View source for Ellipsoid

{{short description|Quadric surface that looks like a deformed sphere}}
[[File:Ellipsoide.svg|400px|thumb|Examples of ellipsoids with equation {{math|{{sfrac|''x''<sup>2</sup>|''a''<sup>2</sup>}} + {{sfrac|''y''<sup>2</sup>|''b''<sup>2</sup>}} + {{sfrac|''z''<sup>2</sup>|''c''<sup>2</sup>}} {{=}} 1}}: {{ubl
| ''[[Sphere]]'', {{math|''a'' {{=}} ''b'' {{=}} ''c'' {{=}} 4}}, ''top'';
| ''[[Spheroid]]'', {{math|''a'' {{=}} ''b'' {{=}} 5}}, {{math|''c'' {{=}} 3}}, ''bottom left'';
| ''Tri-axial'' ellipsoid, {{math|''a'' {{=}} 4.5}}, {{math|''b'' {{=}} 6}}; {{math|''c'' {{=}} 3}}, ''bottom right''
}}]]

An '''ellipsoid''' is a surface that can be obtained from a [[sphere]] by deforming it by means of directional [[Scaling (geometry)|scaling]]s, or more generally, of an [[affine transformation]].

An ellipsoid is a [[quadric surface]];  that is, a [[Surface (mathematics)|surface]] that may be defined as the [[zero set]] of a [[polynomial]] of degree two in three variables. Among quadric surfaces, an ellipsoid is characterized by either of the two following properties. Every planar [[Cross section (geometry)|cross section]] is either an [[ellipse]], or is empty, or is reduced to a single point (this explains the name, meaning "ellipse-like"). It is [[Bounded set|bounded]], which means that it may be enclosed in a sufficiently large sphere.

An ellipsoid has three pairwise [[perpendicular]] [[Rotational symmetry|axes of symmetry]] which intersect at a [[Central symmetry|center of symmetry]], called the center of the ellipsoid. The [[line segment]]s that are delimited on the axes of symmetry by the ellipsoid are called the ''principal axes'', or simply axes of the ellipsoid. If the three axes have different lengths, the figure is a '''triaxial ellipsoid''' (rarely '''scalene ellipsoid'''), and the axes are uniquely defined.

If two of the axes have the same length, then the ellipsoid is an ''ellipsoid of [[Surface of revolution|revolution]]'', also called a ''[[spheroid]]''. In this case, the ellipsoid is invariant under a [[rotation]] around the third axis, and there are thus infinitely many ways of choosing the two perpendicular axes of the same length. In the case of two axes being the same length:

* If the third axis is shorter, the ellipsoid is a sphere that has been flattened (called an ''[[oblate spheroid]]'').
* If the third axis is longer, it is a sphere that has been lengthened (called a ''[[prolate spheroid]]''). 

If the three axes have the same length, the ellipsoid is a sphere.

==Standard equation==
The general ellipsoid, also known as triaxial ellipsoid, is a quadratic surface which is defined in [[Cartesian coordinate system|Cartesian coordinates]] as:

:<math>\frac{x^2}{a^2} + \frac{y^2}{b^2} + \frac{z^2}{c^2} = 1,</math>
where <math>a</math>, <math>b</math> and <math>c</math> are the length of the semi-axes.  

The points <math>(a, 0, 0)</math>, <math>(0, b, 0)</math> and <math>(0, 0, c)</math> lie on the surface. The line segments from the origin to these points are called the principal semi-axes of the ellipsoid, because {{math|''a'', ''b'', ''c''}} are half the length of the principal axes. They correspond to the [[semi-major axis]] and [[semi-minor axis]] of an [[ellipse]].

In [[spherical coordinate system]] for which <math>(x,y,z)=(r\sin\theta\cos\varphi, r\sin\theta\sin\varphi,r\cos\theta)</math>, the general ellipsoid is defined as:

:<math>{r^2\sin^2\theta\cos^2\varphi\over a^2}+{r^2\sin^2\theta\sin^2\varphi \over b^2}+{r^2\cos^2\theta \over c^2}=1,</math>

where <math>\theta</math> is the polar angle and <math>\varphi</math> is the azimuthal angle.

When <math>a=b=c</math>, the ellipsoid is a sphere.

When <math>a=b\neq c</math>, the ellipsoid is a spheroid or ellipsoid of revolution.  In particular, if <math>a = b > c</math>, it is an [[oblate spheroid]]; if <math>a = b < c</math>, it is a [[prolate spheroid]].

== Parameterization ==
The ellipsoid may be parameterized in several ways, which are simpler to express when the ellipsoid axes coincide with coordinate axes. A common choice is
:<math>\begin{align}
  x &= a\sin\theta\cos\varphi,\\
  y &= b\sin\theta\sin\varphi,\\
  z &= c\cos\theta,
\end{align}\,\!</math>

where
:<math>
  0 \le \theta \le \pi,\qquad
  0 \le \varphi < 2\pi.
</math>

These parameters may be interpreted as [[spherical coordinates]], where {{mvar|&theta;}} is the polar angle and {{mvar|&phi;}} is the azimuth angle of the point {{math|(''x'', ''y'', ''z'')}} of the ellipsoid.<ref>{{harvtxt|Kreyszig|1972|pp=455–456}}</ref>

Measuring from the equator rather than a pole,
:<math>\begin{align}
  x &= a\cos\theta\cos\lambda,\\
  y &= b\cos\theta\sin\lambda,\\
  z &= c\sin\theta,
\end{align}\,\!</math>

where
:<math>
  -\tfrac{\pi}2 \le \theta \le \tfrac{\pi}2,\qquad
  0 \le \lambda < 2\pi,
</math>

{{mvar|&theta;}} is the [[reduced latitude]], [[parametric latitude]], or [[eccentric anomaly]] and {{mvar|&lambda;}} is azimuth or longitude.

Measuring angles directly to the surface of the ellipsoid, not to the circumscribed sphere,
:<math>\begin{bmatrix}
    x \\ y \\ z
  \end{bmatrix} =
  R \begin{bmatrix}
    \cos\gamma\cos\lambda\\
    \cos\gamma\sin\lambda\\
    \sin\gamma
  \end{bmatrix}
\,\!</math>

where
:<math>\begin{align}
  R ={} &\frac{abc}{\sqrt{c^2 \left(b^2\cos^2\lambda + a^2\sin^2\lambda\right) \cos^2\gamma
               + a^2 b^2\sin^2\gamma}}, \\[3pt]
        &-\tfrac{\pi}2 \le \gamma \le \tfrac{\pi}2,\qquad
           0 \le \lambda < 2\pi.
\end{align}</math>

{{mvar|&gamma;}} would be [[geocentric latitude]] on the Earth, and {{mvar|&lambda;}} is longitude. These are true spherical coordinates with the origin at the center of the ellipsoid.{{citation needed|date=April 2020}}

In [[geodesy]], the [[geodetic latitude]] is most commonly used, as the angle between the vertical and the equatorial plane, defined for a biaxial ellipsoid. For a more general triaxial ellipsoid, see [[ellipsoidal latitude]].

==Volume==
The [[volume]] bounded by the ellipsoid is 
:<math>V = \tfrac{4}{3}\pi abc.</math>
In terms of the principal [[diameter]]s {{math|''A'', ''B'', ''C''}} (where {{math|''A'' {{=}} 2''a''}}, {{math|''B'' {{=}} 2''b''}}, {{math|''C'' {{=}} 2''c''}}), the volume is
:<math>V = \tfrac16 \pi ABC</math>.
This equation reduces to that of the volume of a sphere when all three elliptic radii are equal, and to that of an [[oblate spheroid|oblate]] or [[prolate spheroid]] when two of them are equal.

The [[volume]] of an ellipsoid is {{sfrac|2|3}} the volume of a [[circumscribed]] [[elliptic cylinder]], and {{sfrac|{{math|&pi;}}|6}} the volume of the circumscribed box. The [[volumes]] of the [[inscribed]] and circumscribed [[rectangular cuboid|boxes]] are respectively:
:<math>
  V_\text{inscribed} = \frac{8}{3\sqrt{3}} abc,\qquad
  V_\text{circumscribed} = 8abc.
</math>

==Surface area==
{{see also|Area of a geodesic polygon}}

The [[surface area]] of a general (triaxial) ellipsoid is<ref>F.W.J. Olver, D.W. Lozier, R.F. Boisvert, and C.W. Clark, editors, 2010, ''NIST Handbook of Mathematical Functions'' ([[Cambridge University Press]]), Section 19.33 {{cite web |url=http://dlmf.nist.gov/19.33 |title=Triaxial Ellipsoids |access-date=2012-01-08 }}</ref> 
:<math>S = 2\pi c^2 + \frac{2\pi ab}{\sin(\varphi)}\left(E(\varphi, k)\,\sin^2(\varphi) + F(\varphi, k)\,\cos^2(\varphi)\right),</math>

where
:<math> 
  \cos(\varphi) = \frac{c}{a},\qquad
  k^2 = \frac{a^2\left(b^2 - c^2\right)}{b^2\left(a^2 - c^2\right)},\qquad
  a \ge b \ge c,
</math>

and where {{math|''F''(''φ'', ''k'')}} and {{math|''E''(''φ'', ''k'')}} are incomplete [[elliptic integral]]s of the first and second kind respectively.<ref>{{Cite web|url=http://dlmf.nist.gov/19.2|title = DLMF: 19.2 Definitions}}</ref> 

The surface area of this general ellipsoid can also be expressed in terms of {{tmath|R_G}}, one of the [[Carlson symmetric form]]s of elliptic integrals:<ref>{{Cite web |title=Surface Area of an Ellipsoid |url=https://analyticphysics.com/Mathematical%20Methods/Surface%20Area%20of%20an%20Ellipsoid.htm |access-date=2024-07-23 |website=analyticphysics.com}}</ref>
:<math>S = 4\pi bc R_{G} \left( \frac{a^2}{b^2} , \frac{a^2}{c^2} , 1\right).</math>

Simplifying the above formula using properties of {{math|''R''<sub>''G''</sub>}},<ref>{{Cite web |title=DLMF: §19.20 Special Cases ‣ Symmetric Integrals ‣ Chapter 19 Elliptic Integrals |url=https://dlmf.nist.gov/19.20#ii |access-date=2024-07-23 |website=dlmf.nist.gov}}</ref> this can also be expressed in terms of the volume of the ellipsoid {{math|''V''}}:
:<math>S = 3VR_{G}\left(a^{-2},b^{-2},c^{-2}\right).</math>

Unlike the expression with {{math|''F''(''φ'', ''k'')}} and {{math|''E''(''φ'', ''k'')}}, the equations in terms of {{math|''R''<sub>''G''</sub>}} do not depend on the choice of an order on {{math|''a''}}, {{math|''b''}}, and {{math|''c''}}. 

The surface area of an ellipsoid of revolution (or spheroid) may be expressed in terms of [[elementary function]]s:
:<math>
   S_\text{oblate} = 2\pi a^2\left(1 + \frac{c^2}{ea^2} \operatorname{artanh}e\right),
                   \qquad\text{where }e^2 = 1 - \frac{c^2}{a^2}\text{ and }(c < a), </math>
or
:<math>
 S_\text{oblate} = 2\pi a^2\left(1 + \frac{1 - e^2}{e} \operatorname{artanh}e\right)</math>

or
:<math>
S_\text{oblate} = 2\pi a^2\ + \frac{\pi c^2}{e}\ln\frac{1+e}{1-e}</math>

and 
:<math>
  S_\text{prolate} = 2\pi a^2\left(1 + \frac{c}{ae}  \arcsin e\right)
                   \qquad\text{where } e^2 = 1 - \frac{a^2}{c^2}\text{ and } (c > a),
</math>

which, as follows from basic trigonometric identities, are equivalent expressions (i.e. the formula for {{math|''S''<sub>oblate</sub>}} can be used to calculate the surface area of a prolate ellipsoid and vice versa). In both cases {{mvar|e}} may again be identified as the [[eccentricity (mathematics)|eccentricity]] of the ellipse formed by the cross section through the symmetry axis. (See [[ellipse]]). Derivations of these results may be found in standard sources, for example [[Mathworld]].<ref>{{cite web |url=http://mathworld.wolfram.com/ProlateSpheroid.html |title=Prolate Spheroid |first=Weisstein, Eric |last=W. |website=mathworld.wolfram.com |access-date=25 March 2018 |url-status=live |archive-url=https://web.archive.org/web/20170803085757/http://mathworld.wolfram.com/ProlateSpheroid.html |archive-date=3 August 2017}}</ref>

=== Approximate formula ===
: <math>S \approx 4\pi \sqrt[p]{\frac{a^p b^p + a^p c^p + b^p c^p}{3}}.\,\!</math>

Here {{math|''p'' ≈ 1.6075}} yields a relative error of at most 1.061%;<ref>[http://www.numericana.com/answer/ellipsoid.htm#thomsen Final answers] {{webarchive |url=https://web.archive.org/web/20110930084035/http://www.numericana.com/answer/ellipsoid.htm |date=2011-09-30}} by Gerard P. Michon (2004-05-13). See Thomsen's formulas and Cantrell's comments.</ref> a value of {{math|1=''p'' = {{sfrac|8|5}} = 1.6}} is optimal for nearly spherical ellipsoids, with a relative error of at most 1.178%.

In the "flat" limit of {{mvar|c}} much smaller than {{mvar|a}} and {{mvar|b}}, the area is approximately {{math|2π''ab''}}, equivalent to {{math|1=''p'' = log<sub>2</sub>3 ≈ 1.5849625007}}.

== Plane sections ==

{{see also|Earth section}}
[[File:Ellipsoid-ebener-Schnitt.svg|300px|thumb|Plane section of an ellipsoid]]
The intersection of a plane and a sphere is a circle (or is reduced to a single point, or is empty). Any ellipsoid is the image of the unit sphere under some affine transformation, and any plane is the image of some other plane under the same transformation. So, because affine transformations map circles to ellipses, the intersection of a plane with an ellipsoid is an ellipse or a single point, or is empty.<ref>{{citation|first=Abraham Adrian|last=Albert|title=Solid Analytic Geometry|year=2016|orig-year=1949|publisher=Dover|isbn=978-0-486-81026-3|page=117}}</ref> Obviously, spheroids contain circles. This is also true, but less obvious, for triaxial ellipsoids (see [[Circular section]]).

=== Determining the ellipse of a plane section ===
[[File:Ellipso-eb-beisp.svg|thumb|Plane section of an ellipsoid (see example)]]
'''Given:''' Ellipsoid {{math|{{sfrac|''x''<sup>2</sup>|''a''<sup>2</sup>}} + {{sfrac|''y''<sup>2</sup>|''b''<sup>2</sup>}} + {{sfrac|''z''<sup>2</sup>|''c''<sup>2</sup>}} {{=}} 1}} and the plane with equation {{math|''n<sub>x</sub>x'' + ''n<sub>y</sub>y'' + ''n<sub>z</sub>z'' {{=}} ''d''}}, which have an ellipse in common.

'''Wanted:''' Three vectors {{math|'''f'''<sub>0</sub>}} (center) and {{math|'''f'''<sub>1</sub>}}, {{math|'''f'''<sub>2</sub>}} (conjugate vectors), such that the ellipse can be represented by the parametric equation 
:<math>\mathbf x = \mathbf f_0 + \mathbf f_1\cos t + \mathbf f_2\sin t</math>
(see [[Ellipse#Ellipse as an affine image of the unit circle x²+y²=1|ellipse]]).

[[File:Ellipso-eb-ku.svg|300px|thumb|Plane section of the unit sphere (see example)]]
'''Solution:''' The scaling {{math|1=''u'' = {{sfrac|''x''|''a''}}, ''v'' = {{sfrac|''y''|''b''}}, ''w'' = {{sfrac|''z''|''c''}}}} transforms the ellipsoid onto the unit sphere {{math|''u''<sup>2</sup> + ''v''<sup>2</sup> + ''w''<sup>2</sup> {{=}} 1}} and the given plane onto the plane with equation
:<math>\ n_x au + n_y bv + n_z cw = d. </math>
Let {{math|''m<sub>u</sub>u'' + ''m<sub>v</sub>v'' + ''m<sub>w</sub>w'' {{=}} ''&delta;''}} be the [[Hesse normal form]] of the new plane and
:<math>\;\mathbf m = \begin{bmatrix} m_u \\ m_v \\ m_w \end{bmatrix}\;</math>
its unit normal vector. Hence
:<math>\mathbf e_0 = \delta \mathbf m \;</math>
is the  ''center'' of the intersection circle and
:<math>\;\rho = \sqrt{1 - \delta^2}\;</math>
its radius (see diagram).

Where {{math|''m<sub>w</sub>'' {{=}} ±1}} (i.e. the plane is horizontal), let
:<math>\ \mathbf e_1 = \begin{bmatrix} \rho \\ 0 \\ 0 \end{bmatrix},\qquad \mathbf e_2 = \begin{bmatrix} 0 \\ \rho \\ 0 \end{bmatrix}.</math>

Where {{math|''m<sub>w</sub>'' ≠ ±1}}, let
:<math >\mathbf e_1 = \frac{\rho}{\sqrt{m_u^2 + m_v^2}}\, \begin{bmatrix} m_v \\ -m_u \\ 0 \end{bmatrix}\, ,\qquad \mathbf e_2 = \mathbf m \times \mathbf e_1\ .</math>

In any case, the vectors {{math|'''e'''<sub>1</sub>, '''e'''<sub>2</sub>}} are orthogonal, parallel to the intersection plane and have length {{mvar|&rho;}} (radius of the circle). Hence the intersection circle can be described by the parametric equation
:<math>\;\mathbf u = \mathbf e_0 + \mathbf e_1\cos t + \mathbf e_2\sin t\;.</math>

The reverse scaling (see above) transforms the unit sphere back to the ellipsoid and the vectors {{math|'''e'''<sub>0</sub>, '''e'''<sub>1</sub>, '''e'''<sub>2</sub>}} are mapped onto vectors {{math|'''f'''<sub>0</sub>, '''f'''<sub>1</sub>, '''f'''<sub>2</sub>}}, which were wanted for the parametric representation of the intersection ellipse. 

How to find the vertices and semi-axes of the ellipse is described in [[Ellipse#Ellipse as an affine image of the unit circle x²+y²=1|ellipse]].

'''Example:''' The diagrams show an ellipsoid with the semi-axes {{math|1=''a'' = 4, ''b'' = 5, ''c'' = 3}} which is cut by the plane {{math|1=''x'' + ''y'' + ''z'' = 5}}.
{{clear}}

== Pins-and-string construction ==
[[File:Ellipse-gaertner-k.svg|upright=1|thumb|Pins-and-string construction of an ellipse:<br>
{{math|{{abs|''S''<sub>1</sub> ''S''<sub>2</sub>}}}}, length of the string (red)]]
[[File:Fokalks-ellipsoid.svg|thumb|upright=1.2|Pins-and-string construction of an ellipsoid, blue: focal conics]]
[[File:Fokalks-ellipsoid-xyz.svg|thumb|upright=1.2|Determination of the semi axis of the ellipsoid]]
The pins-and-string construction of an ellipsoid is a transfer of the idea constructing an ellipse using two [[ellipse#Pins-and-string method|pins and a string]] (see diagram).

A pins-and-string construction of an [[ellipsoid of revolution]] is given by the pins-and-string construction of the rotated ellipse. 

The construction of points of a ''triaxial ellipsoid'' is more complicated. First ideas are due to the Scottish physicist [[James Clerk Maxwell|J. C. Maxwell]] (1868).<ref> W. Böhm: ''Die FadenKonstruktion der Flächen zweiter Ordnung'', Mathemat. Nachrichten 13, 1955, S. 151</ref> Main investigations and the extension to quadrics was done by the German mathematician O. Staude in 1882, 1886 and 1898.<ref>Staude, O.: ''Ueber Fadenconstructionen des Ellipsoides''. Math. Ann. 20, 147–184 (1882)</ref><ref> Staude, O.: ''Ueber neue Focaleigenschaften der Flächen 2. Grades.'' Math. Ann. 27, 253–271 (1886).</ref><ref> Staude, O.: ''Die algebraischen Grundlagen der Focaleigenschaften der Flächen 2. Ordnung'' Math. Ann. 50, 398 - 428 (1898).</ref> The description of the pins-and-string construction of ellipsoids and hyperboloids is contained in the book ''Geometry and the imagination'' written by [[David Hilbert|D. Hilbert]] & S. Vossen,<ref>D. Hilbert & S Cohn-Vossen: ''Geometry and the imagination'', Chelsea New York, 1952, {{ISBN|0-8284-1087-9}}, p. 20 .</ref> too.

=== Steps of the construction ===
# Choose an ''ellipse'' {{mvar|E}} and a ''hyperbola'' {{mvar|H}}, which are a pair of [[focal conics]]: <math display="block">\begin{align}
E(\varphi) &= (a\cos\varphi, b\sin\varphi, 0) \\
H(\psi) &= (c\cosh\psi, 0, b\sinh\psi),\quad c^2 = a^2 - b^2
\end{align} </math> with the vertices and foci of the ellipse <math display="block">S_1 = (a, 0, 0),\quad F_1 = (c, 0, 0),\quad F_2 = (-c, 0, 0),\quad S_2 = (-a, 0, 0)</math> and a ''string'' (in diagram red) of length {{mvar|l}}.
# Pin one end of the string to [[vertex (curve)|vertex]] {{math|''S''<sub>1</sub>}} and the other to focus {{math|''F''<sub>2</sub>}}. The string is kept tight at a point {{mvar|P}} with positive {{mvar|y}}- and {{mvar|z}}-coordinates, such that the string runs from {{math|''S''<sub>1</sub>}} to {{mvar|P}} behind the upper part of the hyperbola (see diagram) and is free to slide on the hyperbola. The part of the string from {{mvar|P}} to {{math|''F''<sub>2</sub>}} runs and slides in front of the ellipse. The string runs through that point of the hyperbola, for which the distance {{math|{{abs|''S''<sub>1</sub> ''P''}}}} over any hyperbola point is at a minimum. The analogous statement on the second part of the string and the ellipse has to be true, too.
# Then: {{mvar|P}} is a point of the ellipsoid with equation  <math display="block">\begin{align}
 &\frac{x^2}{r_x^2} + \frac{y^2}{r_y^2} + \frac{z^2}{r_z^2} = 1 \\
 &r_x = \tfrac{1}{2}(l - a + c), \quad
  r_y = {\textstyle \sqrt{r^2_x - c^2}}, \quad
  r_z = {\textstyle \sqrt{r^2_x - a^2}}.
\end{align}</math>
# The remaining points of the ellipsoid can be constructed by suitable changes of the string at the focal conics.

=== Semi-axes ===
Equations for the semi-axes of the generated ellipsoid can be derived by special choices for point {{mvar|P}}:
:<math>Y = (0, r_y, 0),\quad Z = (0, 0, r_z).</math>

The lower part of the diagram shows that {{math|''F''<sub>1</sub>}} and {{math|''F''<sub>2</sub>}} are the foci of the ellipse in the {{mvar|xy}}-plane, too. Hence, it is [[Confocal conic sections|confocal]] to the given ellipse and the length of the string is {{math|''l'' {{=}} 2''r<sub>x</sub>'' + (''a'' − ''c'')}}. Solving for {{mvar|r<sub>x</sub>}} yields {{math|''r<sub>x</sub>'' {{=}} {{sfrac|1|2}}(''l'' − ''a'' + ''c'')}}; furthermore {{math|''r''{{su|p=2|b=''y''}} {{=}} ''r''{{su|p=2|b=''x''}} − ''c''<sup>2</sup>}}.

From the upper diagram we see that {{math|''S''<sub>1</sub>}} and {{math|''S''<sub>2</sub>}} are the foci of the ellipse section of the ellipsoid in the {{mvar|xz}}-plane and that {{math|''r''{{su|p=2|b=''z''}} {{=}} ''r''{{su|p=2|b=''x''}} − ''a''<sup>2</sup>}}.

=== Converse ===
If, conversely, a triaxial ellipsoid is given by its equation, then from the equations in step 3 one can derive the parameters {{mvar|a}}, {{mvar|b}}, {{mvar|l}} for a pins-and-string construction. 

=== Confocal ellipsoids ===
If {{overline|{{mathcal|E}}}} is an ellipsoid [[Confocal quadrics|confocal]] to {{mathcal|E}} with the squares of its semi-axes 
: <math>\overline r_x^2 = r_x^2 - \lambda, \quad
  \overline r_y^2 = r_y^2 - \lambda, \quad
  \overline r_z^2 = r_z^2 - \lambda</math>

then from the equations of {{mathcal|E}}
: <math>  r_x^2 - r_y^2 = c^2, \quad
  r_x^2 - r_z^2 = a^2, \quad
  r_y^2 - r_z^2 = a^2 - c^2 = b^2</math>

one finds, that the corresponding focal conics used for the pins-and-string construction have the same semi-axes {{math|''a'', ''b'', ''c''}} as ellipsoid {{mathcal|E}}. Therefore (analogously to the foci of an ellipse) one considers the focal conics of a triaxial ellipsoid as the (infinite many) foci and calls them the '''focal curves''' of the ellipsoid.<ref>O. Hesse: ''Analytische Geometrie des Raumes'', Teubner, Leipzig 1861, p.&nbsp;287</ref>

The converse statement is true, too: if one chooses a second string of length {{math|{{overline|''l''}}}} and defines
:<math>\lambda = r^2_x - \overline r^2_x</math>
then the equations
:<math>\overline r_y^2 = r_y^2 - \lambda,\quad \overline r_z^2 = r_z^2 - \lambda</math>
are valid, which means the two ellipsoids are confocal.

=== Limit case, ellipsoid of revolution ===
In case of {{math|''a'' {{=}} ''c''}} (a [[spheroid]]) one gets {{math|''S''<sub>1</sub> {{=}} ''F''<sub>1</sub>}} and {{math|''S''<sub>2</sub> {{=}} ''F''<sub>2</sub>}}, which means that the focal ellipse degenerates to a line segment and the focal hyperbola collapses to two infinite line segments on the {{mvar|x}}-axis. The ellipsoid is [[Rotational symmetry|rotationally symmetric]] around the {{mvar|x}}-axis and
:<math>r_x = \tfrac12l,\quad r_y = r_z = {\textstyle \sqrt{r^2_x - c^2}}</math>.

=== Properties of the focal hyperbola ===
[[File:Ellipsoid-pk-zk.svg|thumb|upright=1.5|'''Top:''' 3-axial Ellipsoid with its focal hyperbola.<br>
'''Bottom:''' parallel and central projection of the ellipsoid such that it looks like a sphere, i.e. its apparent shape is a circle]]
; True curve
: If one views an ellipsoid from an external point {{mvar|V}} of its focal hyperbola, then it seems to be a sphere, that is its apparent shape is a circle. Equivalently, the tangents of the ellipsoid containing point {{mvar|V}} are the lines of a circular [[cone]], whose axis of rotation is the [[Tangent (geometry)|tangent line]] of the hyperbola at {{mvar|V}}.<ref>D. Hilbert & S Cohn-Vossen: ''Geometry and the Imagination'', p. 24</ref><ref>O. Hesse: ''Analytische Geometrie des Raumes'', p.&nbsp;301</ref> If one allows the center {{mvar|V}} to disappear into infinity, one gets an [[Orthogonal projection|orthogonal]] [[parallel projection]] with the corresponding [[asymptote]] of the focal hyperbola as its direction. The ''true curve of shape'' (tangent points) on the ellipsoid is not a circle.{{paragraph}} The lower part of the diagram shows on the left a parallel projection of an ellipsoid (with semi-axes 60, 40, 30) along an asymptote and on the right a central projection with center {{mvar|V}} and main point {{mvar|H}} on the tangent of the hyperbola at point {{mvar|V}}. ({{mvar|H}} is the foot of the perpendicular from {{mvar|V}} onto the image plane.) For both projections the apparent shape is a circle. In the parallel case the image of the origin {{mvar|O}} is the circle's center; in the central case main point {{mvar|H}} is the center.
; Umbilical points
: The focal hyperbola intersects the ellipsoid at its four [[umbilical point]]s.<ref>W. Blaschke: ''Analytische Geometrie'', p. 125</ref>

=== Property of the focal ellipse ===
The focal ellipse together with its inner part can be considered as the limit surface (an infinitely thin ellipsoid) of the [[Pencil (mathematics)|pencil]] of confocal ellipsoids determined by {{math|''a'', ''b''}} for {{math|''r<sub>z</sub>'' → 0}}. For the limit case one gets
:<math>r_x = a,\quad r_y = b,\quad l = 3a - c.</math>

== In higher dimensions and general position {{anchor|In higher dimensions|Ellipsoids in higher dimensions and general position}} ==


A '''hyperellipsoid''', or ellipsoid of dimension <math>n - 1</math> in a [[Euclidean space]] of dimension <math>n</math>, is a [[quadric hypersurface]] defined by a polynomial of degree two that has a [[homogeneous polynomial|homogeneous part]] of degree two which is a [[positive definite quadratic form]].

One can also define a hyperellipsoid as the image of a sphere under an invertible [[affine  transformation]]. The spectral theorem can again be used to obtain a standard equation of the form 
:<math>\frac{x_1^2}{a_1^2}+\frac{x_2^2}{a_2^2}+\cdots + \frac{x_n^2}{a_n^2}=1.</math>

The volume of an {{mvar|n}}-dimensional ''hyperellipsoid'' can be obtained by replacing {{mvar|R<sup>n</sup>}} by the product of the semi-axes {{math|''a''<sub>1</sub>''a''<sub>2</sub>...''a<sub>n</sub>''}} in the formula for the [[Volume of an n-ball#The volume|volume of a hypersphere]]:
:<math>V = \frac{\pi^\frac{n}{2}}{\Gamma\left(\frac{n}{2} + 1\right)}a_1a_2\cdots a_n \approx  \frac{1}{\sqrt{\pi n}} \cdot \left(\frac{2 e \pi}{n}\right)^{n/2} a_1a_2\cdots a_n </math>
(where {{math|&Gamma;}} is the [[gamma function]]).

=== As a quadric ===
If {{mvar|'''A'''}} is a real, symmetric, {{mvar|n}}-by-{{mvar|n}} [[positive-definite matrix]], and {{mvar|'''v'''}} is a vector in <math>\R^n,</math> then the set of points {{math|'''x'''}} that satisfy the equation
:<math>(\mathbf{x}-\mathbf{v})^\mathsf{T}\! \boldsymbol{A}\, (\mathbf{x}-\mathbf{v}) = 1</math>
is an ''n''-dimensional ellipsoid centered at {{mvar|'''v'''}}. The expression <math>(\mathbf{x}-\mathbf{v})^\mathsf{T}\! \boldsymbol{A}\, (\mathbf{x}-\mathbf{v}) </math> is also called the '''ellipsoidal norm''' of {{math|'''x''' - '''v'''}}. For every ellipsoid, there are unique {{mvar|'''A'''}} and {{math|'''v'''}} that satisfy the above equation.<ref name=":0" />{{Rp|page=67|location=}}

The [[eigenvector]]s of {{mvar|'''A'''}} are the principal axes of the ellipsoid, and the [[eigenvalue]]s of {{mvar|'''A'''}} are the reciprocals of the squares of the semi-axes (in three dimensions these are {{math|''a''<sup>−2</sup>}}, {{math|''b''<sup>−2</sup>}} and {{math|''c''<sup>−2</sup>}}).<ref>{{cite web |url=http://see.stanford.edu/materials/lsoeldsee263/15-symm.pdf |title=Archived copy |access-date=2013-10-12 |url-status=live |archive-url=https://web.archive.org/web/20130626233838/http://see.stanford.edu/materials/lsoeldsee263/15-symm.pdf |archive-date=2013-06-26}} pp. 17–18.</ref> In particular:

* The [[diameter]] of the ellipsoid is twice the longest semi-axis, which is twice the square-root of the reciprocal of the largest eigenvalue of {{mvar|'''A'''}}.
* The [[width]] of the ellipsoid is twice the shortest semi-axis, which is twice the square-root of the reciprocal of the smallest eigenvalue of {{mvar|'''A'''}}.

An invertible [[linear transformation]] applied to a sphere produces an ellipsoid, which can be brought into the above standard form by a suitable [[rotation]], a consequence of the [[polar decomposition]] (also, see [[spectral theorem]]). If the linear transformation is represented by a [[symmetric matrix|symmetric 3&nbsp;×&nbsp;3 matrix]], then the eigenvectors of the matrix are orthogonal (due to the [[spectral theorem]]) and represent the directions of the axes of the ellipsoid; the lengths of the semi-axes are computed from the eigenvalues. The [[singular value decomposition]] and [[polar decomposition]] are matrix decompositions closely related to these geometric observations.

For every positive definite matrix <math>\boldsymbol{A}</math>, there exists a unique positive definite matrix denoted {{math|'''''A'''''<sup>1/2</sup>}}, such that  <math>\boldsymbol{A} = \boldsymbol{A}^{1/ 2}\boldsymbol{A}^{1/ 2}; </math> this notation  is motivated by the fact that this matrix can be seen as the "positive square root" of <math>\boldsymbol{A}.</math> The ellipsoid defined by <math>(\mathbf{x}-\mathbf{v})^\mathsf{T}\! \boldsymbol{A}\, (\mathbf{x}-\mathbf{v}) = 1</math> can also be presented as<ref name=":0">{{Cite Geometric Algorithms and Combinatorial Optimization}}</ref>{{Rp|page=67|location=}}<blockquote><math>A^{-1/2}\cdot S(\mathbf{0},1) + \mathbf{v}</math></blockquote>where S('''0''',1) is the [[unit sphere]] around the origin.

=== Parametric representation ===
[[File:Ellipsoid-affin.svg|300px|thumb|ellipsoid as an affine image of the unit sphere]]
The key to a parametric representation of an ellipsoid in general position is the alternative definition:
: ''An ellipsoid is an affine image of the unit sphere.''

An [[affine transformation]] can be represented by a translation with a vector {{math|'''f'''<sub>0</sub>}} and a regular 3&nbsp;×&nbsp;3 matrix {{math|'''''A'''''}}:
: <math>\mathbf x \mapsto \mathbf f_0 + \boldsymbol A \mathbf x = \mathbf f_0 + x\mathbf f_1 + y\mathbf f_2 + z\mathbf f_3</math>

where {{math|'''f'''<sub>1</sub>, '''f'''<sub>2</sub>, '''f'''<sub>3</sub>}} are the column vectors of matrix {{math|'''''A'''''}}.

A parametric representation of an ellipsoid in general position can be obtained by the parametric representation of a unit sphere (see above) and an affine transformation:
: <math>\mathbf x(\theta, \varphi) = \mathbf f_0 + \mathbf f_1 \cos\theta \cos\varphi + \mathbf f_2 \cos\theta \sin\varphi + \mathbf f_3 \sin\theta, \qquad -\tfrac{\pi}{2} < \theta < \tfrac{\pi}{2},\quad 0 \le \varphi < 2\pi</math>.

If the vectors {{math|'''f'''<sub>1</sub>, '''f'''<sub>2</sub>, '''f'''<sub>3</sub>}} form an orthogonal system, the six points with vectors {{math|'''f'''<sub>0</sub> ± '''f'''<sub>1,2,3</sub>}} are the vertices of the ellipsoid and {{math|{{abs|'''f'''<sub>1</sub>}}, {{abs|'''f'''<sub>2</sub>}}, {{abs|'''f'''<sub>3</sub>}}}} are the semi-principal axes.

A surface normal vector at point {{math|'''x'''(''&theta;'', ''&phi;'')}} is
: <math>\mathbf n(\theta, \varphi) = \mathbf f_2 \times \mathbf f_3\cos\theta\cos\varphi + \mathbf f_3 \times \mathbf f_1\cos\theta\sin\varphi + \mathbf f_1 \times \mathbf f_2\sin\theta.</math>

For any ellipsoid there exists an [[Implicit surface|implicit representation]] {{math|''F''(''x'', ''y'', ''z'') {{=}} 0}}. If for simplicity the center of the ellipsoid is the origin, {{math|'''f'''<sub>0</sub> {{=}} '''0'''}}, the following equation describes the ellipsoid above:<ref>[http://www.mathematik.tu-darmstadt.de/~ehartmann/cdg-skript-1998.pdf ''Computerunterstützte Darstellende und Konstruktive Geometrie.''] {{webarchive |url=https://web.archive.org/web/20131110190049/http://www.mathematik.tu-darmstadt.de/~ehartmann/cdg-skript-1998.pdf |date=2013-11-10}} Uni Darmstadt (PDF; 3,4&nbsp;MB), S.&nbsp;88.</ref>
: <math>F(x, y, z) = \operatorname{det}\left(\mathbf x, \mathbf f_2, \mathbf f_3\right)^2 + \operatorname{det}\left(\mathbf f_1,\mathbf x, \mathbf f_3\right)^2 + \operatorname{det}\left(\mathbf f_1, \mathbf f_2, \mathbf x\right)^2 - \operatorname{det}\left(\mathbf f_1, \mathbf f_2, \mathbf f_3\right)^2 = 0</math>

==Applications==
The ellipsoidal shape finds many practical applications:

;[[Geodesy]]
* [[Earth ellipsoid]], a mathematical figure approximating the shape of the [[Earth]].
* [[Reference ellipsoid]], a mathematical figure approximating the shape of [[planetary body|planetary bodies]] in general.

;[[Mechanics]]
* [[Poinsot's ellipsoid]], a geometrical method for visualizing the [[Torque-free precession|torque-free motion]] of a rotating [[rigid body]].
* [[Lamé's stress ellipsoid]], an alternative to [[Mohr's circle]] for the graphical representation of the [[Stress (mechanics)|stress]] state at a point.
* [[Manipulability ellipsoid]], used to describe a robot's freedom of motion.
* [[Jacobi ellipsoid]], a triaxial ellipsoid formed by a rotating fluid

;[[Crystallography]]
* [[Index ellipsoid]], a diagram of an ellipsoid that depicts the orientation and relative magnitude of [[Refractive index|refractive indices]] in a [[crystal]].
* [[Thermal ellipsoid]], ellipsoids used in crystallography to indicate the magnitudes and directions of the [[thermal vibration]] of atoms in [[crystal structure]]s.

=== Computer science ===

* [[Ellipsoid method]], a [[convex optimization]] algorithm of theoretical significance
;Lighting
* [[Ellipsoidal reflector floodlight]]
* [[Ellipsoidal reflector spotlight]]

;Medicine
* Measurements obtained from [[MRI]] imaging of the [[prostate]] can be used to determine the volume of the gland using the approximation {{math|''L'' × ''W'' × ''H'' × 0.52}} (where 0.52 is an approximation for {{sfrac|{{math|π}}|6}})<ref>{{cite journal |last1=Bezinque |first1=Adam |display-authors=etal |title=Determination of Prostate Volume: A Comparison of Contemporary Methods |journal=Academic Radiology |volume=25 |issue=12 |pages=1582–1587 |doi=10.1016/j.acra.2018.03.014 |pmid=29609953 |year=2018|s2cid=4621745 }}</ref>

===Dynamical properties===
The [[mass]] of an ellipsoid of uniform [[density]] {{mvar|ρ}} is
:<math>m = V \rho = \tfrac{4}{3} \pi abc \rho.</math>

The [[Moment of Inertia|moments of inertia]] of an ellipsoid of uniform density are
:<math>\begin{align}
  I_\mathrm{xx} &= \tfrac{1}{5}m\left(b^2 + c^2\right), &
    I_\mathrm{yy} &= \tfrac{1}{5}m\left(c^2 + a^2\right), &
    I_\mathrm{zz} &= \tfrac{1}{5}m\left(a^2 + b^2\right), \\[3pt]
  I_\mathrm{xy} &=  I_\mathrm{yz} = I_\mathrm{zx} = 0.
\end{align}</math>

For {{math|1=''a'' = ''b'' = ''c''}} these moments of inertia reduce to those for a sphere of uniform density.

[[File:2003EL61art.jpg|right|thumb|Artist's conception of {{dp|Haumea}}, a Jacobi-ellipsoid [[dwarf planet]], with its two moons]]
Ellipsoids and [[cuboid]]s rotate stably along their major or minor axes, but not along their median axis. This can be seen experimentally by throwing an eraser with some spin. In addition, [[moment of inertia]] considerations mean that rotation along the major axis is more easily perturbed than rotation along the minor axis.<ref>Goldstein, H G (1980). ''Classical Mechanics'', (2nd edition) Chapter 5.</ref>

One practical effect of this is that scalene astronomical bodies such as {{dp|Haumea}} generally rotate along their minor axes (as does Earth, which is merely [[oblate spheroid|oblate]]); in addition, because of [[tidal locking]], moons in [[synchronous orbit]] such as [[Mimas (moon)|Mimas]] orbit with their major axis aligned radially to their planet.

A spinning body of homogeneous self-gravitating fluid will assume the form of either a [[Maclaurin spheroid]] (oblate spheroid) or [[Jacobi ellipsoid]] (scalene ellipsoid) when in [[hydrostatic equilibrium]], and for moderate rates of rotation.  At faster rotations, non-ellipsoidal [[:wikt:pyriform|piriform]] or [[oviform]] shapes can be expected, but these are not stable.

===Fluid dynamics===
The ellipsoid is the most general shape for which it has been possible to calculate the [[creeping flow]] of fluid around the solid shape. The calculations include the force required to translate through a fluid and to rotate within it. Applications include determining the size and shape of large molecules, the sinking rate of small particles, and the swimming abilities of [[microorganisms]].<ref>Dusenbery, David B. (2009).''Living at Micro Scale'', Harvard University Press, Cambridge, Massachusetts {{isbn|978-0-674-03116-6}}.</ref>

===In probability and statistics===
The [[elliptical distribution]]s, which generalize the [[multivariate normal distribution]] and are used in [[finance]], can be defined in terms of their [[density function]]s. When they exist, the density functions {{mvar|f}} have the structure:
:<math>f(x) = k \cdot g\left((\mathbf x - \boldsymbol\mu)\boldsymbol\Sigma^{-1}(\mathbf x - \boldsymbol\mu)^\mathsf{T}\right)</math>

where {{mvar|k}} is a scale factor, {{math|'''x'''}} is an {{mvar|n}}-dimensional [[random vector|random row vector]] with median vector {{math|'''&mu;'''}} (which is also the mean vector if the latter exists), {{math|'''&Sigma;'''}} is a [[positive definite matrix]] which is proportional to the [[covariance matrix]] if the latter exists, and {{mvar|g}} is a function mapping from the non-negative reals to the non-negative reals giving a finite area under the curve.<ref>Frahm, G., Junker, M., & Szimayer, A. (2003). Elliptical copulas: applicability and limitations. Statistics & Probability Letters, 63(3), 275–286.</ref> The multivariate normal distribution is the special case in which {{math|''g''(''z'') {{=}} exp(−{{sfrac|''z''|2}})}} for quadratic form {{mvar|z}}.

Thus the density function is a scalar-to-scalar transformation of a quadric expression. Moreover, the equation for any [[contour line|iso-density surface]] states that the quadric expression equals some constant specific to that value of the density, and the iso-density surface is an ellipsoid.

== See also ==
* [[Ellipsoidal dome]]
* [[Ellipsoidal coordinates]]
* [[Elliptical distribution]], in statistics
* [[Flattening]], also called ''[[ellipticity]]'' and ''[[oblateness]]'', is a measure of the compression of a circle or sphere along a diameter to form an ellipse or an ellipsoid of revolution (spheroid), respectively.
* [[Focaloid]], a shell bounded by two concentric, confocal ellipsoids
* [[Geodesics on an ellipsoid]]
* [[Geodetic datum]], the gravitational Earth modeled by a best-fitted ellipsoid
* [[Homoeoid]], a shell bounded by two concentric similar ellipsoids
* [[John ellipsoid]], the smallest ellipsoid containing a given convex set.
* [[List of surfaces]]
* [[Superellipsoid]]

== Notes ==
<references />

== References ==
* {{citation |last1=Kreyszig |first1=Erwin |author-link=Erwin Kreyszig |title=Advanced Engineering Mathematics |publisher=[[John Wiley & Sons|Wiley]] |location=New York |edition=3rd |year=1972 |isbn=0-471-50728-8 |url-access=registration |url=https://archive.org/details/advancedengineer00krey}}

== External links ==
{{Commons category|Ellipsoids}}
* "[http://demonstrations.wolfram.com/Ellipsoid/ Ellipsoid]" by Jeff Bryant, [[Wolfram Demonstrations Project]], 2007.
* [http://mathworld.wolfram.com/Ellipsoid.html Ellipsoid] and [http://mathworld.wolfram.com/QuadraticSurface.html Quadratic Surface], [[MathWorld]].

[[Category:Ellipsoids|*]]

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