipse (see diagram). : For an arbitrary point P of the ellipse, the quotient of the distance to one focus and to the corresponding directrix (see diagram) is equal to the eccentricity: :: \frac = \frac = e = \frac\ . The proof for the pair F_1, l_1 follows from the fact that \left, PF_1\^2 = (x - c)^2 + y^2,\ \left, Pl_1\^2 = \left(x - \tfrac\right)^2 and y^2 = b^2 - \tfracx^2 satisfy the equation :\left, PF_1\^2 - \frac\left, Pl_1\^2 = 0\ . The second case is proven analogously. The converse is also true and can be used to define an ellipse (in a manner similar to the definition of a parabola): : For any point F (focus), any line l (directrix) not through F, and any real number e with 0 < e < 1, the ellipse is the locus of points for which the quotient of the distances to the point and to the line is e, that is: :: E = \left\. The extension to e = 0, which is the eccentricity of a circle, is not allowed in this context in the Euclidean plane. However, one may consider the directrix of a circle to be the line at infinity in the projective plane. (The choice e = 1 yields a parabola, and if e > 1, a hyperbola.) ;Proof Let F = (f,\, 0),\ e > 0, and assume (0,\, 0) is a point on the curve. The directrix l has equation x = -\tfrac. With P = (x,\, y), the relation , PF, ^2 = e^2, Pl, ^2 produces the equations :(x - f)^2 + y^2 = e^2\left(x + \frac\right)^2 = (ex + f)^2 and x^2\left(e^2 - 1\right) + 2xf(1 + e) - y^2 = 0. The substitution p = f(1 + e) yields : x^2\left(e^2 - 1\right) + 2px - y^2 = 0. This is the equation of an ''ellipse'' (e < 1), or a ''parabola'' (e = 1), or a ''hyperbola'' (e > 1). All of these non-degenerate conics have, in common, the origin as a vertex (see diagram). If e < 1, introduce new parameters a,\, b so that 1 - e^2 = \tfrac, \text\ p = \tfrac, and then the equation above becomes :\frac + \frac = 1\ , which is the equation of an ellipse with center (a,\, 0), the ''x''-axis as major axis, and the major/minor semi axis a,\, b. ;Construction of a directrix Because of c\cdot\tfrac=a^2 point L_1 of directrix l_1 (see diagram) and focus F_1 are inverse with respect to the circle inversion at circle x^2+y^2=a^2 (in diagram green). Hence L_1 can be constructed as shown in the diagram. Directrix l_1 is the perpendicular to the main axis at point L_1. ;General ellipse If the focus is F = \left(f_1,\, f_2\right) and the directrix ux + vy + w = 0, one obtains the equation :\left(x - f_1\right)^2 + \left(y - f_2\right)^2 = e^2 \frac\ . (The right side of the equation uses the Hesse normal form of a line to calculate the distance , Pl, .)


Focus-to-focus reflection property

An ellipse possesses the following property: : The normal at a point P bisects the angle between the lines \overline,\, \overline. ; Proof Because the tangent is perpendicular to the normal, the statement is true for the tangent and the supplementary angle of the angle between the lines to the foci (see diagram), too. Let L be the point on the line \overline with the distance 2a to the focus F_2, a is the semi-major axis of the ellipse. Let line w be the bisector of the supplementary angle to the angle between the lines \overline,\, \overline. In order to prove that w is the tangent line at point P, one checks that any point Q on line w which is different from P cannot be on the ellipse. Hence w has only point P in common with the ellipse and is, therefore, the tangent at point P. From the diagram and the triangle inequality one recognizes that 2a = \left, LF_2\ < \left, QF_2\ + \left, QL\ = \left, QF_2\ + \left, QF_1\ holds, which means: \left, QF_2\ + \left, QF_1\ > 2a . The equality \left, QL\ = \left, QF_1\ is true from the
Angle bisector theorem In geometry, the angle bisector theorem is concerned with the relative lengths of the two segments that a triangle's side is divided into by a line that bisects the opposite angle. It equates their relative lengths to the relative lengths of ...
because \frac=\frac and \left, PL\=\left, PF_1\ . But if Q is a point of the ellipse, the sum should be 2a. ; Application The rays from one focus are reflected by the ellipse to the second focus. This property has optical and acoustic applications similar to the reflective property of a parabola (see whispering gallery).


Conjugate diameters


Definition of conjugate diameters

A circle has the following property: : The midpoints of parallel chords lie on a diameter. An affine transformation preserves parallelism and midpoints of line segments, so this property is true for any ellipse. (Note that the parallel chords and the diameter are no longer orthogonal.) ; Definition: Two diameters d_1,\, d_2 of an ellipse are ''conjugate'' if the midpoints of chords parallel to d_1 lie on d_2\ . From the diagram one finds: : Two diameters \overline,\, \overline of an ellipse are conjugate whenever the tangents at P_1 and Q_1 are parallel to \overline. Conjugate diameters in an ellipse generalize orthogonal diameters in a circle. In the parametric equation for a general ellipse given above, : \vec x = \vec p(t) = \vec f\!_0 +\vec f\!_1 \cos t + \vec f\!_2 \sin t, any pair of points \vec p(t),\ \vec p(t + \pi) belong to a diameter, and the pair \vec p\left(t + \tfrac\right),\ \vec p\left(t - \tfrac\right) belong to its conjugate diameter. For the common parametric representation (a\cos t,b\sin t) of the ellipse with equation \tfrac+\tfrac=1 one gets: The points :(x_1,y_1)=(\pm a\cos t,\pm b\sin t)\quad (signs: (+,+) or (-,-) ) :(x_2,y_2)=( a\sin t,\pm b\cos t)\quad (signs: (-,+) or (+,-) ) :are conjugate and :\frac+\frac=0\ . In case of a circle the last equation collapses to x_1x_2+y_1y_2=0\ .


Theorem of Apollonios on conjugate diameters

For an ellipse with semi-axes a,\, b the following is true: : Let c_1 and c_2 be halves of two conjugate diameters (see diagram) then :# c_1^2 + c_2^2 = a^2 + b^2. :# The ''triangle'' O,P_1,P_2 with sides c_1,\, c_2 (see diagram) has the constant area A_\Delta = \fracab, which can be expressed by A_\Delta=\tfrac 1 2 c_2d_1=\tfrac 1 2 c_1c_2\sin\alpha, too. d_1 is the altitude of point P_1 and \alpha the angle between the half diameters. Hence the area of the ellipse (see section metric properties) can be written as A_=\pi ab=\pi c_2d_1=\pi c_1c_2\sin\alpha. :# The parallelogram of tangents adjacent to the given conjugate diameters has the \text_ = 4ab\ . ; Proof: Let the ellipse be in the canonical form with parametric equation : \vec p(t) = (a\cos t,\, b\sin t). The two points \vec c_1 = \vec p(t),\ \vec c_2 = \vec p\left(t + \frac\right) are on conjugate diameters (see previous section). From trigonometric formulae one obtains \vec c_2 = (-a\sin t,\, b\cos t)^\mathsf and : \left, \vec c_1\^2 + \left, \vec c_2\^2 = \cdots = a^2 + b^2\ . The area of the triangle generated by \vec c_1,\, \vec c_2 is : A_\Delta = \frac\det\left(\vec c_1,\, \vec c_2\right) = \cdots = \fracab and from the diagram it can be seen that the area of the parallelogram is 8 times that of A_\Delta. Hence : \text_ = 4ab\ .


Orthogonal tangents

For the ellipse \tfrac+\tfrac=1 the intersection points of ''orthogonal'' tangents lie on the circle x^2+y^2=a^2+b^2. This circle is called ''orthoptic'' or
director circle In geometry, the director circle of an ellipse or hyperbola (also called the orthoptic circle or Fermat–Apollonius circle) is a circle consisting of all points where two perpendicular tangent lines to the ellipse or hyperbola cross each oth ...
of the ellipse (not to be confused with the circular directrix defined above).


Drawing ellipses

Ellipses appear in descriptive geometry as images (parallel or central projection) of circles. There exist various tools to draw an ellipse. Computers provide the fastest and most accurate method for drawing an ellipse. However, technical tools ('' ellipsographs'') to draw an ellipse without a computer exist. The principle of ellipsographs were known to Greek mathematicians such as Archimedes and
Proklos Proclus Lycius (; 8 February 412 – 17 April 485), called Proclus the Successor ( grc-gre, Πρόκλος ὁ Διάδοχος, ''Próklos ho Diádokhos''), was a Greek Neoplatonist philosopher, one of the last major classical philosoph ...
. If there is no ellipsograph available, one can draw an ellipse using an approximation by the four osculating circles at the vertices. For any method described below, knowledge of the axes and the semi-axes is necessary (or equivalently: the foci and the semi-major axis). If this presumption is not fulfilled one has to know at least two conjugate diameters. With help of
Rytz's construction The Rytz’s axis construction is a basic method of descriptive geometry to find the axes, the semi-major axis and semi-minor axis and the vertices of an ellipse, starting from two conjugated half-diameters. If the center and the semi axis of ...
the axes and semi-axes can be retrieved.


de La Hire's point construction

The following construction of single points of an ellipse is due to de La Hire. It is based on the standard parametric representation (a\cos t,\, b\sin t) of an ellipse: # Draw the two ''circles'' centered at the center of the ellipse with radii a,b and the axes of the ellipse. # Draw a ''line through the center'', which intersects the two circles at point A and B, respectively. # Draw a ''line'' through A that is parallel to the minor axis and a ''line'' through B that is parallel to the major axis. These lines meet at an ellipse point (see diagram). # Repeat steps (2) and (3) with different lines through the center. Elliko-sk.svg, de La Hire's method Parametric ellipse.gif, Animation of the method


Pins-and-string method

The characterization of an ellipse as the locus of points so that sum of the distances to the foci is constant leads to a method of drawing one using two drawing pins, a length of string, and a pencil. In this method, pins are pushed into the paper at two points, which become the ellipse's foci. A string is tied at each end to the two pins; its length after tying is 2a. The tip of the pencil then traces an ellipse if it is moved while keeping the string taut. Using two pegs and a rope, gardeners use this procedure to outline an elliptical flower bed—thus it is called the ''gardener's ellipse''. A similar method for drawing confocal ellipses with a ''closed'' string is due to the Irish bishop Charles Graves.


Paper strip methods

The two following methods rely on the parametric representation (see section ''
parametric representation In mathematics, a parametric equation defines a group of quantities as functions of one or more independent variables called parameters. Parametric equations are commonly used to express the coordinates of the points that make up a geometric o ...
'', above): : (a\cos t,\, b\sin t) This representation can be modeled technically by two simple methods. In both cases center, the axes and semi axes a,\, b have to be known. ;Method 1 The first method starts with : a strip of paper of length a + b. The point, where the semi axes meet is marked by P. If the strip slides with both ends on the axes of the desired ellipse, then point P traces the ellipse. For the proof one shows that point P has the parametric representation (a\cos t,\, b\sin t), where parameter t is the angle of the slope of the paper strip. A technical realization of the motion of the paper strip can be achieved by a Tusi couple (see animation). The device is able to draw any ellipse with a ''fixed'' sum a + b, which is the radius of the large circle. This restriction may be a disadvantage in real life. More flexible is the second paper strip method. Elliko-pap1.svg, Ellipse construction: paper strip method 1 Tusi couple vs Paper strip plus Ellipses horizontal.gif, Ellipses with Tusi couple. Two examples: red and cyan. A variation of the paper strip method 1 uses the observation that the midpoint N of the paper strip is moving on the circle with center M (of the ellipse) and radius \tfrac. Hence, the paperstrip can be cut at point N into halves, connected again by a joint at N and the sliding end K fixed at the center M (see diagram). After this operation the movement of the unchanged half of the paperstrip is unchanged. This variation requires only one sliding shoe. Ellipse-papsm-1a.svg, Variation of the paper strip method 1 Ellipses with SliderCrank inner Ellipses.gif, Animation of the variation of the paper strip method 1 ; Method 2: The second method starts with : a strip of paper of length a. One marks the point, which divides the strip into two substrips of length b and a - b. The strip is positioned onto the axes as described in the diagram. Then the free end of the strip traces an ellipse, while the strip is moved. For the proof, one recognizes that the tracing point can be described parametrically by (a\cos t,\, b\sin t), where parameter t is the angle of slope of the paper strip. This method is the base for several ''ellipsographs'' (see section below). Similar to the variation of the paper strip method 1 a ''variation of the paper strip method 2'' can be established (see diagram) by cutting the part between the axes into halves. File:Archimedes Trammel.gif,
Trammel of Archimedes A trammel of Archimedes is a mechanism that generates the shape of an ellipse. () It consists of two shuttles which are confined ("trammeled") to perpendicular channels or rails and a rod which is attached to the shuttles by pivots at fixed pos ...
(principle) File:L-Ellipsenzirkel.png, Ellipsograph due to
Benjamin Bramer Benjamin Bramer (15 February 1588 – 17 March 1652) was a German mathematician, architect, inventor, and adviser. Early life Bramer was born on 15 February 1588 in Felsberg, Germany to a Protestant minister father. The minister later died when ...
File:Ellipses with SliderCrank Ellipses at Slider Side.gif, Variation of the paper strip method 2
Most ellipsograph drafting instruments are based on the second paperstrip method.


Approximation by osculating circles

From ''Metric properties'' below, one obtains: * The radius of curvature at the vertices V_1,\, V_2 is: \tfrac * The radius of curvature at the co-vertices V_3,\, V_4 is: \tfrac\ . The diagram shows an easy way to find the centers of curvature C_1 = \left(a - \tfrac, 0\right),\, C_3 = \left(0, b - \tfrac\right) at vertex V_1 and co-vertex V_3, respectively: # mark the auxiliary point H = (a,\, b) and draw the line segment V_1 V_3\ , # draw the line through H, which is perpendicular to the line V_1 V_3\ , # the intersection points of this line with the axes are the centers of the osculating circles. (proof: simple calculation.) The centers for the remaining vertices are found by symmetry. With help of a French curve one draws a curve, which has smooth contact to the osculating circles.


Steiner generation

The following method to construct single points of an ellipse relies on the Steiner generation of a conic section: : Given two
pencils A pencil () is a writing or drawing implement with a solid pigment core in a protective casing that reduces the risk of core breakage, and keeps it from marking the user's hand. Pencils create marks by physical abrasion, leaving a tra ...
B(U),\, B(V) of lines at two points U,\, V (all lines containing U and V, respectively) and a projective but not perspective mapping \pi of B(U) onto B(V), then the intersection points of corresponding lines form a non-degenerate projective conic section. For the generation of points of the ellipse \tfrac + \tfrac = 1 one uses the pencils at the vertices V_1,\, V_2. Let P = (0,\, b) be an upper co-vertex of the ellipse and A = (-a,\, 2b),\, B = (a,\,2b). P is the center of the rectangle V_1,\, V_2,\, B,\, A. The side \overline of the rectangle is divided into n equal spaced line segments and this division is projected parallel with the diagonal AV_2 as direction onto the line segment \overline and assign the division as shown in the diagram. The parallel projection together with the reverse of the orientation is part of the projective mapping between the pencils at V_1 and V_2 needed. The intersection points of any two related lines V_1 B_i and V_2 A_i are points of the uniquely defined ellipse. With help of the points C_1,\, \dotsc the points of the second quarter of the ellipse can be determined. Analogously one obtains the points of the lower half of the ellipse. Steiner generation can also be defined for hyperbolas and parabolas. It is sometimes called a ''parallelogram method'' because one can use other points rather than the vertices, which starts with a parallelogram instead of a rectangle.


As hypotrochoid

The ellipse is a special case of the hypotrochoid when R = 2r, as shown in the adjacent image. The special case of a moving circle with radius r inside a circle with radius R = 2r is called a Tusi couple.


Inscribed angles and three-point form


Circles

A circle with equation \left(x - x_\circ\right)^2 + \left(y - y_\circ\right)^2 = r^2 is uniquely determined by three points \left(x_1, y_1\right),\; \left(x_2,\,y_2\right),\; \left(x_3,\, y_3\right) not on a line. A simple way to determine the parameters x_\circ,y_\circ,r uses the '' inscribed angle theorem'' for circles: : For four points P_i = \left(x_i,\, y_i\right),\ i = 1,\, 2,\, 3,\, 4,\, (see diagram) the following statement is true: : The four points are on a circle if and only if the angles at P_3 and P_4 are equal. Usually one measures inscribed angles by a degree or radian ''θ,'' but here the following measurement is more convenient: : In order to measure the angle between two lines with equations y = m_1x + d_1,\ y = m_2x + d_2,\ m_1 \ne m_2, one uses the quotient: :: \frac = \cot\theta\ .


Inscribed angle theorem for circles

For four points P_i = \left(x_i,\, y_i\right),\ i = 1,\, 2,\, 3,\, 4,\, no three of them on a line, we have the following (see diagram): : The four points are on a circle, if and only if the angles at P_3 and P_4 are equal. In terms of the angle measurement above, this means: :: \frac = \frac . At first the measure is available only for chords not parallel to the y-axis, but the final formula works for any chord.


Three-point form of circle equation

: As a consequence, one obtains an equation for the circle determined by three non-colinear points P_i = \left(x_i,\, y_i\right): :: \frac = \frac . For example, for P_1 = (2,\, 0),\; P_2 = (0,\, 1),\; P_3 = (0,\,0) the three-point equation is: : \frac = 0, which can be rearranged to (x - 1)^2 + \left(y - \tfrac\right)^2 = \tfrac\ . Using vectors,
dot product In mathematics, the dot product or scalar productThe term ''scalar product'' means literally "product with a scalar as a result". It is also used sometimes for other symmetric bilinear forms, for example in a pseudo-Euclidean space. is an alg ...
s and
determinant In mathematics, the determinant is a scalar value that is a function of the entries of a square matrix. It characterizes some properties of the matrix and the linear map represented by the matrix. In particular, the determinant is nonzero if a ...
s this formula can be arranged more clearly, letting \vec x = (x,\, y): : \frac = \frac . The center of the circle \left(x_\circ,\, y_\circ\right) satisfies: : \begin 1 & \frac \\ \frac & 1 \end \begin x_\circ \\ y_\circ \end = \begin \frac \\ \frac \end. The radius is the distance between any of the three points and the center. : r = \sqrt = \sqrt = \sqrt.


Ellipses

This section, we consider the family of ellipses defined by equations \tfrac + \tfrac = 1 with a ''fixed'' eccentricity e. It is convenient to use the parameter: : = \frac = \frac, and to write the ellipse equation as: : \left(x - x_\circ\right)^2 + \, \left(y - y_\circ\right)^2 = a^2, where ''q'' is fixed and x_\circ,\, y_\circ,\, a vary over the real numbers. (Such ellipses have their axes parallel to the coordinate axes: if q < 1, the major axis is parallel to the ''x''-axis; if q > 1, it is parallel to the ''y''-axis.) Like a circle, such an ellipse is determined by three points not on a line. For this family of ellipses, one introduces the following q-analog angle measure, which is ''not'' a function of the usual angle measure ''θ'': : In order to measure an angle between two lines with equations y = m_1x + d_1,\ y = m_2x + d_2,\ m_1 \ne m_2 one uses the quotient: :: \frac\ .


Inscribed angle theorem for ellipses

: Given four points P_i = \left(x_i,\, y_i\right),\ i = 1,\, 2,\, 3,\, 4, no three of them on a line (see diagram). : The four points are on an ellipse with equation (x - x_\circ)^2 + \, (y - y_\circ)^2 = a^2 if and only if the angles at P_3 and P_4 are equal in the sense of the measurement above—that is, if :: \frac = \frac \ . At first the measure is available only for chords which are not parallel to the y-axis. But the final formula works for any chord. The proof follows from a straightforward calculation. For the direction of proof given that the points are on an ellipse, one can assume that the center of the ellipse is the origin.


Three-point form of ellipse equation

: A consequence, one obtains an equation for the ellipse determined by three non-colinear points P_i = \left(x_i,\, y_i\right): :: \frac = \frac \ . For example, for P_1 = (2,\, 0),\; P_2 = (0,\,1),\; P_3 = (0,\, 0) and q = 4 one obtains the three-point form : \frac = 0 and after conversion \frac + \frac = 1. Analogously to the circle case, the equation can be written more clearly using vectors: : \frac = \frac , where * is the modified
dot product In mathematics, the dot product or scalar productThe term ''scalar product'' means literally "product with a scalar as a result". It is also used sometimes for other symmetric bilinear forms, for example in a pseudo-Euclidean space. is an alg ...
\vec u*\vec v = u_x v_x + \,u_y v_y.


Pole-polar relation

Any ellipse can be described in a suitable coordinate system by an equation \tfrac + \tfrac = 1. The equation of the tangent at a point P_1 = \left(x_1,\, y_1\right) of the ellipse is \tfrac + \tfrac = 1. If one allows point P_1 = \left(x_1,\, y_1\right) to be an arbitrary point different from the origin, then : point P_1 = \left(x_1,\, y_1\right) \neq (0,\, 0) is mapped onto the line \tfrac + \tfrac = 1, not through the center of the ellipse. This relation between points and lines is a
bijection In mathematics, a bijection, also known as a bijective function, one-to-one correspondence, or invertible function, is a function between the elements of two sets, where each element of one set is paired with exactly one element of the other ...
. The inverse function maps * line y = mx + d,\ d \ne 0 onto the point \left(-\tfrac,\, \tfrac\right) and * line x = c,\ c \ne 0 onto the point \left(\tfrac,\, 0\right). Such a relation between points and lines generated by a conic is called '' pole-polar relation'' or ''polarity''. The pole is the point; the polar the line. By calculation one can confirm the following properties of the pole-polar relation of the ellipse: * For a point (pole) ''on'' the ellipse, the polar is the tangent at this point (see diagram: P_1,\, p_1). * For a pole P ''outside'' the ellipse, the intersection points of its polar with the ellipse are the tangency points of the two tangents passing P (see diagram: P_2,\, p_2). * For a point ''within'' the ellipse, the polar has no point with the ellipse in common (see diagram: F_1,\, l_1). # The intersection point of two polars is the pole of the line through their poles. # The foci (c,\, 0) and (-c,\, 0), respectively, and the directrices x = \tfrac and x = -\tfrac, respectively, belong to pairs of pole and polar. Because they are even polar pairs with respect to the circle x^2+y^2=a^2, the directrices can be constructed by compass and straightedge (see Inversive geometry). Pole-polar relations exist for hyperbolas and parabolas as well.


Metric properties

All metric properties given below refer to an ellipse with equation except for the section on the area enclosed by a tilted ellipse, where the generalized form of Eq.() will be given.


Area

The
area Area is the quantity that expresses the extent of a region on the plane or on a curved surface. The area of a plane region or ''plane area'' refers to the area of a shape or planar lamina, while '' surface area'' refers to the area of an op ...
A_\text enclosed by an ellipse is: where a and b are the lengths of the semi-major and semi-minor axes, respectively. The area formula \pi a b is intuitive: start with a circle of radius b (so its area is \pi b^2) and stretch it by a factor a/b to make an ellipse. This scales the area by the same factor: \pi b^2(a/b) = \pi a b. However, using the same approach for the circumference would be fallacious – compare the
integrals In mathematics, an integral assigns numbers to functions in a way that describes displacement, area, volume, and other concepts that arise by combining infinitesimal data. The process of finding integrals is called integration. Along with d ...
\int f(x)\, dx and \int \sqrt\, dx. It is also easy to rigorously prove the area formula using integration as follows. Equation () can be rewritten as y(x)= b \sqrt. For x\in a,a this curve is the top half of the ellipse. So twice the integral of y(x) over the interval a,a/math> will be the area of the ellipse: : \begin A_\text &= \int_^a 2b\sqrt\,dx\\ &= \frac ba \int_^a 2\sqrt\,dx. \end The second integral is the area of a circle of radius a, that is, \pi a^2. So : A_\text = \frac\pi a^2 = \pi ab. An ellipse defined implicitly by Ax^2+ Bxy + Cy^2 = 1 has area 2\pi / \sqrt. The area can also be expressed in terms of eccentricity and the length of the semi-major axis as a^2\pi\sqrt (obtained by solving for
flattening Flattening is a measure of the compression of a circle or sphere along a diameter to form an ellipse or an ellipsoid of revolution ( spheroid) respectively. Other terms used are ellipticity, or oblateness. The usual notation for flattening ...
, then computing the semi-minor axis). So far we have dealt with ''erect'' ellipses, whose major and minor axes are parallel to the x and y axes. However, some applications require ''tilted'' ellipses. In charged-particle beam optics, for instance, the enclosed area of an erect or tilted ellipse is an important property of the beam, its ''emittance''. In this case a simple formula still applies, namely where y_, x_ are intercepts and x_, y_ are maximum values. It follows directly from Apollonios's theorem.


Circumference

The circumference C of an ellipse is: : C \,=\, 4a\int_0^\sqrt \ d\theta \,=\, 4 a \,E(e) where again a is the length of the semi-major axis, e=\sqrt is the eccentricity, and the function E is the
complete elliptic integral of the second kind In integral calculus, an elliptic integral is one of a number of related functions defined as the value of certain integrals, which were first studied by Giulio Fagnano and Leonhard Euler (). Their name originates from their originally arising i ...
, : E(e) \,=\, \int_0^\sqrt \ d\theta which is in general not an elementary function. The circumference of the ellipse may be evaluated in terms of E(e) using Gauss's arithmetic-geometric mean; this is a quadratically converging iterative method (see
here Here is an adverb that means "in, on, or at this place". It may also refer to: Software * Here Technologies, a mapping company * Here WeGo (formerly Here Maps), a mobile app and map website by Here Television * Here TV (formerly "here!"), a ...
for details). The exact infinite series is: :\begin C &= 2\pi a \left
right Rights are legal, social, or ethical principles of freedom or entitlement; that is, rights are the fundamental normative rules about what is allowed of people or owed to people according to some legal system, social convention, or ethical ...
\\ &= 2\pi a \left - \sum_^\infty \left(\frac\right)^2 \frac\right\\ &= -2\pi a \sum_^\infty \left(\frac\right)^2 \frac, \end where n!! is the double factorial (extended to negative odd integers by the recurrence relation (2n-1)!! = (2n+1)!!/(2n+1), for n \le 0). This series converges, but by expanding in terms of h = (a-b)^2 / (a+b)^2,
James Ivory James Francis Ivory (born June 7, 1928) is an American film director, producer, and screenwriter. For many years, he worked extensively with Indian-born film producer Ismail Merchant, his domestic as well as professional partner, and with scree ...
and Bessel derived an expression that converges much more rapidly: :\begin C &= \pi (a+b) \sum_^\infty \left(\frac\right)^2 h^n \\ &= \pi (a+b) \left + \frac + \sum_^\infty \left(\frac\right)^2 h^n\right\\ &= \pi (a+b) \left + \sum_^\infty \left(\frac\right)^2 \frac\right \end Srinivasa Ramanujan gave two close approximations for the circumference in §16 of "Modular Equations and Approximations to \pi"; they are : C \approx \pi \biggl (a + b) - \sqrt \biggr= \pi \biggl (a + b) - \sqrt\biggr/math> and : C\approx\pi\left(a+b\right)\left(1+\frac\right), where h takes on the same meaning as above. The errors in these approximations, which were obtained empirically, are of order h^3 and h^5, respectively.


Arc length

More generally, the
arc length ARC may refer to: Business * Aircraft Radio Corporation, a major avionics manufacturer from the 1920s to the '50s * Airlines Reporting Corporation, an airline-owned company that provides ticket distribution, reporting, and settlement services * ...
of a portion of the circumference, as a function of the angle subtended (or -coordinates of any two points on the upper half of the ellipse), is given by an incomplete elliptic integral. The upper half of an ellipse is parameterized by : y=b\sqrt. Then the arc length s from x_ to x_ is: : s = -b\int_^ \sqrt \, dz. This is equivalent to : s = b\left \; 1 - \frac\right)\right_ where E(z \mid m) is the incomplete elliptic integral of the second kind with parameter m=k^. Some lower and upper bounds on the circumference of the canonical ellipse x^2/a^2 + y^2/b^2 = 1 with a\geq b are : \begin 2\pi b &\le C \le 2\pi a, \\ \pi (a+b) &\le C \le 4(a+b), \\ 4\sqrt &\le C \le \sqrt \pi \sqrt . \end Here the upper bound 2\pi a is the circumference of a circumscribed concentric circle passing through the endpoints of the ellipse's major axis, and the lower bound 4\sqrt is the perimeter of an
inscribed {{unreferenced, date=August 2012 An inscribed triangle of a circle In geometry, an inscribed planar shape or solid is one that is enclosed by and "fits snugly" inside another geometric shape or solid. To say that "figure F is inscribed in figu ...
rhombus with vertices at the endpoints of the major and the minor axes.


Curvature

The
curvature In mathematics, curvature is any of several strongly related concepts in geometry. Intuitively, the curvature is the amount by which a curve deviates from being a straight line, or a surface deviates from being a plane. For curves, the can ...
is given by \kappa = \frac\left(\frac+\frac\right)^\ , radius of curvature at point (x,y): : \rho = a^2 b^2 \left(\frac + \frac\right)^\frac = \frac \sqrt \ . Radius of curvature at the two ''vertices'' (\pm a,0) and the centers of curvature: : \rho_0 = \frac=p\ , \qquad \left(\pm\frac\,\bigg, \,0\right)\ . Radius of curvature at the two ''co-vertices'' (0,\pm b) and the centers of curvature: : \rho_1 = \frac\ , \qquad \left(0\,\bigg, \,\pm\frac\right)\ .


In triangle geometry

Ellipses appear in triangle geometry as #
Steiner ellipse In geometry, the Steiner ellipse of a triangle, also called the Steiner circumellipse to distinguish it from the Steiner inellipse, is the unique circumellipse (ellipse that touches the triangle at its vertices) whose center is the triangle's c ...
: ellipse through the vertices of the triangle with center at the centroid, #
inellipse In triangle geometry, an inellipse is an ellipse that touches the three sides of a triangle. The simplest example is the incircle. Further important inellipses are the Steiner inellipse, which touches the triangle at the midpoints of its side ...
s: ellipses which touch the sides of a triangle. Special cases are the Steiner inellipse and the
Mandart inellipse In geometry, the Mandart inellipse of a triangle is an ellipse inscribed within the triangle, tangent to its sides at the contact points of its excircles (which are also the vertices of the extouch triangle and the endpoints of the splitter ...
.


As plane sections of quadrics

Ellipses appear as plane sections of the following quadrics: * Ellipsoid * Elliptic cone * Elliptic
cylinder A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an ...
* Hyperboloid of one sheet * Hyperboloid of two sheets Ellipsoid Quadric.png, Ellipsoid Quadric Cone.jpg, Elliptic cone Elliptic Cylinder Quadric.png, Elliptic cylinder Hyperboloid1.png, Hyperboloid of one sheet Hyperboloid2.png, Hyperboloid of two sheets


Applications


Physics


Elliptical reflectors and acoustics

If the water's surface is disturbed at one focus of an elliptical water tank, the circular waves of that disturbance, after reflecting off the walls, converge simultaneously to a single point: the ''second focus''. This is a consequence of the total travel length being the same along any wall-bouncing path between the two foci. Similarly, if a light source is placed at one focus of an elliptic
mirror A mirror or looking glass is an object that reflects an image. Light that bounces off a mirror will show an image of whatever is in front of it, when focused through the lens of the eye or a camera. Mirrors reverse the direction of the im ...
, all light rays on the plane of the ellipse are reflected to the second focus. Since no other smooth curve has such a property, it can be used as an alternative definition of an ellipse. (In the special case of a circle with a source at its center all light would be reflected back to the center.) If the ellipse is rotated along its major axis to produce an ellipsoidal mirror (specifically, a prolate spheroid), this property holds for all rays out of the source. Alternatively, a cylindrical mirror with elliptical cross-section can be used to focus light from a linear
fluorescent lamp A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet, ult ...
along a line of the paper; such mirrors are used in some
document scanner An image scanner—often abbreviated to just scanner—is a device that optically scans images, printed text, handwriting or an object and converts it to a digital image. Commonly used in offices are variations of the desktop ''flatbed scanner'' w ...
s. Sound waves are reflected in a similar way, so in a large elliptical room a person standing at one focus can hear a person standing at the other focus remarkably well. The effect is even more evident under a vaulted roof shaped as a section of a prolate spheroid. Such a room is called a ''
whisper chamber file:St Paul's Cathedral Whispering Gallery.jpg, The Whispering Gallery of St Paul's Cathedral, London A whispering gallery is usually a circular, Sphere#Hemisphere, hemispherical, Ellipse, elliptical or ellipsoidal enclosure, often beneath a ell ...
''. The same effect can be demonstrated with two reflectors shaped like the end caps of such a spheroid, placed facing each other at the proper distance. Examples are the National Statuary Hall at the United States Capitol (where John Quincy Adams is said to have used this property for eavesdropping on political matters); the Mormon Tabernacle at Temple Square in
Salt Lake City Salt Lake City (often shortened to Salt Lake and abbreviated as SLC) is the capital and most populous city of Utah, United States. It is the seat of Salt Lake County, the most populous county in Utah. With a population of 200,133 in 2020, th ...
,
Utah Utah ( , ) is a state in the Mountain West subregion of the Western United States. Utah is a landlocked U.S. state bordered to its east by Colorado, to its northeast by Wyoming, to its north by Idaho, to its south by Arizona, and to its ...
; at an exhibit on sound at the Museum of Science and Industry in
Chicago (''City in a Garden''); I Will , image_map = , map_caption = Interactive Map of Chicago , coordinates = , coordinates_footnotes = , subdivision_type = List of sovereign states, Count ...
; in front of the
University of Illinois at Urbana–Champaign The University of Illinois Urbana-Champaign (U of I, Illinois, University of Illinois, or UIUC) is a public land-grant research university in Illinois in the twin cities of Champaign and Urbana. It is the flagship institution of the Un ...
Foellinger Auditorium; and also at a side chamber of the Palace of Charles V, in the Alhambra.


Planetary orbits

In the 17th century,
Johannes Kepler Johannes Kepler (; ; 27 December 1571 – 15 November 1630) was a German astronomer, mathematician, astrologer, natural philosopher and writer on music. He is a key figure in the 17th-century Scientific Revolution, best known for his laws ...
discovered that the orbits along which the planets travel around the Sun are ellipses with the Sun pproximatelyat one focus, in his first law of planetary motion. Later,
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 ...
explained this as a corollary of his law of universal gravitation. More generally, in the gravitational two-body problem, if the two bodies are bound to each other (that is, the total energy is negative), their orbits are similar ellipses with the common barycenter being one of the foci of each ellipse. The other focus of either ellipse has no known physical significance. The orbit of either body in the reference frame of the other is also an ellipse, with the other body at the same focus. Keplerian elliptical orbits are the result of any radially directed attraction force whose strength is inversely proportional to the square of the distance. Thus, in principle, the motion of two oppositely charged particles in empty space would also be an ellipse. (However, this conclusion ignores losses due to
electromagnetic radiation In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
and quantum effects, which become significant when the particles are moving at high speed.) For elliptical orbits, useful relations involving the eccentricity e are: : \begin e &= \frac = \frac \\ r_a &= (1 + e)a \\ r_p &= (1 - e)a \end where * r_a is the radius at apoapsis (the farthest distance) * r_p is the radius at periapsis (the closest distance) * a is the length of the semi-major axis Also, in terms of r_a and r_p, the semi-major axis a is their
arithmetic mean In mathematics and statistics, the arithmetic mean ( ) or arithmetic average, or just the '' mean'' or the ''average'' (when the context is clear), is the sum of a collection of numbers divided by the count of numbers in the collection. The co ...
, the semi-minor axis b is their geometric mean, and 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 ...
\ell is their harmonic mean. In other words, :\begin a &= \frac \\ pt b &= \sqrt \\ pt \ell &= \frac = \frac \end.


Harmonic oscillators

The general solution for a
harmonic oscillator In classical mechanics, a harmonic oscillator is a system that, when displaced from its equilibrium position, experiences a restoring force ''F'' proportional to the displacement ''x'': \vec F = -k \vec x, where ''k'' is a positive const ...
in two or more
dimension In physics and mathematics, the dimension of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus, a line has a dimension of one (1D) because only one coord ...
s is also an ellipse. Such is the case, for instance, of a long pendulum that is free to move in two dimensions; of a mass attached to a fixed point by a perfectly elastic
spring Spring(s) may refer to: Common uses * Spring (season), a season of the year * Spring (device), a mechanical device that stores energy * Spring (hydrology), a natural source of water * Spring (mathematics), a geometric surface in the shape of a h ...
; or of any object that moves under influence of an attractive force that is directly proportional to its distance from a fixed attractor. Unlike Keplerian orbits, however, these "harmonic orbits" have the center of attraction at the geometric center of the ellipse, and have fairly simple equations of motion.


Phase visualization

In
electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
, the relative phase of two sinusoidal signals can be compared by feeding them to the vertical and horizontal inputs of an oscilloscope. If the Lissajous figure display is an ellipse, rather than a straight line, the two signals are out of phase.


Elliptical gears

Two
non-circular gear A non-circular gear (NCG) is a special gear design with special characteristics and purpose. While a regular gear is optimized to transmit torque to another engaged member with minimum noise and wear and with maximum efficiency, a non-circular ge ...
s with the same elliptical outline, each pivoting around one focus and positioned at the proper angle, turn smoothly while maintaining contact at all times. Alternatively, they can be connected by a link chain or timing belt, or in the case of a bicycle the main chainring may be elliptical, or an ovoid similar to an ellipse in form. Such elliptical gears may be used in mechanical equipment to produce variable
angular speed Angular may refer to: Anatomy * Angular artery, the terminal part of the facial artery * Angular bone, a large bone in the lower jaw of amphibians and reptiles * Angular incisure, a small anatomical notch on the stomach * Angular gyrus, a region ...
or
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
from a constant rotation of the driving axle, or in the case of a bicycle to allow a varying crank rotation speed with inversely varying mechanical advantage. Elliptical bicycle gears make it easier for the chain to slide off the cog when changing gears. An example gear application would be a device that winds thread onto a conical bobbin on a spinning machine. The bobbin would need to wind faster when the thread is near the apex than when it is near the base.


Optics

* In a material that is optically anisotropic ( birefringent), the
refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, ...
depends on the direction of the light. The dependency can be described by an index ellipsoid. (If the material is optically
isotropic Isotropy is uniformity in all orientations; it is derived . Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence '' anisotropy''. ''Anisotropy'' is also used to describ ...
, this ellipsoid is a sphere.) * In lamp- pumped solid-state lasers, elliptical cylinder-shaped reflectors have been used to direct light from the pump lamp (coaxial with one ellipse focal axis) to the active medium rod (coaxial with the second focal axis). * In laser-plasma produced EUV light sources used in microchip
lithography Lithography () is a planographic method of printing originally based on the immiscibility of oil and water. The printing is from a stone (lithographic limestone) or a metal plate with a smooth surface. It was invented in 1796 by the German a ...
, EUV light is generated by plasma positioned in the primary focus of an ellipsoid mirror and is collected in the secondary focus at the input of the lithography machine.


Statistics and finance

In
statistics Statistics (from German: '' Statistik'', "description of a state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, indust ...
, a bivariate random vector (X, Y) is jointly elliptically distributed if its iso-density contours—loci of equal values of the density function—are ellipses. The concept extends to an arbitrary number of elements of the random vector, in which case in general the iso-density contours are ellipsoids. A special case is the multivariate normal distribution. The elliptical distributions are important in
finance Finance is the study and discipline of money, currency and capital assets. It is related to, but not synonymous with economics, the study of production, distribution, and consumption of money, assets, goods and services (the discipline of f ...
because if rates of return on assets are jointly elliptically distributed then all portfolios can be characterized completely by their mean and variance—that is, any two portfolios with identical mean and variance of portfolio return have identical distributions of portfolio return.


Computer graphics

Drawing an ellipse as a graphics primitive is common in standard display libraries, such as the MacIntosh QuickDraw API, and Direct2D on Windows. Jack Bresenham at IBM is most famous for the invention of 2D drawing primitives, including line and circle drawing, using only fast integer operations such as addition and branch on carry bit. M. L. V. Pitteway extended Bresenham's algorithm for lines to conics in 1967. Another efficient generalization to draw ellipses was invented in 1984 by Jerry Van Aken. In 1970 Danny Cohen presented at the "Computer Graphics 1970" conference in England a linear algorithm for drawing ellipses and circles. In 1971, L. B. Smith published similar algorithms for all conic sections and proved them to have good properties. These algorithms need only a few multiplications and additions to calculate each vector. It is beneficial to use a parametric formulation in computer graphics because the density of points is greatest where there is the most curvature. Thus, the change in slope between each successive point is small, reducing the apparent "jaggedness" of the approximation. ;Drawing with Bézier paths:
Composite Bézier curve In geometric modelling and in computer graphics, a composite Bézier curve or Bézier spline is a spline made out of Bézier curves that is at least C^0 continuous. In other words, a composite Bézier curve is a series of Bézier curves joined ...
s may also be used to draw an ellipse to sufficient accuracy, since any ellipse may be construed as an
affine transformation In Euclidean geometry, an affine transformation or affinity (from the Latin, ''affinis'', "connected with") is a geometric transformation that preserves lines and parallelism, but not necessarily Euclidean distances and angles. More generall ...
of a circle. The spline methods used to draw a circle may be used to draw an ellipse, since the constituent Bézier curves behave appropriately under such transformations.


Optimization theory

It is sometimes useful to find the minimum bounding ellipse on a set of points. The
ellipsoid method In mathematical optimization, the ellipsoid method is an iterative method for minimizing convex functions. When specialized to solving feasible linear optimization problems with rational data, the ellipsoid method is an algorithm which finds ...
is quite useful for solving this problem.


See also

* Cartesian oval, a generalization of the ellipse *
Circumconic and inconic In Euclidean geometry, a circumconic is a conic section that passes through the three vertices of a triangle, and an inconic is a conic section inscribed in the sides, possibly extended, of a triangle.Weisstein, Eric W. "Inconic." From MathWorld ...
*
Distance of closest approach of ellipses The distance of closest approach of two objects is the distance between their centers when they are externally tangent. The objects may be geometric shapes or physical particles with well-defined boundaries. The distance of closest approach is s ...
* Ellipse fitting * Elliptic coordinates, an orthogonal coordinate system based on families of ellipses and
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 ...
e *
Elliptic partial differential equation Second-order linear partial differential equations (PDEs) are classified as either elliptic, hyperbolic, or parabolic. Any second-order linear PDE in two variables can be written in the form :Au_ + 2Bu_ + Cu_ + Du_x + Eu_y + Fu +G= 0,\, whe ...
*
Elliptical distribution In probability and statistics, an elliptical distribution is any member of a broad family of probability distributions that generalize the multivariate normal distribution. Intuitively, in the simplified two and three dimensional case, the joint ...
, in statistics * Elliptical dome * Geodesics on an ellipsoid * Great ellipse * Kepler's laws of planetary motion * ''n''-ellipse, a generalization of the ellipse for ''n'' foci * Oval *
Spheroid A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has ...
, the ellipsoid obtained by rotating an ellipse about its major or minor axis * Stadium (geometry), a two-dimensional geometric shape constructed of a rectangle with semicircles at a pair of opposite sides * Steiner circumellipse, the unique ellipse circumscribing a triangle and sharing its centroid * Superellipse, a generalization of an ellipse that can look more rectangular or more "pointy" *
True True most commonly refers to truth, the state of being in congruence with fact or reality. True may also refer to: Places * True, West Virginia, an unincorporated community in the United States * True, Wisconsin, a town in the United States * ...
,
eccentric Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
, and mean anomaly


Notes


References

* * * * *


External links

* * * * *
Apollonius' Derivation of the Ellipse
at Convergence
''The Shape and History of The Ellipse in Washington, D.C.''
by
Clark Kimberling Clark Kimberling (born November 7, 1942 in Hinsdale, Illinois) is a mathematician, musician, and composer. He has been a mathematics professor since 1970 at the University of Evansville. His research interests include triangle centers, integer ...

Ellipse circumference calculator


*
Trammel according Frans van Schooten
* by Matt Parker {{Authority control Conic sections Plane curves Elementary shapes Algebraic curves