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ellipse, closed plane curve consisting of all points for which the sum of the distances between a point on the curve and two fixed points (foci) is the same. It is the conic section formed by a plane cutting all the elements of the cone in the same nappe. The center of an ellipse is the point halfway between its foci. The major axis is the chord that passes through the foci. The minor axis is the chord that passes through the center perpendicular to the major axis. The *latus rectum* is the chord through either focus perpendicular to the major axis. The vertices are the two points of intersection of the major axis with the curve. The eccentricity of an ellipse, a ratio of two lengths, is a measure of its flatness; it is the distance from the center to either focus divided by the distance from the center to either vertex. The circle may be considered an ellipse of eccentricity zero, i.e., one in which the center and the two foci all coincide.

The Columbia Electronic Encyclopedia Copyright © 2004.

Licensed from Columbia University Press

Licensed from Columbia University Press

Closed curve, one of the conic sections of analytic geometry, consisting of all points whose distances from each of two fixed points (foci) add up to the same value. The midpoint between the foci is the center. One property of an ellipse is that the reflection off its boundary of a line from one focus will pass through the other. In an elliptical room, a person whispering at one focus is easily heard by someone at the other. An oval may or may not fit the definition of an ellipse.

Learn more about ellipse with a free trial on Britannica.com.

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

In mathematics, an ellipse (from the Greek ἔλλειψις, literally absence) is a conic section, the locus of points in a plane such that the sum of the distances to two fixed points is a constant. The two fixed points are called foci (singular- focus). An alternate definition would be that an ellipse is the path traced out by a point whose distance from a fixed point, called the focus, maintains a constant ratio less than one with its distance from a straight line not passing through the focus, called the directrix.

Algebraically, an ellipse is a curve in the Cartesian plane defined by an equation of the form

- $A\; x^2\; +\; B\; xy\; +\; C\; y^2\; +\; D\; x\; +\; E\; y\; +\; F\; =\; 0\; ,$

The line segment AB, that passes through the foci and terminates on the ellipse, is called the major axis. The major axis is the longest segment that can be obtained by joining two points on the ellipse. The line segment CD, which passes through the center (halfway between the foci), perpendicular to the major axis, and terminates on the ellipse, is called the minor axis. The semimajor axis (denoted by a in the figure) is one half the major axis: the line segment from the center, through a focus, and to the edge of the ellipse. Likewise, the semiminor axis (denoted by b in the figure) is one half the minor axis.

If the two foci coincide, then the ellipse is a circle; in other words, a circle is a special case of an ellipse, one where the eccentricity is zero.

An ellipse centered at the origin can be viewed as the image of the unit circle under a linear map associated with a symmetric matrix $A\; =\; PDP^T$, $D$ being a diagonal matrix with the eigenvalues of $A$, both of which are real positive, along the main diagonal, and $P$ being a real unitary matrix having as columns the eigenvectors of $A$. Then the axes of the ellipse will lie along the eigenvectors of $A$, and the (1 over the square root of the) eigenvalues are the lengths of the semimajor and semiminor axes, which are one-half of the lengths of the major and minor axes respectively.

An ellipse can be produced by multiplying the x coordinates of all points on a circle by a constant, without changing the y coordinates. This is equivalent to stretching the circle out in the x-direction.

- $varepsilon=sin(o!varepsilon)!!:;,o!varepsilon=arccosleft(frac\{b\}\{a\}right);,!$

The greater the eccentricity is, the larger the ratio of a to b, and therefore the more elongated the ellipse.

If c equals the distance from the center to either focus, then

- $varepsilon\; =\; frac\{c\}\{a\}!!:;,c=asin(o!varepsilon)=sqrt\{a^2-b^2\};,!$

An ellipse can be inscribed within a rectangle using two pins, a loop of string, and a pencil. The pins are placed at the foci and the pins and pencil are enclosed inside the loop. The pencil is placed on the paper inside the loop and the string made taut. The string will form a triangle. If the pencil is moved around with the string kept taut, the sum of the distances from the pencil to the pins will remain constant, thus satisfying the definition of an ellipse.

Consider the center of the rectangle to be the origin and the lengths of its sides to be 2a and 2b, with a being larger than b. The major axis then passes through the origin and is parallel to the longer side. The two pins are placed the distance c away from the origin in each direction along the major axis. The required length of the string can be seen in the particular case of the point of the ellipse lying on the major axis a. Here the length of the string is the distance from the first focus to the center c plus the distance from the center to the point on the ellipse a plus the distance from that point back to the second focus a-c, all together 2a.

- $frac\{(x-h)^\{2\}\}\{a^\{2\}\}+frac\{(y-k)^\{2\}\}\{b^\{2\}\}=1;,!$

This ellipse can be expressed parametrically as

- $x=h+a,cos\; t;,!$

- $y=k+b,sin\; t;,!$

Parametric form of an ellipse rotated counterclockwise by an angle $phi,!$:

- $x=h+a,cos\; t,cos\; phi\; -\; b,sin\; t,sin\; phi,;!$

- $y=k+b,sin\; t,cos\; phi+a,cos\; t,sinphi,;!$

The formula for the directrices is

- $x=hpmfrac\{a^2\}\{c\}=hpm\; a;csc(o!varepsilon)=hpmfrac\{a\}\{sin(o!varepsilon)\};,!$

If $h$ = 0 and $k$ = 0 (i.e., if the center is the origin (0,0)), then we can express this ellipse in polar coordinates by the equation

- $r=frac\{ab\}\{sqrt\{a^2sin^2theta+b^2cos^2theta\}\}=frac\{b\}\{sqrt\{1-varepsilon^2cos^2theta\}\};,!$

With one focus at the origin, the ellipse's polar equation is

- $r=frac\{acdot(1-varepsilon^\{2\})\}\{1+varepsiloncdotcostheta\};,!$

A Gauss-mapped form:

- $left(frac\{acosbeta\}\{sqrt\{a^2cos^2beta+b^2sin^2beta\}\},frac\{bsinbeta\}\{sqrt\{a^2cos^2beta+b^2sin^2beta\}\}right);$

In polar coordinates, an ellipse with one focus at the origin and the other on the negative x-axis is given by the equation

- $l=rcdot(1+sin(o!varepsilon)costheta)=rcdot(1+varepsiloncdotcostheta);,!$

An ellipse can also be thought of as a projection of a circle: a circle on a plane at angle φ to the horizontal projected vertically onto a horizontal plane gives an ellipse of eccentricity sin φ, provided φ is not 90°.

The circumference $C$ of an ellipse is $4\; a\; E(varepsilon)$, where the function $E$ is the complete elliptic integral of the second kind.

The exact infinite series is:

- $C\; =\; 2pi\; a\; left[\{1\; -\; left(\{1over\; 2\}right)^2varepsilon^2\; -\; left(\{1cdot\; 3over\; 2cdot\; 4\}right)^2\{varepsilon^4over\; 3\}\; -\; left(\{1cdot\; 3cdot\; 5over\; 2cdot\; 4cdot\; 6\}right)^2\{varepsilon^6over5\}\; -\; dots\}right];!,$

Or:

- $C\; =\; 2pi\; a\; sum\_\{n=0\}^infty\; \{leftlbrace\; -\; left[prod\_\{m=1\}^n\; left(\{\; 2m-1\; over\; 2m\}right)right]^2\; \{varepsilon^\{2n\}over\; 2n\; -\; 1\}rightrbrace\};,!$

A good approximation is Ramanujan's:

- $C\; approx\; pi\; left[3(a+b)\; -\; sqrt\{(3a+b)(a+3b)\}right]!,$

or better approximation:

- $Capproxpileft(a+bright)left(1+frac\{3left(frac\{a-b\}\{a+b\}right)^2\}\{10+sqrt\{4-3left(frac\{a-b\}\{a+b\}right)^2\}\}right);!,$

For the special case where the minor axis is half the major axis, we can use:

- $C\; approx\; frac\{pi\; a\; (9\; -\; sqrt\{35\})\}\{2\};!,$

Or:

$C\; approx\; frac\{a\}\{2\}\; sqrt\{93\; +\; frac\{1\}\{2\}\; sqrt\{3\}\};!,$ (better approximation).

More generally, the arc length of a portion of the circumference, as a function of the angle subtended, is given by an incomplete elliptic integral. The inverse function, the angle subtended as a function of the arc length, is given by the elliptic functions.

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. Such a room is called a whisper chamber. Examples are the National Statuary Hall at the U.S. Capitol (where John Quincy Adams is said to have used this property for eavesdropping on political matters), 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.

More generally, in the gravitational two-body problem, if the two bodies are bound to each other (i.e., 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. Interestingly, the orbit of either body in the reference frame of the other is also an ellipse, with the other body at one focus.

The general solution for a harmonic oscillator in two or more dimensions is also an ellipse, but this time with the origin of the force located at the center of the ellipse.

In optics, an index ellipsoid describes the refractive index of a material as a function of the direction through that material. This only applies to materials that are optically anisotropic. Also see birefringence.

Drawing an ellipse as a graphics primitive is common in standard display libraries, such as the Macintosh QuickDraw API, the Windows Graphics Device Interface (GDI) and the Windows Presentation Foundation (WPF). Often such libraries are limited and can only draw an ellipse with either the major axis or the minor axis horizontal. 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. An efficient generalization to draw ellipses was invented in 1984 by Jerry Van Aken (IEEE CG&A, Sept. 1984).

The following is example JavaScript code using the parametric formula for an ellipse to calculate a set of points. The ellipse can be then approximated by connecting the points with lines.

- This functions returns an array containing 36 points to draw an
- ellipse. *
- @param x {double} X coordinate
- @param y {double} Y coordinate
- @param a {double} Semimajor axis
- @param b {double} Semiminor axis
- @param angle {double} Angle of the ellipse
- /

function calculateEllipse(x, y, a, b, angle, steps) {

if (steps == null)

steps = 36;

var points = [];

// Angle is given by Degree Value

var beta = -angle * (Math.PI / 180); //(Math.PI/180) converts Degree Value into Radians

var sinbeta = Math.sin(beta);

var cosbeta = Math.cos(beta);

for (var i = 0; i < 360; i += 360 / steps)

{

var alpha = i * (Math.PI / 180) ;

var sinalpha = Math.sin(alpha);

var cosalpha = Math.cos(alpha);

var X = x + (a * cosalpha * cosbeta - b * sinalpha * sinbeta);

var Y = y + (a * cosalpha * sinbeta + b * sinalpha * cosbeta);

points.push(new OpenLayers.Geometry.Point(X, Y));}

return points;}

One beneficial consequence of using the parametric formula is that 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.

- Ellipsoid, a higher dimensional analog of an ellipse
- Spheroid, the ellipsoids obtained by rotating an ellipse about its major or minor axis.
- Superellipse, a generalization of an ellipse that can look more rectangular or more "pointy"
- Hyperbola
- Parabola
- Oval
- True, eccentric, and mean anomalies
- Matrix representation of conic sections
- Kepler's Laws of Planetary Motion
- Ellipse/Proofs

- Charles D.Miller, Margaret L.Lial, David I.Schneider: Fundamentals of College Algebra. 3rd Edition Scott Foresman/Little 1990. ISBN 0-673-38638-4. Page 381
- Coxeter, H. S. M.: Introduction to Geometry, 2nd ed. New York: Wiley, pp. 115-119, 1969.
- Ellipse at the Encyclopedia of Mathematics (Springer)
- Ellipse at Planetmath

- Apollonius' Derivation of the Ellipse at Convergence
- Ellipse & Hyperbola Construction - An interactive sketch showing how to trace the curves of the ellipse and hyperbola. (Requires Java.)
- Ellipse Construction - Another interactive sketch, this time showing a different method of tracing the ellipse. (Requires Java.)
- The Shape and History of The Ellipse in Washington, D.C. by Clark Kimberling
- Collection of animated ellipse demonstrations. Ellipse, axes, semi-axes, area, perimeter, tangent, foci.
- Woodworking videos showing how to work with ellipses in wood.
- Ellipse as hypotrochoid

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Last updated on Friday October 10, 2008 at 13:48:34 PDT (GMT -0700)

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This article is licensed under the GNU Free Documentation License.

Last updated on Friday October 10, 2008 at 13:48:34 PDT (GMT -0700)

View this article at Wikipedia.org - Edit this article at Wikipedia.org - Donate to the Wikimedia Foundation

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