Osculating circle

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at a given point $M$ of a curve $l$

The circle that has contact of order $n\geq2$ with $l$ at $M$ (see Osculation). If the curvature of $l$ at $M$ is zero, then the osculating circle degenerates into a straight line. The radius of the osculating circle is called the radius of curvature of $l$ at $M$, and its centre the centre of curvature (see Fig.). If $l$ is the plane curve given by an equation $y=f(x)$, then the radius of the osculating circle is given by


Figure: o070540a

If $l$ is the spatial curve given by equations

$$x=x(u),\quad y=y(u),\quad z=z(u),$$

then the radius of the osculating circle is given by


(where the primes denote differentiation with respect to $u$).


[a1] R.S. Millman, G.D. Parker, "Elements of differential geometry" , Prentice-Hall (1977) pp. 39
[a2] D.J. Struik, "Lectures on classical differential geometry" , Dover, reprint (1988) pp. 14

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Osculating circle. Encyclopedia of Mathematics. URL:
This article was adapted from an original article by BSE-3 (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article