In the differential geometry of curves, the evolute of a curve is the locus of all its centers of curvature. That is to say that when the center of curvature of each point on a curve is drawn, the resultant shape will be the evolute of that curve. The evolute of a circle is therefore a single point at its center.[1] Equivalently, an evolute is the envelope of the normals to a curve.

The evolute of a curve (blue parabola) is the locus of all its centers of curvature (red).
The evolute of a curve (in this case, an ellipse) is the envelope of its normals.

The evolute of a curve, a surface, or more generally a submanifold, is the caustic of the normal map. Let M be a smooth, regular submanifold in Rn. For each point p in M and each vector v, based at p and normal to M, we associate the point p + v. This defines a Lagrangian map, called the normal map. The caustic of the normal map is the evolute of M.[2]

Evolutes are closely connected to involutes: A curve is the evolute of any of its involutes.


Apollonius (c. 200 BC) discussed evolutes in Book V of his Conics. However, Huygens is sometimes credited with being the first to study them (1673). Huygens formulated his theory of evolutes sometime around 1659 to help solve the problem of finding the tautochrone curve, which in turn helped him construct an isochronous pendulum. This was because the tautochrone curve is a cycloid, and the cycloid has the unique property that its evolute is also a cycloid. The theory of evolutes, in fact, allowed Huygens to achieve many results that would later be found using calculus.[3]

Evolute of a parametric curveEdit

If   is the parametric representation of a regular curve in the plane with its curvature nowhere 0 and   its curvature radius and   the unit normal pointing to the curvature center, then

describes the evolute of the given curve.

For   and   one gets


Properties of the evoluteEdit

The normal at point P is the tangent at the curvature center C.

In order to derive properties of a regular curve it is advantageous to use the arc length   of the given curve as its parameter, because of   and   (see Frenet–Serret formulas). Hence the tangent vector of the evolute   is:

From this equation one gets the following properties of the evolute:
  • At points with   the evolute is not regular. That means: at points with maximal or minimal curvature (vertices of the given curve) the evolute has cusps. (See the diagrams of the evolutes of the parabola, the ellipse, the cycloid and the nephroid.)
  • For any arc of the evolute that does not include a cusp, the length of the arc equals the difference between the radii of curvature at its endpoints. This fact leads to an easy proof of the Tait–Kneser theorem on nesting of osculating circles.[4]
  • The normals of the given curve at points of nonzero curvature are tangents to the evolute, and the normals of the curve at points of zero curvature are asymptotes to the evolute. Hence: the evolute is the envelope of the normals of the given curve.
  • At sections of the curve with   or   the curve is an involute of its evolute. (In the diagram: The blue parabola is an involute of the red semicubic parabola, which is actually the evolute of the blue parabola.)

Proof of the last property:
Let be   at the section of consideration. An involute of the evolute can be described as follows:

where   is a fixed string extension (see Involute of a parameterized curve ).
With   and   one gets
That means: For the string extension   the given curve is reproduced.
  • Parallel curves have the same evolute.

Proof: A parallel curve with distance   off the given curve has the parametric representation   and the radius of curvature   (see parallel curve). Hence the evolute of the parallel curve is



Evolute of a parabolaEdit

For the parabola with the parametric representation   one gets from the formulae above the equations:

which describes a semicubic parabola
Evolute (red) of an ellipse

Evolute of an ellipseEdit

For the ellipse with the parametric representation   one gets:[5]

These are the equations of a non symmetric astroid. Eliminating parameter   leads to the implicit representation
Cycloid (blue), its osculating circle (red) and evolute (green).

Evolute of a cycloidEdit

For the cycloid with the parametric representation   the evolute will be:[6]

which describes a transposed replica of itself.
The evolute of the large nephroid (blue) is the small nephroid (red).

Evolutes of some curvesEdit

The evolute

Radial curveEdit

A curve with a similar definition is the radial of a given curve. For each point on the curve take the vector from the point to the center of curvature and translate it so that it begins at the origin. Then the locus of points at the end of such vectors is called the radial of the curve. The equation for the radial is obtained by removing the x and y terms from the equation of the evolute. This produces



  1. ^ Weisstein, Eric W. "Circle Evolute". MathWorld.
  2. ^ Arnold, V. I.; Varchenko, A. N.; Gusein-Zade, S. M. (1985). The Classification of Critical Points, Caustics and Wave Fronts: Singularities of Differentiable Maps, Vol 1. Birkhäuser. ISBN 0-8176-3187-9.
  3. ^ Yoder, Joella G. (2004). Unrolling Time: Christiaan Huygens and the Mathematization of Nature. Cambridge University Press.
  4. ^ Ghys, Étienne; Tabachnikov, Sergei; Timorin, Vladlen (2013). "Osculating curves: around the Tait-Kneser theorem". The Mathematical Intelligencer. 35 (1): 61–66. arXiv:1207.5662. doi:10.1007/s00283-012-9336-6. MR 3041992.
  5. ^ R.Courant: Vorlesungen über Differential- und Integralrechnung. Band 1, Springer-Verlag, 1955, S. 268.
  6. ^ Weisstein, Eric W. "Cycloid Evolute". MathWorld.