In 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. As such, it generalizes a circle, 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).
An ellipse has a simple algebraic solution for its area, but only approximations for its perimeter, 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:
: $\backslash frac+\backslash frac\; =\; 1\; .$
Assuming $a\; \backslash ge\; b$, the foci are $(\backslash pm\; c,\; 0)$ for $c\; =\; \backslash sqrt$. The standard parametric equation is:
: $(x,y)\; =\; (a\backslash cos(t),b\backslash sin(t))\; \backslash quad\; \backslash text\; \backslash quad\; 0\backslash leq\; t\backslash leq\; 2\backslash pi.$
Ellipses are the closed type of conic section: 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 hyperbolas, both of which are open and unbounded. An angled cross section of a cylinder 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:
: $e\; =\; \backslash frac\; =\; \backslash sqrt$.
Ellipses are common in physics, astronomy and engineering. For example, the orbit of each planet in the solar system 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 ellipsoids. A circle viewed from a side angle looks like an ellipse: that is, the ellipse is the image of a circle under parallel 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.
The name, (, "omission"), was given by Apollonius of Perga 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 $|PF\_1|,\backslash \; |PF\_2|$ is equal to $2a$:$E\; =\; \backslash \backslash \; .$
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 $e=\backslash tfrac$ 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 midpoint $F\_2$ 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, one can prove that any plane 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 $F\_1\; =\; (c,\backslash ,\; 0),\backslash \; F\_2=(-c,\backslash ,\; 0)$,
: the vertices are $V\_1\; =\; (a,\backslash ,\; 0),\backslash \; V\_2\; =\; (-a,\backslash ,\; 0)$.
For an arbitrary point $(x,y)$ the distance to the focus $(c,0)$ is
$\backslash sqrt$ and to the other focus $\backslash sqrt$. Hence the point $(x,\backslash ,\; y)$ is on the ellipse whenever:
:$\backslash sqrt\; +\; \backslash sqrt\; =\; 2a\backslash \; .$
Removing the radicals by suitable squarings and using $b^2\; =\; a^2-c^2$ produces the standard equation of the ellipse:
:$\backslash frac\; +\; \backslash frac\; =\; 1,$
or, solved for ''y:''
:$y\; =\; \backslash pm\backslash frac\backslash sqrt\; =\; \backslash pm\; \backslash sqrt.$
The width and height parameters $a,\backslash ;\; b$ are called the semi-major and semi-minor axes. The top and bottom points $V\_3\; =\; (0,\backslash ,\; b),\backslash ;\; V\_4\; =\; (0,\backslash ,\; -b)$ are the ''co-vertices''. The distances from a point $(x,\backslash ,\; y)$ 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. $a\; \backslash ge\; b\; >\; 0\; \backslash \; .$
In principle, the canonical ellipse equation $\backslash tfrac\; +\; \backslash tfrac\; =\; 1$ 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: $c\; =\; \backslash sqrt$.

** Eccentricity **

The eccentricity can be expressed as:
: $e\; =\; \backslash frac\; =\; \backslash sqrt$,
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'' $\backslash ell$. A calculation shows:
: $\backslash ell\; =\; \backslash fraca\; =\; a\; \backslash left(1\; -\; e^2\backslash right).$
The semi-latus rectum $\backslash ell$ is equal to the radius of curvature at the vertices (see section curvature).

** 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 $(x\_1,\backslash ,\; y\_1)$ of the ellipse $\backslash tfrac\; +\; \backslash tfrac\; =\; 1$ has the coordinate equation:
:$\backslash fracx\; +\; \backslash fracy\; =\; 1.$
A vector parametric equation of the tangent is:
: $\backslash vec\; x\; =\; \backslash beginx\_1\; \backslash \backslash \; y\_1\backslash end\; +\; s\backslash begin\; \backslash ;\backslash !\; -y\_1\; a^2\; \backslash \backslash \; \backslash ;\backslash \; \backslash \; \backslash \; x\_1\; b^2\; \backslash end\backslash $ with $\backslash \; s\; \backslash in\; \backslash mathbb\backslash \; .$
Proof:
Let $(x\_1,\backslash ,\; y\_1)$ be a point on an ellipse and $\backslash vec\; =\; \backslash beginx\_1\; \backslash \backslash \; y\_1\backslash end\; +\; s\backslash beginu\; \backslash \backslash \; v\backslash end$ be the equation of any line $g$ containing $(x\_1,\backslash ,\; y\_1)$. Inserting the line's equation into the ellipse equation and respecting $$ yields:
: $\backslash frac\; +\; \backslash frac\; =\; 1\backslash \; \backslash quad\backslash Longrightarrow\backslash quad\; 2s\backslash left(\backslash frac\; +\; \backslash frac\backslash right)\; +\; s^2\backslash left(\backslash frac\; +\; \backslash frac\backslash right)\; =\; 0\backslash \; .$
: There are then cases:
:# $\backslash fracu\; +\; \backslash fracv\; =\; 0.$ Then line $g$ and the ellipse have only point $(x\_1,\backslash ,\; y\_1)$ in common, and $g$ is a tangent. The tangent direction has perpendicular vector $\backslash begin\backslash frac\; \&\; \backslash frac\backslash end$, so the tangent line has equation $\backslash fracx\; +\; \backslash tfracy\; =\; k$ for some $k$. Because $(x\_1,\backslash ,\; y\_1)$ is on the tangent and the ellipse, one obtains $k\; =\; 1$.
:# $\backslash fracu\; +\; \backslash fracv\; \backslash ne\; 0.$ Then line $g$ has a second point in common with the ellipse, and is a secant.
Using (1) one finds that $\backslash begin\; -y\_1a^2\; \&\; x\_1b^2\; \backslash end$ is a tangent vector at point $(x\_1,\backslash ,\; y\_1)$, which proves the vector equation.
If $(x\_1,\; y\_1)$ and $(u,\; v)$ are two points of the ellipse such that $\backslash frac\; +\; \backslash tfrac\; =\; 0$, then the points lie on two ''conjugate diameters'' (see below). (If $a\; =\; b$, the ellipse is a circle and "conjugate" means "orthogonal".)

** Shifted ellipse **

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

Rotated ellipse

* The center is the origin $z\; =\; (0,\; 0)$. * $\backslash theta$ is the angle measured from ''x''-axis. * The parameter $t$ (called the ''eccentric anomaly'' in astronomy) is not the angle of $(x(t),y(t))$ with the ''x''-axis. * $a$ and $b$ are the semi-axes in the x and y directions, respectively. :$\backslash mathbf\; =\backslash mathbf(\backslash theta)\; =\; x\backslash cos\backslash theta\; -\; y\backslash sin\backslash theta$ :$\backslash mathbf\; =\; \backslash mathbf(\backslash theta)\; =\; x\; \backslash sin\backslash theta\; +\; y\; \backslash cos\backslash theta$ Here * $\backslash theta$ is fixed (constant value) * ''t'' is a parameter = independent variable used to parametrize the ellipse :$\backslash mathbf\; =\backslash mathbf\_(t)\; =\; a\backslash cos\backslash \; t\backslash cos\backslash theta\; -\; b\backslash sin\backslash \; t\backslash sin\backslash theta$ :$\backslash mathbf\; =\backslash mathbf\_(t)\; =\; a\backslash cos\backslash \; t\backslash sin\backslash theta\; +\; b\backslash sin\backslash \; t\backslash cos\backslash theta$ So :$\backslash frac+\backslash frac\; =\; 1\; .$ :$\backslash frac\; +\; \backslash frac=1$ :$\backslash frac\; +\; \backslash frac=1$

** General ellipse **

In analytic geometry, the ellipse is defined as a quadric: the set of points $(X,\backslash ,\; Y)$ of the Cartesian plane that, in non-degenerate cases, satisfy the implicit 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
:$\backslash Delta\; =\; \backslash begin\; A\; \&\; \backslash fracB\; \&\; \backslash fracD\; \backslash \backslash \; \backslash fracB\; \&\; C\; \&\; \backslash fracE\; \backslash \backslash \; \backslash fracD\; \&\; \backslash fracE\; \&\; F\; \backslash end\; =\; \backslash left(AC\; -\; \backslash frac\backslash right)\; F\; +\; \backslash frac\; -\; \backslash frac\; -\; \backslash frac.$
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 $\backslash left(x\_\backslash circ,\backslash ,\; y\_\backslash circ\backslash right)$, and rotation angle $\backslash theta$ (the angle from the positive horizontal axis to the ellipse's major axis) using the formulae:
:$\backslash begin\; A\; \&=\; a^2\; \backslash sin^2\backslash theta\; +\; b^2\; \backslash cos^2\backslash theta\; \backslash \backslash \; B\; \&=\; 2\backslash left(b^2\; -\; a^2\backslash right)\; \backslash sin\backslash theta\; \backslash cos\backslash theta\; \backslash \backslash \; C\; \&=\; a^2\; \backslash cos^2\backslash theta\; +\; b^2\; \backslash sin^2\backslash theta\; \backslash \backslash \; D\; \&=\; -2A\; x\_\backslash circ\; -\; B\; y\_\backslash circ\; \backslash \backslash \; E\; \&=\; -\; B\; x\_\backslash circ\; -\; 2C\; y\_\backslash circ\; \backslash \backslash \; F\; \&=\; A\; x\_\backslash circ^2\; +\; B\; x\_\backslash circ\; y\_\backslash circ\; +\; C\; y\_\backslash circ^2\; -\; a^2\; b^2.\; \backslash end$
These expressions can be derived from the canonical equation $\backslash tfrac\; +\; \backslash tfrac\; =\; 1$ by an affine transformation of the coordinates $(x,\backslash ,\; y)$:
:$\backslash begin\; x\; \&=\; \backslash left(X\; -\; x\_\backslash circ\backslash right)\; \backslash cos\backslash theta\; +\; \backslash left(Y\; -\; y\_\backslash circ\backslash right)\; \backslash sin\backslash theta\; \backslash \backslash \; y\; \&=\; -\backslash left(X\; -\; x\_\backslash circ\backslash right)\; \backslash sin\backslash theta\; +\; \backslash left(Y\; -\; y\_\backslash circ\backslash right)\; \backslash cos\backslash theta.\; \backslash end$
Conversely, the canonical form parameters can be obtained from the general form coefficients by the equations:
:$\backslash begin\; a,\; b\; \&=\; \backslash frac\; \backslash \backslash \; x\_\backslash circ\; \&=\; \backslash frac\; \backslash \backslash pty\_\backslash circ\; \&=\; \backslash frac\; \backslash \backslash pt\backslash theta\; \&=\; \backslash begin\; \backslash arctan\backslash left(\backslash frac\backslash left(C\; -\; A\; -\; \backslash sqrt\backslash right)\backslash right)\; \&\; \backslash text\; B\; \backslash ne\; 0\; \backslash \backslash \; 0\; \&\; \backslash text\; B\; =\; 0,\backslash \; A\; <\; C\; \backslash \backslash \; 90^\backslash circ\; \&\; \backslash text\; B\; =\; 0,\backslash \; A\; >\; C\; \backslash \backslash \; \backslash end\; \backslash end$

** Parametric representation **

Standard parametric representation

Using trigonometric functions, a parametric representation of the standard ellipse $\backslash tfrac+\backslash tfrac\; =\; 1$ is: : $(x,\backslash ,\; y)\; =\; (a\; \backslash cos\; t,\backslash ,\; b\; \backslash sin\; t),\backslash \; 0\; \backslash le\; t\; <\; 2\backslash pi\backslash \; .$ 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 $u\; =\; \backslash tan\backslash left(\backslash frac\backslash right)$ and trigonometric formulae one obtains :$\backslash cos\; t\; =\; \backslash frac\backslash \; ,\backslash quad\; \backslash sin\; t\; =\; \backslash frac$ and the ''rational'' parametric equation of an ellipse : $\backslash begin\; x(u)\; \&=\; a\backslash frac\; \backslash \backslash \; y(u)\; \&=\; \backslash frac\; \backslash end\backslash ;,\backslash quad\; -\backslash infty\; <\; u\; <\; \backslash infty\backslash ;,$ which covers any point of the ellipse $\backslash tfrac\; +\; \backslash tfrac\; =\; 1$ except the left vertex $(-a,\backslash ,\; 0)$. For $u\; \backslash in,\backslash ,\; 1$ this formula represents the right upper quarter of the ellipse moving counter-clockwise with increasing $u.$ The left vertex is the limit $\backslash lim\_\; (x(u),\backslash ,\; y(u))\; =\; (-a,\backslash ,\; 0)\backslash ;.$ Rational representations of conic sections are commonly used in computer-aided design (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 $\backslash vec\; x(t)\; =\; (a\; \backslash cos\; t,\backslash ,\; b\; \backslash sin\; t)^\backslash mathsf$: :$\backslash vec\; x\text{'}(t)\; =\; (-a\backslash sin\; t,\backslash ,\; b\backslash cos\; t)^\backslash mathsf\; \backslash quad\; \backslash rightarrow\; \backslash quad\; m\; =\; -\backslash frac\backslash cot\; t\backslash quad\; \backslash rightarrow\; \backslash quad\; \backslash cot\; t\; =\; -\backslash frac.$ With help of trigonometric formulae one obtains: :$\backslash cos\; t\; =\; \backslash frac\; =\; \backslash frac\backslash \; ,\backslash quad\backslash quad\; \backslash sin\; t\; =\; \backslash frac\; =\; \backslash frac.$ Replacing $\backslash cos\; t$ and $\backslash sin\; t$ of the standard representation yields: : $\backslash vec\; c\_\backslash pm(m)\; =\; \backslash left(-\backslash frac,\backslash ;\backslash frac\backslash right),\backslash ,\; m\; \backslash in\; \backslash R.$ Here $m$ is the slope of the tangent at the corresponding ellipse point, $\backslash vec\; c\_+$ is the upper and $\backslash vec\; c\_-$ the lower half of the ellipse. The vertices$(\backslash pm\; a,\backslash ,\; 0)$, having vertical tangents, are not covered by the representation. The equation of the tangent at point $\backslash vec\; c\_\backslash pm(m)$ has the form $y\; =\; mx\; +\; n$. The still unknown $n$ can be determined by inserting the coordinates of the corresponding ellipse point $\backslash vec\; c\_\backslash pm(m)$: : $y\; =\; mx\; \backslash pm\backslash sqrt\backslash ;\; .$ 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 transformations: : 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 $\backslash vec\; x\; \backslash mapsto\; \backslash vec\; f\backslash !\_0\; +\; A\backslash vec\; x$, where $A$ is a regular matrix (with non-zero determinant) and $\backslash vec\; f\backslash !\_0$ is an arbitrary vector. If $\backslash vec\; f\backslash !\_1,\; \backslash vec\; f\backslash !\_2$ are the column vectors of the matrix $A$, the unit circle $(\backslash cos(t),\; \backslash sin(t))$, $0\; \backslash leq\; t\; \backslash leq\; 2\backslash pi$, is mapped onto the ellipse: : $\backslash vec\; x\; =\; \backslash vec\; p(t)\; =\; \backslash vec\; f\backslash !\_0\; +\; \backslash vec\; f\backslash !\_1\; \backslash cos\; t\; +\; \backslash vec\; f\backslash !\_2\; \backslash sin\; t\; \backslash \; .$ Here $\backslash vec\; f\backslash !\_0$ is the center and $\backslash vec\; f\backslash !\_1,\backslash ;\; \backslash vec\; f\backslash !\_2$ are the directions of two conjugate diameters, in general not perpendicular. ;Vertices The four vertices of the ellipse are $\backslash vec\; p(t\_0),\backslash ;\backslash vec\; p\backslash left(t\_0\; \backslash pm\; \backslash tfrac\backslash right),\backslash ;\; \backslash vec\; p\backslash left(t\_0\; +\; \backslash pi\backslash right)$, for a parameter $t\; =\; t\_0$ defined by: : $\backslash cot\; (2t\_0)\; =\; \backslash frac.$ (If $\backslash vec\; f\backslash !\_1\; \backslash cdot\; \backslash vec\; f\backslash !\_2\; =\; 0$, then $t\_0\; =\; 0$.) This is derived as follows. The tangent vector at point $\backslash vec\; p(t)$ is: : $\backslash vec\; p\backslash ,\text{'}(t)\; =\; -\backslash vec\; f\backslash !\_1\backslash sin\; t\; +\; \backslash vec\; f\backslash !\_2\backslash cos\; t\; \backslash \; .$ At a vertex parameter $t\; =\; t\_0$, the tangent is perpendicular to the major/minor axes, so: : $0\; =\; \backslash vec\; p\text{'}(t)\; \backslash cdot\; \backslash left(\backslash vec\; p(t)\; -\backslash vec\; f\backslash !\_0\backslash right)\; =\; \backslash left(-\backslash vec\; f\backslash !\_1\backslash sin\; t\; +\; \backslash vec\; f\backslash !\_2\backslash cos\; t\backslash right)\; \backslash cdot\; \backslash left(\backslash vec\; f\backslash !\_1\; \backslash cos\; t\; +\; \backslash vec\; f\backslash !\_2\; \backslash sin\; t\backslash right).$ Expanding and applying the identities $\backslash cos^2\; t\; -\backslash sin^2\; t=\backslash cos\; 2t,\backslash \; \backslash \; 2\backslash sin\; t\; \backslash cos\; t\; =\; \backslash sin\; 2t$ gives the equation for $t\; =\; t\_0$. ;Implicit representation Solving the parametric representation for $\backslash ;\; \backslash cos\; t,\backslash sin\; t\backslash ;$ by Cramer's rule and using $\backslash ;\backslash cos^2t+\backslash sin^2t\; -1=0\backslash ;$, one gets the implicit representation :$\backslash det(\backslash vec\; x\backslash !-\backslash !\backslash vec\; f\backslash !\_0,\backslash vec\; f\backslash !\_2)^2+\backslash det(\backslash vec\; f\backslash !\_1,\backslash vec\; x\backslash !-\backslash !\backslash vec\; f\backslash !\_0)^2-\backslash det(\backslash vec\; f\backslash !\_1,\backslash vec\; f\backslash !\_2)^2=0$. ;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 $\backslash vec\; f\backslash !\_0,\; \backslash vec\; f\backslash !\_1,\; \backslash vec\; f\backslash !\_2$ to be vectors in space.

** Polar forms **

** Polar form relative to center **

In polar coordinates, with the origin at the center of the ellipse and with the angular coordinate $\backslash theta$ measured from the major axis, the ellipse's equation is
: $r(\backslash theta)\; =\; \backslash frac=\backslash frac$

** Polar form relative to focus **

If instead we use polar coordinates with the origin at one focus, with the angular coordinate $\backslash theta\; =\; 0$ still measured from the major axis, the ellipse's equation is
: $r(\backslash theta)=\backslash frac$
where the sign in the denominator is negative if the reference direction $\backslash theta\; =\; 0$ 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 $\backslash phi$, the polar form is
:$r(\backslash theta)=\backslash frac.$
The angle $\backslash theta$ in these formulas is called the true anomaly of the point. The numerator of these formulas is the semi-latus rectum $\backslash ell=a\; (1-e^2)$.

Eccentricity and the directrix property

Each of the two lines parallel to the minor axis, and at a distance of $d\; =\; \backslash frac\; =\; \backslash frac$ 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: :: $\backslash frac\; =\; \backslash frac\; =\; e\; =\; \backslash frac\backslash \; .$ The proof for the pair $F\_1,\; l\_1$ follows from the fact that $\backslash left|PF\_1\backslash ^2\; =\; (x\; -\; c)^2\; +\; y^2,\backslash \; \backslash left|Pl\_1\backslash ^2\; =\; \backslash left(x\; -\; \backslash tfrac\backslash right)^2$ and $y^2\; =\; b^2\; -\; \backslash tfracx^2$ satisfy the equation :$\backslash left|PF\_1\backslash ^2\; -\; \backslash frac\backslash left|Pl\_1\backslash ^2\; =\; 0\backslash \; .$ 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: :: $E\; =\; \backslash left\backslash .$ The choice $e\; =\; 0$, which is the eccentricity of a circle, is not allowed in this context. One may consider the directrix of a circle to be the line at infinity. (The choice $e\; =\; 1$ yields a parabola, and if $e\; >\; 1$, a hyperbola.) ;Proof Let $F\; =\; (f,\backslash ,\; 0),\backslash \; e\; >\; 0$, and assume $(0,\backslash ,\; 0)$ is a point on the curve. The directrix $l$ has equation $x\; =\; -\backslash tfrac$. With $P\; =\; (x,\backslash ,\; y)$, the relation $|PF|^2\; =\; e^2|Pl|^2$ produces the equations :$(x\; -\; f)^2\; +\; y^2\; =\; e^2\backslash left(x\; +\; \backslash frac\backslash right)^2\; =\; (ex\; +\; f)^2$ and $x^2\backslash left(e^2\; -\; 1\backslash right)\; +\; 2xf(1\; +\; e)\; -\; y^2\; =\; 0.$ The substitution $p\; =\; f(1\; +\; e)$ yields : $x^2\backslash left(e^2\; -\; 1\backslash right)\; +\; 2px\; -\; y^2\; =\; 0.$ 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 $a,\backslash ,\; b$ so that $1\; -\; e^2\; =\; \backslash tfrac,\; \backslash text\backslash \; p\; =\; \backslash tfrac$, and then the equation above becomes :$\backslash frac\; +\; \backslash frac\; =\; 1\backslash \; ,$ which is the equation of an ellipse with center $(a,\backslash ,\; 0)$, the ''x''-axis as major axis, and the major/minor semi axis $a,\backslash ,\; b$. ;General ellipse If the focus is $F\; =\; \backslash left(f\_1,\backslash ,\; f\_2\backslash right)$ and the directrix $ux\; +\; vy\; +\; w\; =\; 0$, one obtains the equation :$\backslash left(x\; -\; f\_1\backslash right)^2\; +\; \backslash left(y\; -\; f\_2\backslash right)^2\; =\; e^2\; \backslash frac\backslash \; .$ (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 $\backslash overline,\backslash ,\; \backslash overline$.
; 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 $\backslash overline$ 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 $\backslash overline,\backslash ,\; \backslash overline$. 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\; =\; \backslash left|LF\_2\backslash \; <\; \backslash left|QF\_2\backslash \; +\; \backslash left|QL\backslash \; =\; \backslash left|QF\_2\backslash \; +\; \backslash left|QF\_1\backslash $ holds, which means: $\backslash left|QF\_2\backslash \; +\; \backslash left|QF\_1\backslash \; >\; 2a$. 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 $d\_1,\backslash ,\; d\_2$ of an ellipse are ''conjugate'' if the midpoints of chords parallel to $d\_1$ lie on $d\_2\backslash \; .$
From the diagram one finds:
: Two diameters $\backslash overline,\backslash ,\; \backslash overline$ of an ellipse are conjugate whenever the tangents at $P\_1$ and $Q\_1$ are parallel to $\backslash overline$.
Conjugate diameters in an ellipse generalize orthogonal diameters in a circle.
In the parametric equation for a general ellipse given above,
: $\backslash vec\; x\; =\; \backslash vec\; p(t)\; =\; \backslash vec\; f\backslash !\_0\; +\backslash vec\; f\backslash !\_1\; \backslash cos\; t\; +\; \backslash vec\; f\backslash !\_2\; \backslash sin\; t,$
any pair of points $\backslash vec\; p(t),\backslash \; \backslash vec\; p(t\; +\; \backslash pi)$ belong to a diameter, and the pair $\backslash vec\; p\backslash left(t\; +\; \backslash tfrac\backslash right),\backslash \; \backslash vec\; p\backslash left(t\; -\; \backslash tfrac\backslash right)$ belong to its conjugate diameter.

** Theorem of Apollonios on conjugate diameters **

For an ellipse with semi-axes $a,\backslash ,\; b$ 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 $c\_1,\backslash ,\; c\_2$ (see diagram) has the constant area $A\_\backslash Delta\; =\; \backslash fracab$, which can be expressed by $A\_\backslash Delta=\backslash tfrac\; 1\; 2\; c\_2d\_1=\backslash tfrac\; 1\; 2\; c\_1c\_2\backslash sin\backslash alpha$, too. $d\_1$ is the altitude of point $P\_1$ and $\backslash alpha$ the angle between the half diameters. Hence the area of the ellipse (see section metric properties) can be written as $A\_=\backslash pi\; ab=\backslash pi\; c\_2d\_1=\backslash pi\; c\_1c\_2\backslash sin\backslash alpha$.
:# The parallelogram of tangents adjacent to the given conjugate diameters has the $\backslash text\_\; =\; 4ab\backslash \; .$
; Proof:
Let the ellipse be in the canonical form with parametric equation
: $\backslash vec\; p(t)\; =\; (a\backslash cos\; t,\backslash ,\; b\backslash sin\; t)$.
The two points $\backslash vec\; c\_1\; =\; \backslash vec\; p(t),\backslash \; \backslash vec\; c\_2\; =\; \backslash vec\; p\backslash left(t\; +\; \backslash frac\backslash right)$ are on conjugate diameters (see previous section). From trigonometric formulae one obtains $\backslash vec\; c\_2\; =\; (-a\backslash sin\; t,\backslash ,\; b\backslash cos\; t)^\backslash mathsf$ and
: $\backslash left|\backslash vec\; c\_1\backslash ^2\; +\; \backslash left|\backslash vec\; c\_2\backslash ^2\; =\; \backslash cdots\; =\; a^2\; +\; b^2\backslash \; .$
The area of the triangle generated by $\backslash vec\; c\_1,\backslash ,\; \backslash vec\; c\_2$ is
: $A\_\backslash Delta\; =\; \backslash frac\backslash det\backslash left(\backslash vec\; c\_1,\backslash ,\; \backslash vec\; c\_2\backslash right)\; =\; \backslash cdots\; =\; \backslash fracab$
and from the diagram it can be seen that the area of the parallelogram is 8 times that of $A\_\backslash Delta$. Hence
: $\backslash text\_\; =\; 4ab\backslash \; .$

** Orthogonal tangents **

For the ellipse $\backslash tfrac+\backslash tfrac=1$ 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 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.
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 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 $(a\backslash cos\; t,\backslash ,\; b\backslash sin\; t)$ 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'', above):
: $(a\backslash cos\; t,\backslash ,\; b\backslash sin\; t)$
This representation can be modeled technically by two simple methods. In both cases center, the axes and semi axes $a,\backslash ,\; b$ 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 $(a\backslash cos\; t,\backslash ,\; b\backslash sin\; t)$, 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 $\backslash tfrac$. 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 $(a\backslash cos\; t,\backslash ,\; b\backslash sin\; t)$, 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 (principle)
File:L-Ellipsenzirkel.png|Ellipsograph due to Benjamin Bramer
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 $V\_1,\backslash ,\; V\_2$ is: $\backslash tfrac$
* The radius of curvature at the co-vertices $V\_3,\backslash ,\; V\_4$ is: $\backslash tfrac\backslash \; .$
The diagram shows an easy way to find the centers of curvature $C\_1\; =\; \backslash left(a\; -\; \backslash tfrac,\; 0\backslash right),\backslash ,\; C\_3\; =\; \backslash left(0,\; b\; -\; \backslash tfrac\backslash right)$ at vertex $V\_1$ and co-vertex $V\_3$, respectively:
# mark the auxiliary point $H\; =\; (a,\backslash ,\; b)$ and draw the line segment $V\_1\; V\_3\backslash \; ,$
# draw the line through $H$, which is perpendicular to the line $V\_1\; V\_3\backslash \; ,$
# 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 $B(U),\backslash ,\; B(V)$ of lines at two points $U,\backslash ,\; V$ (all lines containing $U$ and $V$, respectively) and a projective but not perspective mapping $\backslash pi$ 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 $\backslash tfrac\; +\; \backslash tfrac\; =\; 1$ one uses the pencils at the vertices $V\_1,\backslash ,\; V\_2$. Let $P\; =\; (0,\backslash ,\; b)$ be an upper co-vertex of the ellipse and $A\; =\; (-a,\backslash ,\; 2b),\backslash ,\; B\; =\; (a,\backslash ,2b)$.
$P$ is the center of the rectangle $V\_1,\backslash ,\; V\_2,\backslash ,\; B,\backslash ,\; A$. The side $\backslash overline$ 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 $\backslash overline$ 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 $C\_1,\backslash ,\; \backslash dotsc$ 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 $\backslash left(x\; -\; x\_\backslash circ\backslash right)^2\; +\; \backslash left(y\; -\; y\_\backslash circ\backslash right)^2\; =\; r^2$ is uniquely determined by three points $\backslash left(x\_1,\; y\_1\backslash right),\backslash ;\; \backslash left(x\_2,\backslash ,y\_2\backslash right),\backslash ;\; \backslash left(x\_3,\backslash ,\; y\_3\backslash right)$ not on a line. A simple way to determine the parameters $x\_\backslash circ,y\_\backslash circ,r$ uses the ''inscribed angle theorem'' for circles:
: For four points $P\_i\; =\; \backslash left(x\_i,\backslash ,\; y\_i\backslash right),\backslash \; i\; =\; 1,\backslash ,\; 2,\backslash ,\; 3,\backslash ,\; 4,\backslash ,$ (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 $y\; =\; m\_1x\; +\; d\_1,\backslash \; y\; =\; m\_2x\; +\; d\_2,\backslash \; m\_1\; \backslash ne\; m\_2,$ one uses the quotient:
:: $\backslash frac\; =\; \backslash cot\backslash theta\backslash \; .$

Inscribed angle theorem for circles

For four points $P\_i\; =\; \backslash left(x\_i,\backslash ,\; y\_i\backslash right),\backslash \; i\; =\; 1,\backslash ,\; 2,\backslash ,\; 3,\backslash ,\; 4,\backslash ,$ 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: :: $\backslash frac\; =\; \backslash frac\; .$ 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 $P\_i\; =\; \backslash left(x\_i,\backslash ,\; y\_i\backslash right)$: :: $\backslash frac\; =\; \backslash frac\; .$ For example, for $P\_1\; =\; (2,\backslash ,\; 0),\backslash ;\; P\_2\; =\; (0,\backslash ,\; 1),\backslash ;\; P\_3\; =\; (0,\backslash ,0)$ the three-point equation is: : $\backslash frac\; =\; 0$, which can be rearranged to $(x\; -\; 1)^2\; +\; \backslash left(y\; -\; \backslash tfrac\backslash right)^2\; =\; \backslash tfrac\backslash \; .$ Using vectors, dot products and determinants this formula can be arranged more clearly, letting $\backslash vec\; x\; =\; (x,\backslash ,\; y)$: : $\backslash frac\; =\; \backslash frac\; .$ The center of the circle $\backslash left(x\_\backslash circ,\backslash ,\; y\_\backslash circ\backslash right)$ satisfies: : $\backslash begin\; 1\; \&\; \backslash frac\; \backslash \backslash \; \backslash frac\; \&\; 1\; \backslash end\; \backslash begin\; x\_\backslash circ\; \backslash \backslash \; y\_\backslash circ\; \backslash end\; =\; \backslash begin\; \backslash frac\; \backslash \backslash \; \backslash frac\; \backslash end.$ The radius is the distance between any of the three points and the center. : $r\; =\; \backslash sqrt\; =\; \backslash sqrt\; =\; \backslash sqrt.$

** Ellipses **

This section, we consider the family of ellipses defined by equations $\backslash tfrac\; +\; \backslash tfrac\; =\; 1$ with a ''fixed'' eccentricity $e$. It is convenient to use the parameter:
: $=\; \backslash frac\; =\; \backslash frac,$
and to write the ellipse equation as:
: $\backslash left(x\; -\; x\_\backslash circ\backslash right)^2\; +\; \backslash ,\; \backslash left(y\; -\; y\_\backslash circ\backslash right)^2\; =\; a^2,$
where ''q'' is fixed and $x\_\backslash circ,\backslash ,\; y\_\backslash circ,\backslash ,\; a$ 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 $y\; =\; m\_1x\; +\; d\_1,\backslash \; y\; =\; m\_2x\; +\; d\_2,\backslash \; m\_1\; \backslash ne\; m\_2$ one uses the quotient:
:: $\backslash frac\backslash \; .$

Inscribed angle theorem for ellipses

: Given four points $P\_i\; =\; \backslash left(x\_i,\backslash ,\; y\_i\backslash right),\backslash \; i\; =\; 1,\backslash ,\; 2,\backslash ,\; 3,\backslash ,\; 4$, no three of them on a line (see diagram). : The four points are on an ellipse with equation $(x\; -\; x\_\backslash circ)^2\; +\; \backslash ,\; (y\; -\; y\_\backslash circ)^2\; =\; a^2$ if and only if the angles at $P\_3$ and $P\_4$ are equal in the sense of the measurement above—that is, if :: $\backslash frac\; =\; \backslash frac\; \backslash \; .$ 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 $P\_i\; =\; \backslash left(x\_i,\backslash ,\; y\_i\backslash right)$: :: $\backslash frac\; =\; \backslash frac\; \backslash \; .$ For example, for $P\_1\; =\; (2,\backslash ,\; 0),\backslash ;\; P\_2\; =\; (0,\backslash ,1),\backslash ;\; P\_3\; =\; (0,\backslash ,\; 0)$ and $q\; =\; 4$ one obtains the three-point form : $\backslash frac\; =\; 0$ and after conversion $\backslash frac\; +\; \backslash frac\; =\; 1.$ Analogously to the circle case, the equation can be written more clearly using vectors: : $\backslash frac\; =\; \backslash frac\; ,$ where $*$ is the modified dot product $\backslash vec\; u*\backslash vec\; v\; =\; u\_x\; v\_x\; +\; \backslash ,u\_y\; v\_y.$

** Pole-polar relation **

Any ellipse can be described in a suitable coordinate system by an equation $\backslash tfrac\; +\; \backslash tfrac\; =\; 1$. The equation of the tangent at a point $P\_1\; =\; \backslash left(x\_1,\backslash ,\; y\_1\backslash right)$ of the ellipse is $\backslash tfrac\; +\; \backslash tfrac\; =\; 1.$ If one allows point $P\_1\; =\; \backslash left(x\_1,\backslash ,\; y\_1\backslash right)$ to be an arbitrary point different from the origin, then
: point $P\_1\; =\; \backslash left(x\_1,\backslash ,\; y\_1\backslash right)\; \backslash neq\; (0,\backslash ,\; 0)$ is mapped onto the line $\backslash tfrac\; +\; \backslash tfrac\; =\; 1$, not through the center of the ellipse.
This relation between points and lines is a bijection.
The inverse function maps
* line $y\; =\; mx\; +\; d,\backslash \; d\; \backslash ne\; 0$ onto the point $\backslash left(-\backslash tfrac,\backslash ,\; \backslash tfrac\backslash right)$ and
* line $x\; =\; c,\backslash \; c\; \backslash ne\; 0$ onto the point $\backslash left(\backslash tfrac,\backslash ,\; 0\backslash right).$
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: $P\_1,\backslash ,\; p\_1$).
* 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: $P\_2,\backslash ,\; p\_2$).
* For a point ''within'' the ellipse, the polar has no point with the ellipse in common (see diagram: $F\_1,\backslash ,\; l\_1$).
# The intersection point of two polars is the pole of the line through their poles.
# The foci $(c,\backslash ,\; 0)$ and $(-c,\backslash ,\; 0)$, respectively, and the directrices $x\; =\; \backslash tfrac$ and $x\; =\; -\backslash tfrac$, 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 $A\_\backslash text$ enclosed by an ellipse is:
where $a$ and $b$ are the lengths of the semi-major and semi-minor axes, respectively. The area formula $\backslash pi\; a\; b$ is intuitive: start with a circle of radius $b$ (so its area is $\backslash pi\; b^2$) and stretch it by a factor $a/b$ to make an ellipse. This scales the area by the same factor: $\backslash pi\; b^2(a/b)\; =\; \backslash pi\; a\; b.$ It is also easy to rigorously prove the area formula using integration as follows. Equation () can be rewritten as $y(x)=\; b\; \backslash sqrt.$ For $x\backslash ina,a$ this curve is the top half of the ellipse. So twice the integral of $y(x)$ over the interval $a,a/math>\; will\; be\; the\; area\; of\; the\; ellipse:\; :$ \backslash begin\; A\_\backslash text\; \&=\; \backslash int\_^a\; 2b\backslash sqrt\backslash ,dx\backslash \backslash \; \&=\; \backslash frac\; ba\; \backslash int\_^a\; 2\backslash sqrt\backslash ,dx.\; \backslash end$The\; second\; integral\; is\; the\; area\; of\; a\; circle\; of\; radius$ a,$that\; is,$ \backslash pi\; a^2.$So\; :$ A\_\backslash text\; =\; \backslash frac\backslash pi\; a^2\; =\; \backslash pi\; ab.$An\; ellipse\; defined\; implicitly\; by$ Ax^2+\; Bxy\; +\; Cy^2\; =\; 1$has\; area$ 2\backslash pi\; /\; \backslash sqrt.$The\; area\; can\; also\; be\; expressed\; in\; terms\; of\; eccentricity\; and\; the\; length\; of\; the\; semi-major\; axis\; as$ a^2\backslash pi\backslash sqrt$(obtained\; by\; solving\; for\; flattening,\; then\; computing\; the\; semi-minor\; axis).The\; area\; enclosed\; by\; a\; tilted\; ellipse\; is$ \backslash pi\backslash ;\; y\_\backslash text\backslash ,\; x\_\backslash text$.So\; far\; we\; have\; dealt\; with\; \text{'}\text{'}erect\text{'}\text{'}\; ellipses,\; whose\; major\; and\; minor\; axes\; are\; parallel\; to\; the$ x$and$ y$axes.\; However,\; some\; applications\; require\; \text{'}\text{'}tilted\text{'}\text{'}\; ellipses.\; In\; charged-particle\; beam\; optics,\; for\; instance,\; the\; enclosed\; area\; of\; an\; erect\; or\; tilted\; ellipse\; is\; an\; important\; property\; of\; the\; beam,\; its\; \text{'}\text{'}emittance\text{'}\text{'}.\; In\; this\; case\; a\; simple\; formula\; still\; applies,\; namely\; where$ y\_$,$ x\_$are\; intercepts\; and$ x\_$,$ y\_$are\; maximum\; values.\; It\; follows\; directly\; fromAppolonio\text{'}s\; theorem.$

Circumference

The circumference $C$ of an ellipse is: : $C\; \backslash ,=\backslash ,\; 4a\backslash int\_0^\backslash sqrt\; \backslash \; d\backslash theta\; \backslash ,=\backslash ,\; 4\; a\; \backslash ,E(e)$ where again $a$ is the length of the semi-major axis, $e=\backslash sqrt$ is the eccentricity, and the function $E$ is the complete elliptic integral of the second kind, : $E(e)\; \backslash ,=\backslash ,\; \backslash int\_0^\backslash sqrt\; \backslash \; d\backslash theta$ 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. The exact infinite series is: :$\backslash begin\; C\; \&=\; 2\backslash pi\; a\; \backslash leftright\backslash \backslash \; \&=\; 2\backslash pi\; a\; \backslash left-\; \backslash sum\_^\backslash infty\; \backslash left(\backslash frac\backslash right)^2\; \backslash frac\backslash right\backslash \backslash \; \&=\; -2\backslash pi\; a\; \backslash sum\_^\backslash infty\; \backslash left(\backslash frac\backslash right)^2\; \backslash frac,\; \backslash end$ where $n!!$ is the double factorial (extended to negative odd integers by the recurrence relation $(2n-1)!!\; =\; (2n+1)!!/(2n+1)$, for $n\; \backslash le\; 0$). This series converges, but by expanding in terms of $h\; =\; (a-b)^2\; /\; (a+b)^2,$ James Ivory and Bessel derived an expression that converges much more rapidly: :$\backslash begin\; C\; \&=\; \backslash pi\; (a+b)\; \backslash sum\_^\backslash infty\; \backslash left(\backslash frac\backslash right)^2\; h^n\; \backslash \backslash \; \&=\; \backslash pi\; (a+b)\; \backslash left+\; \backslash frac\; +\; \backslash sum\_^\backslash infty\; \backslash left(\backslash frac\backslash right)^2\; h^n\backslash right\backslash \backslash \; \&=\; \backslash pi\; (a+b)\; \backslash left+\; \backslash sum\_^\backslash infty\; \backslash left(\backslash frac\backslash right)^2\; \backslash frac\backslash right\backslash end$ Srinivasa Ramanujan gives two close approximations for the circumference in §16 of "Modular Equations and Approximations to $\backslash pi$"; they are : $C\; \backslash approx\; \backslash pi\; \backslash biggl(a\; +\; b)\; -\; \backslash sqrt\; \backslash biggr=\; \backslash pi\; \backslash biggl(a\; +\; b)\; -\; \backslash sqrt\backslash biggr/math>\; and\; :$ C\backslash approx\backslash pi\backslash left(a+b\backslash right)\backslash left(1+\backslash frac\backslash right).$The\; errors\; in\; these\; approximations,\; which\; were\; obtained\; empirically,\; are\; of\; order$ h^3$and$ h^5,$respectively.\; More\; generally,\; thearc\; lengthof\; a\; portion\; of\; the\; circumference,\; as\; a\; function\; of\; the\; angle\; subtended\; (or\; -coordinates\; of\; any\; two\; points\; on\; the\; upper\; half\; of\; the\; ellipse),\; is\; given\; by\; an\; incompleteelliptic\; integral.\; The\; upper\; half\; of\; an\; ellipse\; is\; parameterized\; by\; :$ y=b\backslash sqrt.$Then\; the\; arc\; length$ s$from$ x\_$to$ x\_$is:\; :$ s\; =\; -b\backslash int\_^\; \backslash sqrt\; \backslash ,\; dz.$This\; is\; equivalent\; to\; :$ s\; =\; -b\backslash left\backslash ;\; 1\; -\; \backslash frac\backslash right)\backslash right\_$where$ E(z\; \backslash mid\; m)$is\; the\; incomplete\; elliptic\; integral\; of\; the\; second\; kind\; with\; parameter$ m=k^.$Theinverse\; function,\; the\; angle\; subtended\; as\; a\; function\; of\; the\; arc\; length,\; is\; given\; by\; a\; certainelliptic\; function.\; Some\; lower\; and\; upper\; bounds\; on\; the\; circumference\; of\; the\; canonical\; ellipse$ x^2/a^2\; +\; y^2/b^2\; =\; 1$with$ a\backslash geq\; b$are\; :$ \backslash begin\; 2\backslash pi\; b\; \&\backslash le\; C\; \backslash le\; 2\backslash pi\; a,\; \backslash \backslash \; \backslash pi\; (a+b)\; \&\backslash le\; C\; \backslash le\; 4(a+b),\; \backslash \backslash \; 4\backslash sqrt\; \&\backslash le\; C\; \backslash le\; \backslash sqrt\; \backslash pi\; \backslash sqrt\; .\; \backslash end$Here\; the\; upper\; bound$ 2\backslash pi\; a$is\; the\; circumference\; of\; acircumscribedconcentric\; circlepassing\; through\; the\; endpoints\; of\; the\; ellipse\text{'}s\; major\; axis,\; and\; the\; lower\; bound$ 4\backslash sqrt$is\; theperimeterof\; aninscribedrhombuswithverticesat\; the\; endpoints\; of\; the\; major\; and\; the\; minor\; axes.$

** Curvature **

The curvature is given by $\backslash kappa\; =\; \backslash frac\backslash left(\backslash frac+\backslash frac\backslash right)^\backslash \; ,$
radius of curvature at point $(x,y)$:
: $\backslash rho\; =\; a^2\; b^2\; \backslash left(\backslash frac\; +\; \backslash frac\backslash right)^\backslash frac\; =\; \backslash frac\; \backslash sqrt\; \backslash \; .$
Radius of curvature at the two ''vertices'' $(\backslash pm\; a,0)$ and the centers of curvature:
: $\backslash rho\_0\; =\; \backslash frac=p\backslash \; ,\; \backslash qquad\; \backslash left(\backslash pm\backslash frac\backslash ,\backslash bigg|\backslash ,0\backslash right)\backslash \; .$
Radius of curvature at the two ''co-vertices'' $(0,\backslash pm\; b)$ and the centers of curvature:
: $\backslash rho\_1\; =\; \backslash frac\backslash \; ,\; \backslash qquad\; \backslash left(0\backslash ,\backslash bigg|\backslash ,\backslash pm\backslash frac\backslash right)\backslash \; .$

** In triangle geometry **

Ellipses appear in triangle geometry as
# Steiner ellipse: ellipse through the vertices of the triangle with center at the centroid,
# inellipses: ellipses which touch the sides of a triangle. Special cases are the Steiner inellipse and the Mandart inellipse.

** As plane sections of quadrics **

Ellipses appear as plane sections of the following quadrics:
* Ellipsoid
* Elliptic cone
* Elliptic cylinder
* 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, 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 along a line of the paper; such mirrors are used in some document scanners.
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''. 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, Utah; at an exhibit on sound at the Museum of Science and Industry in Chicago; in front of the University of Illinois at Urbana–Champaign 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 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 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 and quantum effects, which become significant when the particles are moving at high speed.)
For elliptical orbits, useful relations involving the eccentricity $e$ are:
: $\backslash begin\; e\; \&=\; \backslash frac\; =\; \backslash frac\; \backslash \backslash \; r\_a\; \&=\; (1\; +\; e)a\; \backslash \backslash \; r\_p\; \&=\; (1\; -\; e)a\; \backslash end$
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, the semi-minor axis $b$ is their geometric mean, and the semi-latus rectum $\backslash ell$ is their harmonic mean. In other words,
:$\backslash begin\; a\; \&=\; \backslash frac\; \backslash \backslash ptb\; \&=\; \backslash sqrt\; \backslash \backslash pt\backslash ell\; \&=\; \backslash frac\; =\; \backslash frac\; \backslash end$.

** Harmonic oscillators **

The general solution for a harmonic oscillator in two or more dimensions 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; 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 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 gears 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 or torque 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 depends on the direction of the light. The dependency can be described by an index ellipsoid. (If the material is optically isotropic, 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, 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, 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 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 curves may also be used to draw an ellipse to sufficient accuracy, since any ellipse may be construed as an affine transformation 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 is quite useful for solving this problem.

** See also **

* Cartesian oval, a generalization of the ellipse
* Circumconic and inconic
* Distance of closest approach of ellipses
* Ellipse fitting
* Elliptic coordinates, an orthogonal coordinate system based on families of ellipses and hyperbolae
* Elliptic partial differential equation
* Elliptical distribution, 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, 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, eccentric, and mean anomaly

Notes

** References **

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** External links **

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Apollonius' Derivation of the Ellipse

at Convergence

''The Shape and History of The Ellipse in Washington, D.C.''

by Clark Kimberling

Ellipse circumference calculator

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Trammel according Frans van Schooten

{{Authority control Category:Conic sections Category:Plane curves Category:Elementary shapes Category:Algebraic curves

Rotated ellipse

* The center is the origin $z\; =\; (0,\; 0)$. * $\backslash theta$ is the angle measured from ''x''-axis. * The parameter $t$ (called the ''eccentric anomaly'' in astronomy) is not the angle of $(x(t),y(t))$ with the ''x''-axis. * $a$ and $b$ are the semi-axes in the x and y directions, respectively. :$\backslash mathbf\; =\backslash mathbf(\backslash theta)\; =\; x\backslash cos\backslash theta\; -\; y\backslash sin\backslash theta$ :$\backslash mathbf\; =\; \backslash mathbf(\backslash theta)\; =\; x\; \backslash sin\backslash theta\; +\; y\; \backslash cos\backslash theta$ Here * $\backslash theta$ is fixed (constant value) * ''t'' is a parameter = independent variable used to parametrize the ellipse :$\backslash mathbf\; =\backslash mathbf\_(t)\; =\; a\backslash cos\backslash \; t\backslash cos\backslash theta\; -\; b\backslash sin\backslash \; t\backslash sin\backslash theta$ :$\backslash mathbf\; =\backslash mathbf\_(t)\; =\; a\backslash cos\backslash \; t\backslash sin\backslash theta\; +\; b\backslash sin\backslash \; t\backslash cos\backslash theta$ So :$\backslash frac+\backslash frac\; =\; 1\; .$ :$\backslash frac\; +\; \backslash frac=1$ :$\backslash frac\; +\; \backslash frac=1$

Standard parametric representation

Using trigonometric functions, a parametric representation of the standard ellipse $\backslash tfrac+\backslash tfrac\; =\; 1$ is: : $(x,\backslash ,\; y)\; =\; (a\; \backslash cos\; t,\backslash ,\; b\; \backslash sin\; t),\backslash \; 0\; \backslash le\; t\; <\; 2\backslash pi\backslash \; .$ 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 $u\; =\; \backslash tan\backslash left(\backslash frac\backslash right)$ and trigonometric formulae one obtains :$\backslash cos\; t\; =\; \backslash frac\backslash \; ,\backslash quad\; \backslash sin\; t\; =\; \backslash frac$ and the ''rational'' parametric equation of an ellipse : $\backslash begin\; x(u)\; \&=\; a\backslash frac\; \backslash \backslash \; y(u)\; \&=\; \backslash frac\; \backslash end\backslash ;,\backslash quad\; -\backslash infty\; <\; u\; <\; \backslash infty\backslash ;,$ which covers any point of the ellipse $\backslash tfrac\; +\; \backslash tfrac\; =\; 1$ except the left vertex $(-a,\backslash ,\; 0)$. For $u\; \backslash in,\backslash ,\; 1$ this formula represents the right upper quarter of the ellipse moving counter-clockwise with increasing $u.$ The left vertex is the limit $\backslash lim\_\; (x(u),\backslash ,\; y(u))\; =\; (-a,\backslash ,\; 0)\backslash ;.$ Rational representations of conic sections are commonly used in computer-aided design (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 $\backslash vec\; x(t)\; =\; (a\; \backslash cos\; t,\backslash ,\; b\; \backslash sin\; t)^\backslash mathsf$: :$\backslash vec\; x\text{'}(t)\; =\; (-a\backslash sin\; t,\backslash ,\; b\backslash cos\; t)^\backslash mathsf\; \backslash quad\; \backslash rightarrow\; \backslash quad\; m\; =\; -\backslash frac\backslash cot\; t\backslash quad\; \backslash rightarrow\; \backslash quad\; \backslash cot\; t\; =\; -\backslash frac.$ With help of trigonometric formulae one obtains: :$\backslash cos\; t\; =\; \backslash frac\; =\; \backslash frac\backslash \; ,\backslash quad\backslash quad\; \backslash sin\; t\; =\; \backslash frac\; =\; \backslash frac.$ Replacing $\backslash cos\; t$ and $\backslash sin\; t$ of the standard representation yields: : $\backslash vec\; c\_\backslash pm(m)\; =\; \backslash left(-\backslash frac,\backslash ;\backslash frac\backslash right),\backslash ,\; m\; \backslash in\; \backslash R.$ Here $m$ is the slope of the tangent at the corresponding ellipse point, $\backslash vec\; c\_+$ is the upper and $\backslash vec\; c\_-$ the lower half of the ellipse. The vertices$(\backslash pm\; a,\backslash ,\; 0)$, having vertical tangents, are not covered by the representation. The equation of the tangent at point $\backslash vec\; c\_\backslash pm(m)$ has the form $y\; =\; mx\; +\; n$. The still unknown $n$ can be determined by inserting the coordinates of the corresponding ellipse point $\backslash vec\; c\_\backslash pm(m)$: : $y\; =\; mx\; \backslash pm\backslash sqrt\backslash ;\; .$ 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 transformations: : 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 $\backslash vec\; x\; \backslash mapsto\; \backslash vec\; f\backslash !\_0\; +\; A\backslash vec\; x$, where $A$ is a regular matrix (with non-zero determinant) and $\backslash vec\; f\backslash !\_0$ is an arbitrary vector. If $\backslash vec\; f\backslash !\_1,\; \backslash vec\; f\backslash !\_2$ are the column vectors of the matrix $A$, the unit circle $(\backslash cos(t),\; \backslash sin(t))$, $0\; \backslash leq\; t\; \backslash leq\; 2\backslash pi$, is mapped onto the ellipse: : $\backslash vec\; x\; =\; \backslash vec\; p(t)\; =\; \backslash vec\; f\backslash !\_0\; +\; \backslash vec\; f\backslash !\_1\; \backslash cos\; t\; +\; \backslash vec\; f\backslash !\_2\; \backslash sin\; t\; \backslash \; .$ Here $\backslash vec\; f\backslash !\_0$ is the center and $\backslash vec\; f\backslash !\_1,\backslash ;\; \backslash vec\; f\backslash !\_2$ are the directions of two conjugate diameters, in general not perpendicular. ;Vertices The four vertices of the ellipse are $\backslash vec\; p(t\_0),\backslash ;\backslash vec\; p\backslash left(t\_0\; \backslash pm\; \backslash tfrac\backslash right),\backslash ;\; \backslash vec\; p\backslash left(t\_0\; +\; \backslash pi\backslash right)$, for a parameter $t\; =\; t\_0$ defined by: : $\backslash cot\; (2t\_0)\; =\; \backslash frac.$ (If $\backslash vec\; f\backslash !\_1\; \backslash cdot\; \backslash vec\; f\backslash !\_2\; =\; 0$, then $t\_0\; =\; 0$.) This is derived as follows. The tangent vector at point $\backslash vec\; p(t)$ is: : $\backslash vec\; p\backslash ,\text{'}(t)\; =\; -\backslash vec\; f\backslash !\_1\backslash sin\; t\; +\; \backslash vec\; f\backslash !\_2\backslash cos\; t\; \backslash \; .$ At a vertex parameter $t\; =\; t\_0$, the tangent is perpendicular to the major/minor axes, so: : $0\; =\; \backslash vec\; p\text{'}(t)\; \backslash cdot\; \backslash left(\backslash vec\; p(t)\; -\backslash vec\; f\backslash !\_0\backslash right)\; =\; \backslash left(-\backslash vec\; f\backslash !\_1\backslash sin\; t\; +\; \backslash vec\; f\backslash !\_2\backslash cos\; t\backslash right)\; \backslash cdot\; \backslash left(\backslash vec\; f\backslash !\_1\; \backslash cos\; t\; +\; \backslash vec\; f\backslash !\_2\; \backslash sin\; t\backslash right).$ Expanding and applying the identities $\backslash cos^2\; t\; -\backslash sin^2\; t=\backslash cos\; 2t,\backslash \; \backslash \; 2\backslash sin\; t\; \backslash cos\; t\; =\; \backslash sin\; 2t$ gives the equation for $t\; =\; t\_0$. ;Implicit representation Solving the parametric representation for $\backslash ;\; \backslash cos\; t,\backslash sin\; t\backslash ;$ by Cramer's rule and using $\backslash ;\backslash cos^2t+\backslash sin^2t\; -1=0\backslash ;$, one gets the implicit representation :$\backslash det(\backslash vec\; x\backslash !-\backslash !\backslash vec\; f\backslash !\_0,\backslash vec\; f\backslash !\_2)^2+\backslash det(\backslash vec\; f\backslash !\_1,\backslash vec\; x\backslash !-\backslash !\backslash vec\; f\backslash !\_0)^2-\backslash det(\backslash vec\; f\backslash !\_1,\backslash vec\; f\backslash !\_2)^2=0$. ;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 $\backslash vec\; f\backslash !\_0,\; \backslash vec\; f\backslash !\_1,\; \backslash vec\; f\backslash !\_2$ to be vectors in space.

Eccentricity and the directrix property

Each of the two lines parallel to the minor axis, and at a distance of $d\; =\; \backslash frac\; =\; \backslash frac$ 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: :: $\backslash frac\; =\; \backslash frac\; =\; e\; =\; \backslash frac\backslash \; .$ The proof for the pair $F\_1,\; l\_1$ follows from the fact that $\backslash left|PF\_1\backslash ^2\; =\; (x\; -\; c)^2\; +\; y^2,\backslash \; \backslash left|Pl\_1\backslash ^2\; =\; \backslash left(x\; -\; \backslash tfrac\backslash right)^2$ and $y^2\; =\; b^2\; -\; \backslash tfracx^2$ satisfy the equation :$\backslash left|PF\_1\backslash ^2\; -\; \backslash frac\backslash left|Pl\_1\backslash ^2\; =\; 0\backslash \; .$ 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: :: $E\; =\; \backslash left\backslash .$ The choice $e\; =\; 0$, which is the eccentricity of a circle, is not allowed in this context. One may consider the directrix of a circle to be the line at infinity. (The choice $e\; =\; 1$ yields a parabola, and if $e\; >\; 1$, a hyperbola.) ;Proof Let $F\; =\; (f,\backslash ,\; 0),\backslash \; e\; >\; 0$, and assume $(0,\backslash ,\; 0)$ is a point on the curve. The directrix $l$ has equation $x\; =\; -\backslash tfrac$. With $P\; =\; (x,\backslash ,\; y)$, the relation $|PF|^2\; =\; e^2|Pl|^2$ produces the equations :$(x\; -\; f)^2\; +\; y^2\; =\; e^2\backslash left(x\; +\; \backslash frac\backslash right)^2\; =\; (ex\; +\; f)^2$ and $x^2\backslash left(e^2\; -\; 1\backslash right)\; +\; 2xf(1\; +\; e)\; -\; y^2\; =\; 0.$ The substitution $p\; =\; f(1\; +\; e)$ yields : $x^2\backslash left(e^2\; -\; 1\backslash right)\; +\; 2px\; -\; y^2\; =\; 0.$ 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 $a,\backslash ,\; b$ so that $1\; -\; e^2\; =\; \backslash tfrac,\; \backslash text\backslash \; p\; =\; \backslash tfrac$, and then the equation above becomes :$\backslash frac\; +\; \backslash frac\; =\; 1\backslash \; ,$ which is the equation of an ellipse with center $(a,\backslash ,\; 0)$, the ''x''-axis as major axis, and the major/minor semi axis $a,\backslash ,\; b$. ;General ellipse If the focus is $F\; =\; \backslash left(f\_1,\backslash ,\; f\_2\backslash right)$ and the directrix $ux\; +\; vy\; +\; w\; =\; 0$, one obtains the equation :$\backslash left(x\; -\; f\_1\backslash right)^2\; +\; \backslash left(y\; -\; f\_2\backslash right)^2\; =\; e^2\; \backslash frac\backslash \; .$ (The right side of the equation uses the Hesse normal form of a line to calculate the distance $|Pl|$.)

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.

Inscribed angle theorem for circles

For four points $P\_i\; =\; \backslash left(x\_i,\backslash ,\; y\_i\backslash right),\backslash \; i\; =\; 1,\backslash ,\; 2,\backslash ,\; 3,\backslash ,\; 4,\backslash ,$ 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: :: $\backslash frac\; =\; \backslash frac\; .$ 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 $P\_i\; =\; \backslash left(x\_i,\backslash ,\; y\_i\backslash right)$: :: $\backslash frac\; =\; \backslash frac\; .$ For example, for $P\_1\; =\; (2,\backslash ,\; 0),\backslash ;\; P\_2\; =\; (0,\backslash ,\; 1),\backslash ;\; P\_3\; =\; (0,\backslash ,0)$ the three-point equation is: : $\backslash frac\; =\; 0$, which can be rearranged to $(x\; -\; 1)^2\; +\; \backslash left(y\; -\; \backslash tfrac\backslash right)^2\; =\; \backslash tfrac\backslash \; .$ Using vectors, dot products and determinants this formula can be arranged more clearly, letting $\backslash vec\; x\; =\; (x,\backslash ,\; y)$: : $\backslash frac\; =\; \backslash frac\; .$ The center of the circle $\backslash left(x\_\backslash circ,\backslash ,\; y\_\backslash circ\backslash right)$ satisfies: : $\backslash begin\; 1\; \&\; \backslash frac\; \backslash \backslash \; \backslash frac\; \&\; 1\; \backslash end\; \backslash begin\; x\_\backslash circ\; \backslash \backslash \; y\_\backslash circ\; \backslash end\; =\; \backslash begin\; \backslash frac\; \backslash \backslash \; \backslash frac\; \backslash end.$ The radius is the distance between any of the three points and the center. : $r\; =\; \backslash sqrt\; =\; \backslash sqrt\; =\; \backslash sqrt.$

Inscribed angle theorem for ellipses

: Given four points $P\_i\; =\; \backslash left(x\_i,\backslash ,\; y\_i\backslash right),\backslash \; i\; =\; 1,\backslash ,\; 2,\backslash ,\; 3,\backslash ,\; 4$, no three of them on a line (see diagram). : The four points are on an ellipse with equation $(x\; -\; x\_\backslash circ)^2\; +\; \backslash ,\; (y\; -\; y\_\backslash circ)^2\; =\; a^2$ if and only if the angles at $P\_3$ and $P\_4$ are equal in the sense of the measurement above—that is, if :: $\backslash frac\; =\; \backslash frac\; \backslash \; .$ 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 $P\_i\; =\; \backslash left(x\_i,\backslash ,\; y\_i\backslash right)$: :: $\backslash frac\; =\; \backslash frac\; \backslash \; .$ For example, for $P\_1\; =\; (2,\backslash ,\; 0),\backslash ;\; P\_2\; =\; (0,\backslash ,1),\backslash ;\; P\_3\; =\; (0,\backslash ,\; 0)$ and $q\; =\; 4$ one obtains the three-point form : $\backslash frac\; =\; 0$ and after conversion $\backslash frac\; +\; \backslash frac\; =\; 1.$ Analogously to the circle case, the equation can be written more clearly using vectors: : $\backslash frac\; =\; \backslash frac\; ,$ where $*$ is the modified dot product $\backslash vec\; u*\backslash vec\; v\; =\; u\_x\; v\_x\; +\; \backslash ,u\_y\; v\_y.$

Circumference

The circumference $C$ of an ellipse is: : $C\; \backslash ,=\backslash ,\; 4a\backslash int\_0^\backslash sqrt\; \backslash \; d\backslash theta\; \backslash ,=\backslash ,\; 4\; a\; \backslash ,E(e)$ where again $a$ is the length of the semi-major axis, $e=\backslash sqrt$ is the eccentricity, and the function $E$ is the complete elliptic integral of the second kind, : $E(e)\; \backslash ,=\backslash ,\; \backslash int\_0^\backslash sqrt\; \backslash \; d\backslash theta$ 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. The exact infinite series is: :$\backslash begin\; C\; \&=\; 2\backslash pi\; a\; \backslash leftright\backslash \backslash \; \&=\; 2\backslash pi\; a\; \backslash left-\; \backslash sum\_^\backslash infty\; \backslash left(\backslash frac\backslash right)^2\; \backslash frac\backslash right\backslash \backslash \; \&=\; -2\backslash pi\; a\; \backslash sum\_^\backslash infty\; \backslash left(\backslash frac\backslash right)^2\; \backslash frac,\; \backslash end$ where $n!!$ is the double factorial (extended to negative odd integers by the recurrence relation $(2n-1)!!\; =\; (2n+1)!!/(2n+1)$, for $n\; \backslash le\; 0$). This series converges, but by expanding in terms of $h\; =\; (a-b)^2\; /\; (a+b)^2,$ James Ivory and Bessel derived an expression that converges much more rapidly: :$\backslash begin\; C\; \&=\; \backslash pi\; (a+b)\; \backslash sum\_^\backslash infty\; \backslash left(\backslash frac\backslash right)^2\; h^n\; \backslash \backslash \; \&=\; \backslash pi\; (a+b)\; \backslash left+\; \backslash frac\; +\; \backslash sum\_^\backslash infty\; \backslash left(\backslash frac\backslash right)^2\; h^n\backslash right\backslash \backslash \; \&=\; \backslash pi\; (a+b)\; \backslash left+\; \backslash sum\_^\backslash infty\; \backslash left(\backslash frac\backslash right)^2\; \backslash frac\backslash right\backslash end$ Srinivasa Ramanujan gives two close approximations for the circumference in §16 of "Modular Equations and Approximations to $\backslash pi$"; they are : $C\; \backslash approx\; \backslash pi\; \backslash biggl(a\; +\; b)\; -\; \backslash sqrt\; \backslash biggr=\; \backslash pi\; \backslash biggl(a\; +\; b)\; -\; \backslash sqrt\backslash biggr/math>\; and\; :$ C\backslash approx\backslash pi\backslash left(a+b\backslash right)\backslash left(1+\backslash frac\backslash right).$The\; errors\; in\; these\; approximations,\; which\; were\; obtained\; empirically,\; are\; of\; order$ h^3$and$ h^5,$respectively.\; More\; generally,\; thearc\; lengthof\; a\; portion\; of\; the\; circumference,\; as\; a\; function\; of\; the\; angle\; subtended\; (or\; -coordinates\; of\; any\; two\; points\; on\; the\; upper\; half\; of\; the\; ellipse),\; is\; given\; by\; an\; incompleteelliptic\; integral.\; The\; upper\; half\; of\; an\; ellipse\; is\; parameterized\; by\; :$ y=b\backslash sqrt.$Then\; the\; arc\; length$ s$from$ x\_$to$ x\_$is:\; :$ s\; =\; -b\backslash int\_^\; \backslash sqrt\; \backslash ,\; dz.$This\; is\; equivalent\; to\; :$ s\; =\; -b\backslash left\backslash ;\; 1\; -\; \backslash frac\backslash right)\backslash right\_$where$ E(z\; \backslash mid\; m)$is\; the\; incomplete\; elliptic\; integral\; of\; the\; second\; kind\; with\; parameter$ m=k^.$Theinverse\; function,\; the\; angle\; subtended\; as\; a\; function\; of\; the\; arc\; length,\; is\; given\; by\; a\; certainelliptic\; function.\; Some\; lower\; and\; upper\; bounds\; on\; the\; circumference\; of\; the\; canonical\; ellipse$ x^2/a^2\; +\; y^2/b^2\; =\; 1$with$ a\backslash geq\; b$are\; :$ \backslash begin\; 2\backslash pi\; b\; \&\backslash le\; C\; \backslash le\; 2\backslash pi\; a,\; \backslash \backslash \; \backslash pi\; (a+b)\; \&\backslash le\; C\; \backslash le\; 4(a+b),\; \backslash \backslash \; 4\backslash sqrt\; \&\backslash le\; C\; \backslash le\; \backslash sqrt\; \backslash pi\; \backslash sqrt\; .\; \backslash end$Here\; the\; upper\; bound$ 2\backslash pi\; a$is\; the\; circumference\; of\; acircumscribedconcentric\; circlepassing\; through\; the\; endpoints\; of\; the\; ellipse\text{'}s\; major\; axis,\; and\; the\; lower\; bound$ 4\backslash sqrt$is\; theperimeterof\; aninscribedrhombuswithverticesat\; the\; endpoints\; of\; the\; major\; and\; the\; minor\; axes.$

Physics

Statistics and finance

In statistics, 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 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.

Notes

Apollonius' Derivation of the Ellipse

at Convergence

''The Shape and History of The Ellipse in Washington, D.C.''

by Clark Kimberling

Ellipse circumference calculator

*

Trammel according Frans van Schooten

{{Authority control Category:Conic sections Category:Plane curves Category:Elementary shapes Category:Algebraic curves