# Eccentricity (mathematics)

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All types of conic sections, arranged with increasing eccentricity. Note that curvature decreases with eccentricity, and that none of these curves intersect.
Ellipses, hyperbolas with all possible eccentricites from zero to infinity and a parabola on one cubic surface.

In mathematics, the eccentricity, denoted e or $\varepsilon$, is a parameter associated with every conic section. It can be thought of as a measure of how much the conic section deviates from being circular.

In particular,

• The eccentricity of a circle is zero.
• The eccentricity of an ellipse which is not a circle is greater than zero but less than 1.
• The eccentricity of a parabola is 1.
• The eccentricity of a hyperbola is greater than 1.

Furthermore, two conic sections are similar if and only if they have the same eccentricity.

## Definitions

Any conic section can be defined as the locus of points whose distances to a point (the focus) and a line (the directrix) are in a constant ratio. That ratio is called eccentricity, commonly denoted as "e."

The eccentricity can also be defined in terms of the intersection of a plane and a double-napped cone associated with the conic section. If the cone is oriented with its axis being vertical, the eccentricity is

$e=\frac{\sin \alpha}{\sin \beta}$

where α is the angle between the plane and the horizontal and β is the angle between the cone and the horizontal.

The linear eccentricity of a conic section, denoted c (or sometimes f or e), is the distance between its center and either of its two foci. The eccentricity can be defined as the ratio of the linear eccentricity to the semimajor axis a: that is, $e = \frac{c}{a}$.

## Alternative names

The eccentricity is sometimes called first eccentricity to distinguish it from the second eccentricity and third eccentricity defined for ellipses (see below). The eccentricity is also sometimes called numerical eccentricity.

In the case of ellipses and hyperbolas the linear eccentricity is sometimes called half-focal separation.

## Notation

Three notational conventions are in common use:

1. e for the eccentricity and c for the linear eccentricity.
2. $\varepsilon$ for the eccentricity and e for the linear eccentricity.
3. e or $\epsilon$ for the eccentricity and f for the linear eccentricity (mnemonic for half-focal separation).

## Values

conic section equation eccentricity (e) linear eccentricity (c)
circle $x^2+y^2=r^2$ $0$ $0$
ellipse $\frac{x^2}{a^2}+\frac{y^2}{b^2}=1$ $\sqrt{1-\frac{b^2}{a^2}}$ $\sqrt{a^2-b^2}$
parabola $y^2=4ax$ $1$ $a$
hyperbola $\frac{x^2}{a^2}-\frac{y^2}{b^2}=1$ $\sqrt{1+\frac{b^2}{a^2}}$ $\sqrt{a^2+b^2}$

where, when applicable, a is the length of the semi-major axis and b is the length of the semi-minor axis.

When the conic section is given in the general quadratic form

$Ax^2 + Bxy + Cy^2 +Dx + Ey + F = 0, \,$

the following formula gives the eccentricity e if the conic section is not a parabola (which has eccentricity equal to 1), not a degenerate hyperbola or degenerate ellipse, and not an imaginary ellipse:[1]

$e=\sqrt{\frac{2\sqrt{(A-C)^2 + B^2}}{\eta (A+C) + \sqrt{(A-C)^2 + B^2}}}$

where $\eta$= 1 if the determinant of the 3×3 matrix

$\begin{bmatrix}A & B/2 & D/2\\B/2 & C & E/2\\D/2&E/2&F\end{bmatrix}$

is negative or $\eta$= -1 if that determinant is positive.

Ellipse and hyperbola with constant a and changing eccentricity e.

## Ellipses

The eccentricity of an ellipse is strictly less than 1. When circles are counted as ellipses, the eccentricity of an ellipse is greater than or equal to 0; if circles are given a special category and are excluded from the category of ellipses, then the eccentricity of an ellipse is strictly greater than 0.

For any ellipse, let a be the length of its semi-major axis and b be the length of its semi-minor axis.

We define a number of related additional concepts (only for ellipses):

name symbol in terms of a and b in terms of e
first eccentricity   $e\,$   $\sqrt{1-\frac{b^2}{a^2}}$   $e\,$
second eccentricity   $e'\,$   $\sqrt{\frac{a^2}{b^2}-1}$   $\frac{e}{\sqrt{1-e^2}}$
third eccentricity   $e''=\sqrt m$  $\frac{\sqrt{a^2-b^2}}{\sqrt{a^2+b^2}}$   $\frac{e}{\sqrt{2-e^2}}$
angular eccentricity   $\alpha\,\!$   $\cos^{-1}\left(\frac{b}{a}\right)$   $\sin^{-1} e\,\!$

### Other formulas for the eccentricity of an ellipse

The eccentricity of an ellipse is, most simply, the ratio of the distance between its two foci, to the length of the major axis.

The eccentricity is also the ratio of the semimajor axis a to the distance d from the center to the directrix:

$e = \frac{a}{d}.$

The eccentricity can be expressed in terms of the flattening factor g (defined as g = 1 – b/a for semimajor axis a and semiminor axis b):

$e = \sqrt{g(2-g)}.$

Comment: flattening is denoted by f in some subject areas, particularly geodesy.

Define the maximum and minimum radii $r_{max}$ and $r_{min}$ as the maximum and minimum distances from either focus to the ellipse (that is, the distances from either focus to the two ends of the major axis). Then with semimajor axis a, the eccentricity is given by

$e = \frac{r_{max}-r_{min}}{r_{max}+r_{min}} = \frac{r_{max}-r_{min}}{2a}.$

## Hyperbolas

The eccentricity of a hyperbola can be any real number greater than 1, with no upper bound. The eccentricity of a rectangular hyperbola is $\sqrt{2}$.

## Quadrics

The eccentricity of a three-dimensional quadric is the eccentricity of a designated section of it. For example, on a triaxial ellipsoid, the meridional eccentricity is that of the ellipse formed by a section containing both the longest and the shortest axes (one of which will be the polar axis), and the equatorial eccentricity is the eccentricity of the ellipse formed by a section through the centre, perpendicular to the polar axis (i.e. in the equatorial plane).

## Celestial mechanics

In celestial mechanics, for bound orbits in a spherical potential, the definition above is informally generalized. When the apocenter distance is close to the pericenter distance, the orbit is said to have low eccentricity; when they are very different, the orbit is said be eccentric or having eccentricity near unity. This definition coincides with the mathematical definition of eccentricity for ellipses, in Keplerian, i.e., $1/r$ potentials.

## Analogous classifications

A number of classifications in mathematics use derived terminology from the classification of conic sections by eccentricity:

• Classification of elements of SL2(R) as elliptic, parabolic, and hyperbolic – and similarly for classification of elements of PSL2(R), the real Möbius transformations.
• Classification of discrete distributions by variance-to-mean ratio; see cumulants of some discrete probability distributions for details.

## References

1. Ayoub, Ayoub B., "The eccentricity of a conic section," 34(2), March 2003, 116-121.

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