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ellipse
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{{about|the geometric figure}}{{redirect|Elliptical|the syntactic omission of words|Ellipsis (linguistics)|the punctuation mark (â€¦)|Ellipsis}}{{short description|Plane curve: conic section}}File:Ellipse-conic.svg|thumb|An ellipse (red) obtained as the intersection of a conecone(File:Ellipse-def0.svg|300px|thumb|Ellipse: notations)(File:Ellipse-var.svg|thumb|Ellipses: examples with increasing eccentricity)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).Analytically, the equation of a standard ellipse centered at the origin with width 2a and height 2b is:
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frac{x^2}{a^2}+frac{y^2}{b^2} = 1 .
Assuming a â‰¥ b, the foci are (Â±c, 0) for c = sqrt{a^2-b^2}. The standard parametric equation is:
(x,y) = (acos(t),bsin(t)) quad text{for} quad 0leq tleq 2pi.
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 = frac{c}{a} = sqrt{1 - frac{b^2}{a^2}}.
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 Sun{{ndash}}planet 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, ({{transl|grc|Ã©lleipsis}}, "omission"), was given by Apollonius of Perga in his Conics.Definition as locus of points
(File:Ellipse-def-e.svg|thumb|Ellipse: definition by sum of distances to foci)(File:Ellipse-def-dc.svg|thumb|Ellipse: definition by focus and circular directrix)An ellipse can be defined geometrically as a set or locus of points in the Euclidean plane:
Given two fixed points F_1, F_2 called the foci and a distance 2a which is greater than the distance between the foci, the ellipse is the set of points P such that the sum of the distances |PF_1|, |PF_2| is equal to 2a:E = {Pin mathbb{R}^2 ,mid, |PF_2| +|PF_1 | = 2a } .
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 the vertex points V_1,V_2, which have distance a to the center. The distance c of the foci to the center is called the focal distance or linear eccentricity. The quotient e=tfrac{c}{a} 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:
c_2 is called the circular directrix (related to focus F_2) of the ellipse.{{citation|first1=Tom M.|last1=Apostol|first2=Mamikon A.|last2=Mnatsakanian|title=New Horizons in Geometry|year=2012|publisher=The Mathematical Association of America|series=The Dolciani Mathematical Expositions #47|isbn=978-0-88385-354-2|page=251}}The German term for this circle is Leitkreis which can be translated as "Director circle", but that term has a different meaning in the English literature (see Director circle). 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.
|PF_1|=|Pc_2|.
In Cartesian coordinates
(File:Ellipse-param.svg|thumb|Shape parameters:{{ubl| a: semi-major axis,| b: semi-minor axis,| c: linear eccentricity,| p: semi-latus rectum (usually ell).}})Standard equation
The standard form of an ellipse in Cartesian coordinates assumes that the origin is the center of the ellipse, the x-axis is the major axis, and:
the foci are the points F_1 = (c,, 0), F_2=(-c,, 0),
the vertices are V_1 = (a,, 0), V_2 = (-a,, 0).
For an arbitrary point (x,y) the distance to the focus (c,0) is sqrt{(x - c)^2 + y^2 } and to the other focus sqrt{(x + c)^2 + y^2}. Hence the point (x,, y) is on the ellipse whenever:
sqrt{(x - c)^2 + y^2} + sqrt{(x + c)^2 + y^2} = 2a .
Removing the radicals by suitable squarings and using b^2 = a^2-c^2 produces the standard equation of the ellipse:
frac{x^2}{a^2} + frac{y^2}{b^2} = 1,
or, solved for y:
y = pmfrac{b}{a}sqrt{a^2 - x^2} = pm sqrt{left(a^2 - x^2right)left(1 - e^2right)}.
The width and height parameters a,; b are called the semi-major and semi-minor axes. The top and bottom points V_3 = (0,, b),; V_4 = (0,, -b) are the co-vertices. The distances from a point (x,, y) on the ellipse to the left and right foci are a + ex and a - ex.It follows from the equation that the ellipse is symmetric with respect to the coordinate axes and hence with respect to the origin.Parameters
Semi-major and semi-minor axes a â‰¥ b
Throughout this article a is the semi-major axis, i.e. a ge b > 0 . In general the canonical ellipse equation tfrac{x^2}{a^2} + tfrac{y^2}{b^2} = 1 may have a < b (and hence the ellipse would be taller than it is wide); in this form the semi-major axis would be b. 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 c
This is the distance from the center to a focus: c = sqrt{a^2 - b^2}.Eccentricity e
The eccentricity can be expressed as:
e = frac{c}{a} = sqrt{1 - left(frac{b}{a}right)^2},
assuming a > b. An ellipse with equal axes (a = b) has zero eccentricity, and is a circle.Semi-latus rectum l
The length of the chord through one of the foci, perpendicular to the major axis, is called the latus rectum. One half of it is the semi-latus rectum ell. A calculation shows:
ell = frac{b^2}a = a left(1 - e^2right).
The semi-latus rectum ell is equal to the radius of curvature of the osculating circles at the vertices.Tangent
An arbitrary line g intersects an ellipse at 0, 1, or 2 points, respectively called an exterior line, tangent and secant. Through any point of an ellipse there is a unique tangent. The tangent at a point (x_1,, y_1) of the ellipse tfrac{x^2}{a^2} + tfrac{y^2}{b^2} = 1 has the coordinate equation:
frac{x_1}{a^2}x + frac{y_1}{b^2}y = 1.
A vector parametric equation of the tangent is:
vec x = begin{pmatrix}x_1 y_1end{pmatrix} + sbegin{pmatrix}
;! -y_1 a^2
; x_1 b^2
end{pmatrix} with s in mathbb{R} .Proof:Let (x_1,, y_1) be a point on an ellipse and vec{x} = begin{pmatrix}x_1 y_1end{pmatrix} + sbegin{pmatrix}u vend{pmatrix} be the equation of any line g containing (x_1,, y_1). Inserting the line's equation into the ellipse equation and respecting frac{x_1^2}{a^2} + frac{y_1^2}{b^2} = 1 yields:
; x_1 b^2
frac{left(x_1 + suright)^2}{a^2} + frac{left(y_1 + svright)^2}{b^2} = 1 quadLongrightarrowquad
2sleft(frac{x_1u}{a^2} + frac{y_1v}{b^2}right) + s^2left(frac{u^2}{a^2} + frac{v^2}{b^2}right) = 0 .
2sleft(frac{x_1u}{a^2} + frac{y_1v}{b^2}right) + s^2left(frac{u^2}{a^2} + frac{v^2}{b^2}right) = 0 .
There are then cases:
# frac{x_1}{a^2}u + frac{y_1}{b^2}v = 0. Then line g and the ellipse have only point (x_1,, y_1) in common, and g is a tangent. The tangent direction has perpendicular vector begin{pmatrix}frac{x_1}{a^2} & frac{y_1}{b^2}end{pmatrix}, so the tangent line has equation frac{x_1}{a^2}x + tfrac{y_1}{b^2}y = k for some k. Because (x_1,, y_1) is on the tangent and the ellipse, one obtains k = 1.
# frac{x_ 1}{a^2}u + frac{y_1}{b^2}v ne 0. Then line g has a second point in common with the ellipse, and is a secant.
Using (1) one finds that begin{pmatrix} -y_1a^2 & x_1b^2 end{pmatrix} is a tangent vector at point (x_1,, y_1), which proves the vector equation.If (x_1, y_1) and (u, v) are two points of the ellipse such that frac{x_1u}{a^2} + tfrac{y_1v}{b^2} = 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 left(x_circ,, y_circright), its equation is
frac{left(x - x_circright)^2}{a^2} + frac{left(y - y_circright)^2}{b^2} = 1 .
The axes are still parallel to the x- and y-axes.(File:Elliko-sk.svg|thumb|The construction of points based on the parametric equation and the interpretation of parameter t, which is due to de la Hire)General ellipse
In analytic geometry, the ellipse is defined as a quadric: the set of points (X,, Y) of the Cartesian plane that, in non-degenerate cases, satisfy the implicit equationBOOK,weblink Precalculus with Limits, Larson, Ron, Hostetler, Robert P., Falvo, David C., Cengage Learning, 2006, 978-0-618-66089-6, 767, Chapter 10,weblink BOOK,weblink Precalculus, Young, Cynthia Y., John Wiley and Sons, 2010, 978-0-471-75684-2, 831, Chapter 9,weblink
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 âˆ† be the determinant
Delta = begin{vmatrix}
A & frac{1}{2}B & frac{1}{2}D
frac{1}{2}B & C & frac{1}{2}E
frac{1}{2}D & frac{1}{2}E & F
end{vmatrix} = left(AC - frac{B^2}{4}right) F + frac{BED}{4} - frac{CD^2}{4} - frac{AE^2}{4}.
Then the ellipse is a non-degenerate real ellipse if and only if Câˆ† < 0. If Câˆ† > 0, we have an imaginary ellipse, and if âˆ† = 0, we have a point ellipse.Lawrence, J. Dennis, A Catalog of Special Plane Curves, Dover Publ., 1972.{{rp|p.63}}The general equation's coefficients can be obtained from known semi-major axis a, semi-minor axis b, center coordinates left(x_circ,, y_circright), and rotation angle Theta (the angle from the positive horizontal axis to the ellipse's major axis) using the formulae:
frac{1}{2}B & C & frac{1}{2}E
frac{1}{2}D & frac{1}{2}E & F
end{vmatrix} = left(AC - frac{B^2}{4}right) F + frac{BED}{4} - frac{CD^2}{4} - frac{AE^2}{4}.
begin{align}
A &= a^2 (sinTheta)^2 + b^2 (cosTheta)^2
B &= 2left(b^2 - a^2right) sinTheta cosTheta
C &= a^2 (cosTheta)^2 + b^2 (sinTheta)^2
D &= -2A x_circ - B y_circ
E &= - B x_circ - 2C y_circ
F &= A x_circ^2 + B x_circ y_circ + C y_circ^2 - a^2 b^2.
end{align}These expressions can be derived from the canonical equation tfrac{x^2}{a^2} + tfrac{y^2}{b^2} = 1 by an affine transformation of the coordinates (x,, y):
B &= 2left(b^2 - a^2right) sinTheta cosTheta
C &= a^2 (cosTheta)^2 + b^2 (sinTheta)^2
D &= -2A x_circ - B y_circ
E &= - B x_circ - 2C y_circ
F &= A x_circ^2 + B x_circ y_circ + C y_circ^2 - a^2 b^2.
begin{align}
x &= left(X - x_circright) cosTheta + left(Y - y_circright) sinTheta
y &= -left(X - x_circright) sinTheta + left(Y - y_circright) cosTheta.
end{align}Conversely, the canonical form parameters can be obtained from the general form coefficients by the equations:
y &= -left(X - x_circright) sinTheta + left(Y - y_circright) cosTheta.
begin{align}
a, b &= frac{1}{B^2 - 4AC} sqrt{2 left(AE^2 + CD^2 - BDE + left(B^2 - 4ACright) Fright)left((A + C) pm sqrt{(A - C)^2 + B^2}right)} [3pt]
x_circ &= frac{2CD - BE}{B^2 - 4AC} [3pt]
y_circ &= frac{2AE - BD}{B^2 - 4AC} [3pt]
Theta &= begin{cases}
arctanleft(frac{1}{B}left(C - A - sqrt{(A - C)^2 + B^2}right)right)
& text{for } B ne 0
0 & text{for } B = 0, A < C
90^circ = frac{pi}{2}^c & text{for } B = 0, A > C
end{cases}
end{align}x_circ &= frac{2CD - BE}{B^2 - 4AC} [3pt]
y_circ &= frac{2AE - BD}{B^2 - 4AC} [3pt]
Theta &= begin{cases}
arctanleft(frac{1}{B}left(C - A - sqrt{(A - C)^2 + B^2}right)right)
& text{for } B ne 0
0 & text{for } B = 0, A < C
90^circ = frac{pi}{2}^c & text{for } B = 0, A > C
end{cases}
Parametric representation
(File:Ellipse-ratpar.svg|thumb|Ellipse points calculated by the rational representation with equal spaced parameters (Delta u = 0.2).)Standard parametric representation
Using trigonometric functions, a parametric representation of the standard ellipse tfrac{x^2}{a^2}+tfrac{y^2}{b^2} = 1 is:
(x,, y) = (a cos t,, b sin t), 0 le t < 2pi .
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).K. Strubecker: Vorlesungen Ã¼ber Darstellende Geometrie, GÃ–TTINGEN, VANDENHOECK & RUPRECHT, 1967, p. 26Rational representation
With the substitution u = tanleft(frac{2}right) and trigonometric formulae one obtains
cos t = frac{1 - u^2}{u^2 + 1} ,quad sin t = frac{2u}{u^2 + 1}
and the rational parametric equation of an ellipse
begin{align}
x(u) &= afrac{1 - u^2}{u^2 + 1}
y(u) &= frac{2bu}{u^2 + 1}
end{align};,quad -infty < u < infty;,which covers any point of the ellipse tfrac{x^2}{a^2} + tfrac{y^2}{b^2} = 1 except the left vertex (-a,, 0).For u in [0,, 1], this formula represents the right upper quarter of the ellipse moving counter-clockwise with increasing u. The left vertex is the limit lim_{u to pm infty} (x(u),, y(u)) = (-a,, 0);.Rational representations of conic sections are commonly used in Computer Aided Design (see Bezier curve).y(u) &= frac{2bu}{u^2 + 1}
Tangent slope as parameter
A parametric representation, which uses the slope m of the tangent at a point of the ellipse can be obtained from the derivative of the standard representation vec x(t) = (a cos t,, b sin t)^mathsf{T}:
vec x'(t) = (-asin t,, bcos t)^mathsf{T} quad rightarrow quad m = -frac{b}{a}cot tquad rightarrow quad cot t = -frac{ma}{b}.
cos t = frac{cot t}{pmsqrt{1 + cot^2t}} = frac{-ma}{pmsqrt{m^2 a^2 + b^2}} ,quadquad
sin t = frac{1}{pmsqrt{1 + cot^2t}} = frac{b}{pmsqrt{m^2 a^2 + b^2}}.Replacing cos t and sin t of the standard representation yields:
vec c_pm(m) = left(-frac{ma^2}{pmsqrt{m^2 a^2 + b^2}},;frac{b^2}{pmsqrt{m^2a^2 + b^2}}right),, m in R.
Here m is the slope of the tangent at the corresponding ellipse point, vec c_+ is the upper and vec c_- the lower half of the ellipse. The vertices(pm a,, 0), having vertical tangents, are not covered by the representation.The equation of the tangent at point vec c_pm(m) has the form y = mx + n. The still unknown n can be determined by inserting the coordinates of the corresponding ellipse point vec c_pm(m):
y = mx pmsqrt{m^2 a^2 + b^2}; .
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
(File:ellipse-aff.svg|300px|thumb|Ellipse as an affine image of the unit circle)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.
An affine transformation of the Euclidean plane has the form vec x mapsto vec f!_0 + Avec x, where A is a regular matrix (with non-zero determinant) and vec f!_0 is an arbitrary vector. If vec f!_1, vec f!_2 are the column vectors of the matrix A, the unit circle (cos(t), sin(t)), 0 leq t leq 2pi, is mapped onto the ellipse:
vec x = vec p(t) = vec f!_0 + vec f!_1 cos t + vec f!_2 sin t .
Here vec f!_0 is the center and vec f!_1,; vec f!_2 are the directions of two conjugate diameters, in general not perpendicular. The four vertices of the ellipse are vec p(t_0),;vec pleft(t_0 pm tfrac{pi}{2}right),; vec pleft(t_0 + piright), for a parameter t = t_0 defined by:
cot (2t_0) = frac{vec f!_1^{,2} - vec f!_2^{,2}}{2vec f!_1 cdot vec f!_2}.
(If vec f!_1 cdot vec f!_2 = 0, then t_0 = 0.) This is derived as follows. The tangent vector at point vec p(t) is:
vec p,'(t) = -vec f!_1sin t + vec f!_2cos t .
At a vertex parameter t = t_0, the tangent is perpendicular to the major/minor axes, so:
0 = vec p'(t) cdot left(vec p(t) -vec f!_0right) = left(-vec f!_1sin t + vec f!_2cos tright) cdot left(vec f!_1 cos t + vec f!_2 sin tright).
Expanding and applying the identities cos^2 t -sin^2 t=cos 2t, 2sin t cos t = sin 2t gives the equation for t = t_0.This definition gives a parametric representation of an arbitrary ellipse, even in space, if we allow vec f!_0, vec f!_1, vec f!_2 to be vectors in space.Polar forms
Polar form relative to center
(File:Ellipse Polar center.svg|thumb|right|Polar coordinates centered at the center.)In polar coordinates, with the origin at the center of the ellipse and with the angular coordinate theta measured from the major axis, the ellipse's equation is{{rp|p. 75}}
r(theta) = frac{ab}{sqrt{(b cos theta)^2 + (asin theta)^2}}=frac{b}{sqrt{1 - (ecostheta)^2}}
Polar form relative to focus
(File:Ellipse Polar.svg|thumb|right|Polar coordinates centered at focus.)If instead we use polar coordinates with the origin at one focus, with the angular coordinate theta = 0 still measured from the major axis, the ellipse's equation is
r(theta)=frac{a (1-e^2)}{1 pm ecostheta}
where the sign in the denominator is negative if the reference direction theta = 0 points towards the center (as illustrated on the right), and positive if that direction points away from the center.In the slightly more general case of an ellipse with one focus at the origin and the other focus at angular coordinate phi, the polar form is
r=frac{a (1-e^2)}{1 - ecos(theta - phi)}.
The angle theta in these formulas is called the true anomaly of the point. The numerator of these formulas is the semi-latus rectum ell=a (1-e^2).Eccentricity and the directrix property
(File:Ellipse-ll-e.svg|300px|thumb|Ellipse: directrix property)Each of the two lines parallel to the minor axis, and at a distance of d = frac{a^2}{c} = frac{a}{e} 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:
The proof for the pair F_1, l_1 follows from the fact that left|PF_1right|^2 = (x - c)^2 + y^2, left|Pl_1right|^2 = left(x - tfrac{a^2}{c}right)^2 and y^2 = b^2 - tfrac{b^2}{a^2}x^2 satisfy the equation
frac{left|PF_1right|}{left|Pl_1right|} = frac{left|PF_2right|}{left|Pl_2right|} = e = frac{c}{a} .
left|PF_1right|^2 - frac{c^2}{a^2}left|Pl_1right|^2 = 0 .
The second case is proven analogously.The converse is also true and can be used to define an ellipse (in a manner similar to the definition of a parabola):
For any point F (focus), any line l (directrix) not through F, and any real number e with 0 < e < 1, the ellipse is the locus of points for which the quotient of the distances to the point and to the line is e, that is:
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.)(File:Kegelschnitt-schar-ev.svg|thumb|Pencil of conics with a common vertex and common semi-latus rectum)
E = left{P left| frac{|PF|}{|Pl|} = eright.right}.
- Proof
(x - f)^2 + y^2 = e^2left(x + frac{f}{e}right)^2 = (ex + f)^2 and x^2left(e^2 - 1right) + 2xf(1 + e) - y^2 = 0.
The substitution p = f(1 + e) yields
x^2left(e^2 - 1right) + 2px - y^2 = 0.
This is the equation of an ellipse (e < 1), or a parabola (e = 1), or a hyperbola (e > 1). All of these non-degenerate conics have, in common, the origin as a vertex (see diagram).If e < 1, introduce new parameters a,, b so that 1 - e^2 = tfrac{b^2}{a^2}, text{ and } p = tfrac{b^2}{a}, and then the equation above becomes
frac{(x - a)^2}{a^2} + frac{y^2}{b^2} = 1 ,
which is the equation of an ellipse with center (a,, 0), the x-axis as major axis, andthe major/minor semi axis a,, b.
- General ellipse
left(x - f_1right)^2 + left(y - f_2right)^2 = e^2 frac{left(ux + vy + wright)^2}{u^2 + v^2} .
(The right side of the equation uses the Hesse normal form of a line to calculate the distance |Pl|.)Focus-to-focus reflection property
(File:Ellipse-reflex.svg|250px|thumb|Ellipse: the tangent bisects the supplementary angle of the angle between the lines to the foci.)(File:Elli-norm-tang-n.svg|250px|thumb|Rays from one focus reflect off the ellipse to pass through the other focus.)An ellipse possesses the following property:
The normal at a point P bisects the angle between the lines overline{PF_1},, overline{PF_2}.
- Proof
- Application
Conjugate diameters
(File:Parallelproj-kreis-ellipse.svg|400px|thumb|Orthogonal diameters of a circle with a square of tangents, midpoints of parallel chords and an affine image, which is an ellipse with conjugate diameters, a parallelogram of tangents and midpoints of chords.)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 overline{P_1 Q_1},, overline{P_2 Q_2} of an ellipse are conjugate whenever the tangents at P_1 and Q_1 are parallel to overline{P_2 Q_2}.
Conjugate diameters in an ellipse generalize orthogonal diameters in a circle.In the parametric equation for a general ellipse given above,
vec x = vec p(t) = vec f!_0 +vec f!_1 cos t + vec f!_2 sin t,
any pair of points vec p(t), vec p(t + pi) belong to a diameter, and the pair vec pleft(t + tfrac{pi}{2}right), vec pleft(t - tfrac{pi}{2}right) belong to its conjugate diameter.Theorem of Apollonios on conjugate diameters
(File:Elli-apoll-cd.svg|300px|thumb|Ellipse: theorem of Apollonios on conjugate diameters)For an ellipse with semi-axes a,, b the following is true:
Let c_1 and c_2 be halves of two conjugate diameters (see diagram) then
# c_1^2 + c_2^2 = a^2 + b^2,
# the triangle formed by c_1,, c_2 has the constant area A_Delta = frac{1}{2}ab
# the parallelogram of tangents adjacent to the given conjugate diameters has the text{Area}_{12} = 4ab .
- Proof:
vec p(t) = (acos t,, bsin t).
The two points vec c_1 = vec p(t), vec c_2 = vec pleft(t + frac{pi}{2}right) are on conjugate diameters (see previous section). From trigonometric formulae one obtains vec c_2 = (-asin t,, bcos t)^mathsf{T} and
left|vec c_1right|^2 + left|vec c_2right|^2 = cdots = a^2 + b^2 .
The area of the triangle generated by vec c_1,, vec c_2 is
A_Delta = frac{1}{2}detleft(vec c_1,, vec c_2right) = cdots = frac{1}{2}ab
and from the diagram it can be seen that the area of the parallelogram is 8 times that of A_Delta. Hence
text{Area}_{12} = 4ab .
Orthogonal tangents
(File:Orthoptic-ellipse-s.svg|thumb|Ellipse with its orthoptic)For the ellipse tfrac{x^2}{a^2}+tfrac{y^2}{b^2}=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
(File:Zp-turm-tor.svg|thumb|Central projection of circles (gate))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
the knowledge of the axes and the semi-axes is necessary (or equivalent: 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.K. Strubecker: Vorlesungen Ã¼ber Darstellende Geometrie. Vandenhoeck & Ruprecht, GÃ¶ttingen 1967, S. 26. It is based on the standard parametric representation (acos t,, bsin 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.
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 tied at each end to the two pins and the tip of a pencil pulls the loop taut to form a triangle. 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 bedâ€”thus it is called the gardener's ellipse.A similar method for drawing (Confocal conic sections#Graves's theorem: the construction of confocal ellipses by a string|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):
(acos t,, bsin t)
This representation can be modeled technically by two simple methods. In both cases center, the axes and semi axes a,, b have to be known.
- Method 1
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 (acos t,, bsin 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 1Tusi couple vs Paper strip plus Ellipses horizontal.gif|Ellipses with Tusi couple. Two examples: red and cyan.A variation of the paper strip method 1 uses the observation that the midpoint N of the paper strip is moving on the circle with center M (of the ellipse) and radius tfrac{a + b}{2}. 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.J. van Mannen: Seventeenth century instruments for drawing conic sections. In: The Mathematical Gazette. Vol. 76, 1992, p. 222â€“230. This variation requires only one sliding shoe.Ellipse-papsm-1a.svg|Variation of the paper strip method 1Ellipses with SliderCrank inner Ellipses.gif|Animation of the variation of the paper strip method 1(File:Elliko-pap2.svg|250px|thumb|Ellipse construction: paper strip method 2)
- Method 2:
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 (acos t,, bsin 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 BramerFile:Ellipses with SliderCrank Ellipses at Slider Side.gif|Variation of the paper strip method 2Most ellipsograph drafting instruments are based on the second paperstrip method.(File:Elliko-skm.svg|250px|thumb|Approximation of an ellipse with osculating circles)Approximation by osculating circles
From Metric properties below, one obtains:- The radius of curvature at the vertices V_1,, V_2 is: tfrac{b^2}{a}
- The radius of curvature at the co-vertices V_3,, V_4 is: tfrac{a^2}{b} .
- mark the auxiliary point H = (a,, b) and draw the line segment V_1 V_3 ,
- draw the line through H, which is perpendicular to the line V_1 V_3 ,
- the intersection points of this line with the axes are the centers of the osculating circles.
Steiner generation
(File:Ellipse-steiner-e.svg|250px|thumb|Ellipse: Steiner generation)(File:Ellipse construction - parallelogram method.gif|200px|thumb|Ellipse: 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),, B(V) of lines at two points U,, V (all lines containing U and V, respectively) and a projective but not perspective mapping pi of B(U) onto B(V), then the intersection points of corresponding lines form a non-degenerate projective conic section.
For the generation of points of the ellipse tfrac{x^2}{a^2} + tfrac{y^2}{b^2} = 1 one uses the pencils at the vertices V_1,, V_2. Let P = (0,, b) be an upper co-vertex of the ellipse and A = (-a,, 2b),, B = (a,,2b). P is the center of the rectangle V_1,, V_2,, B,, A. The side overline{AB} of the rectangle is divided into n equal spaced line segments and this division is projected parallel with the diagonal AV_2 as direction onto the line segment overline{V_1B} and assign the division as shown in the diagram. The parallel projection together with the reverse of the orientation is part of the projective mapping between the pencils at V_1 and V_2 needed. The intersection points of any two related lines V_1 B_i and V_2 A_i are points of the uniquely defined ellipse. With help of the points C_1,, dotsc the points of the second quarter of the ellipse can be determined. Analogously one obtains the points of the lower half of the ellipse.Steiner generation can also be defined for hyperbolas and parabolas. It is sometimes called a parallelogram method because one can use other points rather than the vertices, which starts with a parallelogram instead of a rectangle.As hypotrochoid
File:Ellipse as hypotrochoid.gif|right|300|thumb|An ellipse (in red) as a special case of the hypotrochoidhypotrochoidThe 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
(File:Inscribe-a-c.svg|thumb|Circle: inscribed angle theorem)A circle with equation left(x - x_circright)^2 + left(y - y_circright)^2 = r^2 is uniquely determined by three points left(x_1, y_1right),; left(x_2,,y_2right),; left(x_3,, y_3right) not on a line. A simple way to determine the parameters x_circ,y_circ,r uses the inscribed angle theorem for circles:
For four points P_i = left(x_i,, y_iright), i = 1,, 2,, 3,, 4,, (see diagram) the following statement is true:
The four points are on a circle if and only if the angles at P_3 and P_4 are equal.
Usually one measures inscribed angles by a degree or radian Î¸, but here the following measurement is more convenient:
In order to measure the angle between two lines with equations y = m_1x + d_1, y = m_2x + d_2, m_1 ne m_2, one uses the quotient:
frac{1 + m_1 m_2}{m_2 - m_1} = cottheta .
Inscribed angle theorem for circles
For four points P_i = left(x_i,, y_iright), i = 1,, 2,, 3,, 4,, no three of them on a line, we have the following (see diagram):
The four points are on a circle, if and only if the angles at P_3 and P_4 are equal. In terms of the angle measurement above, this means:
frac{(x_4 - x_1)(x_4 - x_2) + (y_4 - y_1)(y_4 - y_2)}
{(y_4 - y_1)(x_4 - x_2) - (y_4 - y_2)(x_4 - x_1)} =
frac{(x_3 - x_1)(x_3 - x_2) + (y_3 - y_1)(y_3 - y_2)}
{(y_3 - y_1)(x_3 - x_2) - (y_3 - y_2)(x_3 - x_1)}.
At first the measure is available only for chords not parallel to the y-axis, but the final formula works for any chord.frac{(x_3 - x_1)(x_3 - x_2) + (y_3 - y_1)(y_3 - y_2)}
{(y_3 - y_1)(x_3 - x_2) - (y_3 - y_2)(x_3 - x_1)}.
Three-point form of circle equation
As a consequence, one obtains an equation for the circle determined by three non-colinear points P_i = left(x_i,, y_iright):
frac{({color{red}x} - x_1)({color{red}x} - x_2) + ({color{red}y} - y_1)({color{red}y} - y_2)}
{({color{red}y} - y_1)({color{red}x} - x_2) - ({color{red}y} - y_2)({color{red}x} - x_1)} =
frac{(x_3 - x_1)(x_3 - x_2) + (y_3 - y_1)(y_3 - y_2)}
{(y_3 - y_1)(x_3 - x_2) - (y_3 - y_2)(x_3 - x_1)}.
For example, for P_1 = (2,, 0),; P_2 = (0,, 1),; P_3 = (0,,0) the three-point equation is:
{({color{red}y} - y_1)({color{red}x} - x_2) - ({color{red}y} - y_2)({color{red}x} - x_1)} =
frac{(x_3 - x_1)(x_3 - x_2) + (y_3 - y_1)(y_3 - y_2)}
{(y_3 - y_1)(x_3 - x_2) - (y_3 - y_2)(x_3 - x_1)}.
frac{(x - 2)x + y(y - 1)}{yx - (y - 1)(x - 2)} = 0, which can be rearranged to (x - 1)^2 + left(y - tfrac{1}{2}right)^2 = tfrac{5}{4} .
Using vectors, dot products and determinants this formula can be arranged more clearly, letting vec x = (x,, y):
frac{left({color{red}vec x} - vec x_1right) cdot left({color{red}vec x} - vec x_2right)}
{detleft({color{red}vec x} - vec x_1,{color{red}vec x} - vec x_2right)} =
frac{left(vec x_3 - vec x_1right) cdot left(vec x_3 - vec x_2right)}
{detleft(vec x_3 - vec x_1, vec x_3 - vec x_2right)}.
The center of the circle left(x_circ,, y_circright) satisfies:
{detleft({color{red}vec x} - vec x_1,{color{red}vec x} - vec x_2right)} =
frac{left(vec x_3 - vec x_1right) cdot left(vec x_3 - vec x_2right)}
{detleft(vec x_3 - vec x_1, vec x_3 - vec x_2right)}.
begin{bmatrix}
1 & frac{y_1 - y_2}{x_1 - x_2}
frac{x_1 - x_3}{y_1 - y_3} & 1
end{bmatrix}
begin{bmatrix} x_circ y_circ end{bmatrix}
=
begin{bmatrix}
frac{x_1^2 - x_2^2 + y_1^2 - y_2^2}{2(x_1 - x_2)}
frac{y_1^2 - y_3^2 + x_1^2 - x_3^2}{2(y_1 - y_3)}
end{bmatrix}.
The radius is the distance between any of the three points and the center.
frac{x_1 - x_3}{y_1 - y_3} & 1
end{bmatrix}
begin{bmatrix} x_circ y_circ end{bmatrix}
=
begin{bmatrix}
frac{x_1^2 - x_2^2 + y_1^2 - y_2^2}{2(x_1 - x_2)}
frac{y_1^2 - y_3^2 + x_1^2 - x_3^2}{2(y_1 - y_3)}
end{bmatrix}.
r = sqrt{left(x_1 - x_circright)^2 + left(y_1 - y_circright)^2}
= sqrt{left(x_2 - x_circright)^2 + left(y_2 - y_circright)^2}
= sqrt{left(x_3 - x_circright)^2 + left(y_3 - y_circright)^2}.
= sqrt{left(x_2 - x_circright)^2 + left(y_2 - y_circright)^2}
= sqrt{left(x_3 - x_circright)^2 + left(y_3 - y_circright)^2}.
Ellipses
This section, we consider the family of ellipses defined by equations tfrac{left(x - x_circright)^2}{a^2} + tfrac{left(y - y_circright)^2}{b^2} = 1 with a fixed eccentricity e. It is convenient to use the parameter:
{color{blue}q} = frac{a^2}{b^2} = frac{1}{1 - e^2},
and to write the ellipse equation as:
left(x - x_circright)^2 + {color{blue}q}, left(y - y_circright)^2 = a^2,
where q is fixed and x_circ,, y_circ,, a vary over the real numbers. (Such ellipses have their axes parallel to the coordinate axes: if q < 1, the major axis is parallel to the x-axis; if q > 1, it is parallel to the y-axis.)(File:Inscribe-a-e.svg|thumb|Inscribed angle theorem for an ellipse)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 Î¸:E. Hartmann: Lecture Note 'Planar Circle Geometries', an Introduction to MÃ¶bius-, Laguerre- and Minkowski Planes, p. 55W. Benz, Vorlesungen Ã¼ber Geomerie der Algebren, Springer (1973)
In order to measure an angle between two lines with equations y = m_1x + d_1, y = m_2x + d_2, m_1 ne m_2 one uses the quotient:
frac{1 + {color{blue}q}; m_1 m_2}{m_2 - m_1} .
Inscribed angle theorem for ellipses
Given four points P_i = left(x_i,, y_iright), i = 1,, 2,, 3,, 4, no three of them on a line (see diagram).
The four points are on an ellipse with equation (x - x_circ)^2 + {color{blue}q}, (y - y_circ)^2 = a^2 if and only if the angles at P_3 and P_4 are equal in the sense of the measurement aboveâ€”that is, if
frac{(x_4 - x_1)(x_4 - x_2) + {color{blue}q};(y_4 - y_1)(y_4 - y_2)}
{(y_4 - y_1)(x_4 - x_2) - (y_4 - y_2)(x_4 - x_1)} =
frac{(x_3 - x_1)(x_3 - x_2) + {color{blue}q};(y_3 - y_1)(y_3 - y_2)}
{(y_3 - y_1)(x_3 - x_2) - (y_3 - y_2)(x_3 - x_1)} .
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.frac{(x_3 - x_1)(x_3 - x_2) + {color{blue}q};(y_3 - y_1)(y_3 - y_2)}
{(y_3 - y_1)(x_3 - x_2) - (y_3 - y_2)(x_3 - x_1)} .
Three-point form of ellipse equation
A consequence, one obtains an equation for the ellipse determined by three non-colinear points P_i = left(x_i,, y_iright):
frac{({color{red}x} - x_1)({color{red}x} - x_2) + {color{blue}q};({color{red}y} - y_1)({color{red}y} - y_2)}
{({color{red}y} - y_1)({color{red}x} - x_2) - ({color{red}y} - y_2)({color{red}x} - x_1)} =
frac{(x_3 - x_1)(x_3 - x_2) + {color{blue}q};(y_3 - y_1)(y_3 - y_2)}
{(y_3 - y_1)(x_3 - x_2) - (y_3 - y_2)(x_3 - x_1)} .
For example, for P_1 = (2,, 0),; P_2 = (0,,1),; P_3 = (0,, 0) and q = 4 one obtains the three-point form
frac{(x_3 - x_1)(x_3 - x_2) + {color{blue}q};(y_3 - y_1)(y_3 - y_2)}
{(y_3 - y_1)(x_3 - x_2) - (y_3 - y_2)(x_3 - x_1)} .
frac{(x - 2)x + 4y(y - 1)}{yx - (y - 1)(x - 2)} = 0 and after conversion frac{(x - 1)^2}{2} + frac{left(y - frac{1}{2}right)^2}{frac{1}{2}} = 1.
Analogously to the circle case, the equation can be written more clearly using vectors:
frac{left({color{red}vec x} - vec x_1right)*left({color{red}vec x} - vec x_2right)}
{detleft({color{red}vec x} - vec x_1,{color{red}vec x} - vec x_2right)}
= frac{left(vec x_3 - vec x_1right)*left(vec x_3 - vec x_2right)}
{detleft(vec x_3 - vec x_1, vec x_3 - vec x_2right)},
where * is the modified dot product vec u*vec v = u_x v_x + {color{blue}q},u_y v_y.{detleft({color{red}vec x} - vec x_1,{color{red}vec x} - vec x_2right)}
= frac{left(vec x_3 - vec x_1right)*left(vec x_3 - vec x_2right)}
{detleft(vec x_3 - vec x_1, vec x_3 - vec x_2right)},
Pole-polar relation
(File:Ellipse-pol.svg|250px|thumb|Ellipse: pole-polar relation)Any ellipse can be described in a suitable coordinate system by an equation tfrac{x^2}{a^2} + tfrac{y^2}{b^2} = 1. The equation of the tangent at a point P_1 = left(x_1,, y_1right) of the ellipse is tfrac{x_1x}{a^2} + tfrac{y_1y}{b^2} = 1. If one allows point P_1 = left(x_1,, y_1right) to be an arbitrary point different from the origin, then
point P_1 = left(x_1,, y_1right) neq (0,, 0) is mapped onto the line tfrac{x_1 x}{a^2} + tfrac{y_1 y}{b^2} = 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, d ne 0 onto the point left(-tfrac{ma^2}{d},, tfrac{b^2}{d}right) and
- line x = c, c ne 0 onto the point left(tfrac{a^2}{c},, 0right) .
- For a point (pole) on the ellipse the polar is the tangent at this point (see diagram: P_1,, p_1).
- For a pole P outside the ellipse the intersection points of its polar with the ellipse are the tangency points of the two tangents passing P (see diagram: P_2,, p_2).
- For a point within the ellipse the polar has no point with the ellipse in common. (see diagram: F_1,, l_1).
- The intersection point of two polars is the pole of the line through their poles.
- The foci (c,, 0), and (-c,, 0) respectively and the directrices x = tfrac{a^2}{c} and x = -tfrac{a^2}{c} respectively belong to pairs of pole and polar.
Metric properties
All metric properties given below refer to an ellipse with equation frac{x^2}{a^2}+frac{y^2}{b^2}= 1 .Area
The area A_text{ellipse} enclosed by an ellipse is:
A_text{ellipse} = pi ab
where a and b are the lengths of the semi-major and semi-minor axes, respectively. The area formula pi a b is intuitive: start with a circle of radius b (so its area is pi b^2) and stretch it by a factor a/b to make an ellipse. This scales the area by the same factor: pi b^2(a/b) = pi a b. It is also easy to rigorously prove the area formula using integration as follows. Equation ({{EquationNote|1}}) can be rewritten as y(x)= b sqrt{1 - x^2/a^2}. For xin[-a,a], this curve is the top half of the ellipse. So twice the integral of y(x) over the interval [-a,a] will be the area of the ellipse:
begin{align}
A_text{ellipse} &= int_{-a}^a 2bsqrt{1 - frac{x^2}{a^2}},dx
&= frac ba int_{-a}^a 2sqrt{a^2 - x^2},dx.
end{align}The second integral is the area of a circle of radius a, that is, pi a^2. So
&= frac ba int_{-a}^a 2sqrt{a^2 - x^2},dx.
A_text{ellipse} = frac{b}{a}pi a^2 = pi ab.
An ellipse defined implicitly by Ax^2+ Bxy + Cy^2 = 1 has area 2pi / sqrt{4AC - B^2}.The area can also be expressed in terms of eccentricity and the length of the semi-major axis as a^2pisqrt{1-e^2} (obtained by solving for flattening, then computing the semi-minor axis).Circumference
The circumference C of an ellipse is:
C ,=, 4aint_0^{pi/2}sqrt {1 - e^2 sin^2theta} dtheta ,=, 4 a ,E(e)
where again a is the length of the semi-major axis, e=sqrt{1 - b^2/a^2} is the eccentricity, and the function E is the complete elliptic integral of the second kind,
E(e) ,=, int_0^{pi/2}sqrt {1 - e^2 sin^2theta} dtheta.
The circumference of the ellipse may be evaluated in terms of E(e) using Gauss's arithmetic-geometric mean;{{dlmf|first=B. C.|last=Carlson|id=19.8.E6|title=Elliptic Integrals}} this is a quadratically converging iterative method.{{citation|title = Python code for the circumference of an ellipse in terms of the complete elliptic integral of the second kind|url =http://paulbourke.net/geometry/ellipsecirc/python.code|accessdate = 2013-12-28}}The exact infinite series is:
begin{align}
C &= 2pi a left[{1 - left(frac{1}{2}right)^2e^2 - left(frac{1cdot 3}{2cdot 4}right)^2frac{e^4}{3} - left(frac{1cdot 3cdot 5}{2cdot 4cdot 6}right)^2frac{e^6}{5} - cdots}right]
&= 2pi a left[1 - sum_{n=1}^infty left(frac{(2n-1)!!}{(2n)!!}right)^2 frac{e^{2n}}{2n-1}right],end{align}
where n!! is the double factorial. This series converges, but by expanding in terms of h = (a-b)^2 / (a+b)^2, James Ivory"MEMBERWIDE">IVORY, 1798, |last = Ivory|first = J.|title = A new series for the rectification of the ellipsis|authorlink = James Ivory (mathematician)|journal = Transactions of the Royal Society of Edinburgh|year = 1798|volume = 4|issue = 2|pages = 177{{ndash}}190|url =https://books.google.com/books?id=FaUaqZZYYPAC&pg=PA177|doi=10.1017/s0080456800030817}} and Bessel"MEMBERWIDE">BESSEL, 1825, |last = Bessel|first = F. W.|title = The calculation of longitude and latitude from geodesic measurements (1825)|authorlink = Friedrich Bessel|journal = Astron. Nachr.|year = 2010|volume = 331|number = 8|pages = 852{{ndash}}861|arxiv = 0908.1824|doi = 10.1002/asna.201011352|bibcode = 2010AN....331..852K}} Englisch translation of JOURNAL, F. W., Bessel, 10.1002/asna.18260041601, 1825, 1825AN......4..241B, Ãœber die Berechnung der geographischen LÃ¤ngen und Breiten aus geodÃ¤tischen Vermesssungen, Astron. Nachr., 4, 16, 241{{ndash, 254|arxiv=0908.1823}} derived an expression that converges much more rapidly:
&= 2pi a left[1 - sum_{n=1}^infty left(frac{(2n-1)!!}{(2n)!!}right)^2 frac{e^{2n}}{2n-1}right],end{align}
begin{align}
C &= pi (a+b) left[1 + sum_{n=1}^infty left(frac{(2n-1)!!}{2^n n!}right)^2 frac{h^n}{(2n-1)^2}right].
&= pi (a+b) left[1 + frac{h}{4} + sum_{n=2}^infty left(frac{(2n-3)!!}{2^n n!}right)^2 h^nright].
end{align}Srinivasa Ramanujan gives two close approximations for the circumference in Â§16 of "Modular Equations and Approximations to pi";RAMANUJAN >FIRST=SRINIVASA, Srinivasa Ramanujan, Modular Equations and Approximations to π, Quart. J. Pure App. Math., 45, 350{{ndash, 372|year = 1914|url =weblink|isbn=9780821820766}} they are
&= pi (a+b) left[1 + frac{h}{4} + sum_{n=2}^infty left(frac{(2n-3)!!}{2^n n!}right)^2 h^nright].
C approx pi left[3(a + b) - sqrt{(3a + b)(a + 3b)}right] = pi left[3(a + b) - sqrt{10ab + 3left(a^2 + b^2right)}right]
and
Capproxpileft(a+bright)left(1+frac{3h}{10+sqrt{4-3h}}right).
The errors in these approximations, which were obtained empirically, are of order h^3 and h^5, respectively.More generally, the arc length of a portion of the circumference, as a function of the angle subtended (or {{math|x}}-coordinates of any two points on the upper half of the ellipse), is given by an incomplete elliptic integral. The upper half of an ellipse is parameterized by
y=bsqrt{1-frac{x^{2}}{a^{2}}}.
Then the arc length s from x_{1} to x_{2} is:
s = -bint_{arccos frac{x_{1}}{a}}^{arccos frac{x_{2}}{a}} sqrt{1-left(1-frac{a^{2}}{b^{2}}right)sin^{2}z} , dz.
This is equivalent to
s = -bleft[Eleft(z ;Biggl|; 1 - frac{a^{2}}{b^{2}}right)right]^{arccos frac{x_{2}}{a}}_{arccos frac{x_{1}}{a}}
where E(z mid m) is the incomplete elliptic integral of the second kind with parameter m=k^{2}.{{See also|Meridian arc#Meridian distance on the ellipsoid}}The inverse function, the angle subtended as a function of the arc length, is given by a certain elliptic function.{{Citation needed|date=October 2010}}Some lower and upper bounds on the circumference of the canonical ellipse x^2/a^2 + y^2/b^2 = 1 with ageq b areJOURNAL, Jameson, G.J.O., Inequalities for the perimeter of an ellipse, Mathematical Gazette, 98, 542, 2014, 227â€“234, 10.1017/S002555720000125X,
begin{align}
2pi b &le C le 2pi a,
pi (a+b) &le C le 4(a+b),
4sqrt{a^2+b^2} &le C le sqrt{2} pi sqrt{a^2+b^2} .
end{align}Here the upper bound 2pi a is the circumference of a circumscribed concentric circle passing through the endpoints of the ellipse's major axis, and the lower bound 4sqrt{a^2+b^2} is the perimeter of an inscribed rhombus with vertices at the endpoints of the major and the minor axes.pi (a+b) &le C le 4(a+b),
4sqrt{a^2+b^2} &le C le sqrt{2} pi sqrt{a^2+b^2} .
Curvature
The curvature is given by kappa = frac{1}{a^2 b^2}left(frac{x^2}{a^4}+frac{y^2}{b^4}right)^{-frac{3}{2}} ,radius of curvature at point (x,y):
rho = a^2 b^2 left(frac{x^{2}}{a^4} + frac{y^{2}}{b^4}right)^frac{3}{2} = frac{1}{a^4 b^4} sqrt{left(a^4 y^{2} + b^4 x^{2}right)^3} .
Radius of curvature at the two vertices (pm a,0) and the centers of curvature:
rho_0 = frac{b^2}{a}=p , qquad left(pmfrac{c^2}{a},bigg|,0right) .
Radius of curvature at the two co-vertices (0,pm b) and the centers of curvature:
rho_1 = frac{a^2}{b} , qquad left(0,bigg|,pmfrac{c^2}{b}right) .
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
Applications
Physics
Elliptical reflectors and acoustics
{{See also|Fresnel zone}}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 Urbanaâ€“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 [approximately] at 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:
begin{align}
e &= frac{r_a - r_p}{r_a + r_p} = frac{r_a - r_p}{2a}
r_a &= (1 + e)a
r_p &= (1 - e)a
end{align}where r_a &= (1 + e)a
r_p &= (1 - e)a
- 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
begin{align}
a &= frac{r_a + r_p}{2} [2pt]
b &= sqrt{r_a r_p} [2pt]
ell &= frac{2}{frac{1}{r_a} + frac{1}{r_p}} = frac{2r_ar_p}{r_a + r_p}
end{align}.b &= sqrt{r_a r_p} [2pt]
ell &= frac{2}{frac{1}{r_a} + frac{1}{r_p}} = frac{2r_ar_p}{r_a + r_p}
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.David Drew."Elliptical Gears".weblinkAn 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.BOOK, George B., Grant, A treatise on gear wheels,weblink 1906, Philadelphia Gear Works, 72,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).Encyclopedia of Laser Physics and Technology - lamp-pumped lasers, arc lamps, flash lamps, high-power, Nd:YAG laser
- 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.WEB,weblink Archived copy, 2013-06-20, dead,weblink" title="web.archive.org/web/20130517100847weblink">weblink 2013-05-17,
Statistics and finance
In statistics, a bivariate random vector (X, Y) is jointly elliptically distributed if its iso-density contoursâ€”loci of equal values of the density functionâ€”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 varianceâ€”that is, any two portfolios with identical mean and variance of portfolio return have identical distributions of portfolio return.JOURNAL, Chamberlain, G., A characterization of the distributions that imply meanâ€”Variance utility functions, Journal of Economic Theory, 29, 1, 185â€“201, February 1983, 10.1016/0022-0531(83)90129-1, JOURNAL, Owen, J., Rabinovitch, R., On the class of elliptical distributions and their applications to the theory of portfolio choice, Journal of Finance, 38, 3, 745â€“752, June 1983, 2328079, 10.1111/j.1540-6261.1983.tb02499.x,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.JOURNAL, Pitteway, M.L.V., Algorithm for drawing ellipses or hyperbolae with a digital plotter, The Computer Journal, 10, 3, 282â€“9, 1967, 10.1093/comjnl/10.3.282,weblink Another efficient generalization to draw ellipses was invented in 1984 by Jerry Van Aken.JOURNAL, Van Aken, J.R., An Efficient Ellipse-Drawing Algorithm, IEEE Computer Graphics and Applications, 4, 9, 24â€“35, September 1984, 10.1109/MCG.1984.275994, 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.JOURNAL, Smith, L.B., Drawing ellipses, hyperbolae or parabolae with a fixed number of points, The Computer Journal, 14, 1, 81â€“86, 1971, 10.1093/comjnl/14.1.81,weblink 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 BÃ©zier paths:
Optimization theory
It is sometimes useful to find the minimum bounding ellipse on a set of points. The ellipsoid method is quite useful for attacking this problem.See also
{{Div col|colwidth=30em}}- Apollonius of Perga, the classical authority
- Cartesian oval, a generalization of the ellipse
- Circumconic and inconic
- Conic section
- Ellipse fitting
- Ellipsoid, a higher dimensional analog of an ellipse
- Elliptic coordinates, an orthogonal coordinate system based on families of ellipses and hyperbolae
- Elliptic partial differential equation
- Elliptical distribution, in statistics
- Geodesics on an ellipsoid
- Great ellipse
- Hyperbola
- Kepler's laws of planetary motion
- Matrix representation of conic sections
- n-ellipse, a generalization of the ellipse for n foci
- Oval
- Parabola
- Rytzâ€™s construction, a method for finding the ellipse axes from conjugate diameters or a parallelogram
- 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
- Steiner inellipse, the unique ellipse inscribed in a triangle with tangencies at the sides' midpoints
- Superellipse, a generalization of an ellipse that can look more rectangular or more "pointy"
- True, eccentric, and mean anomaly
Notes
{{Reflist|30em}}References
- BOOK, W.H., Besant, Conic Sections, Chapter III. The Ellipse
- BOOK, Coxeter, H.S.M., Introduction to Geometry, Wiley, New York, 1969, 115â€“9, 2nd,
- {{citation|first=Bruce E.|last=Meserve|title=Fundamental Concepts of Geometry|year=1983|origyear=1959|publisher=Dover|isbn=978-0-486-63415-9}}
- BOOK, Miller, Charles D., Lial, Margaret L., Schneider, David I., Fundamentals of College Algebra, Scott Foresman/Little, 1990, 978-0-673-38638-0, 381, 3rd,
External links
- {{Commons category-inline|Ellipses}}
- {{Britannica|185064|Ellipse (mathematics)}}
- {{PlanetMath |urlname= ellipse|title= ellipse}}
- {{MathWorld |id=Ellipse |title=Ellipse}}
- {{MathWorld |id=Hypotrochoid|title=Ellipse as special case of hypotrochoid}}
- weblink" title="web.archive.org/web/20070715063900weblink">Apollonius' Derivation of the Ellipse at Convergence
- The Shape and History of The Ellipse in Washington, D.C. by Clark Kimberling
- Ellipse circumference calculator
- Collection of animated ellipse demonstrations
- {{springer| title=Ellipse | id=Ellipse&oldid=11394 | last=Ivanov | first=A.B. }}
- Trammel according Frans van Schooten
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