Soddy Circles and David Eppstein's Centers: What Are They?
A Mathematical Droodle

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Soddy Circles and David Eppstein's Centers

The applet suggests that for three circles externally tangent to each other, there exist two circles that touch all circles in the triplet. One of the circles touches the triplet internally, the other touches all of them externally. The circles bear the name of Frederick Soddy (1877-1956), a distinguished chemist and economist, a 1921 Nobel prize winner, who made the circles famous by publishing the result along with a poem "The kiss precise" in Nature, 137 (1936), p. 1021. The circles are correspondingly called the outer and the inner Soddy circles. Their centers are also known as the Soddy points of the triangle formed by the centers of the triplet.

(This is a particular case of the more general Apollonius' problem that asks for a circle tangent to the given three circles (or lines, or passing through given points) in an arbitrary configuration relative to each other.)

The formula that links the radii of four circles that touch each other externally was known to Descartes [Pedoe, p. 158]. It is given in terms of the curvatures, bends in Soddy's terminology. For a circle, the curvature is the reciprocal of its radius, and Descartes' formula appears as ([Pedoe, p. 157, Coxeter, 1.57])

  2(ε12 + ε22 + ε32 + ε42) = (ε1 + ε2 + ε3 + ε4)2,

where εi = 1/Ri is the curvature of the i-th circle (Ri being its radius), i = 1, 2, 3, 4.

If one of the curvatures is taken with the sign "-", then the formula corresponds to the case when one of the circles touches the rest internally, i.e. when one of them -- the outer Soddy circle -- encloses the other three.

In 1826, the famous geometer Jacob Steiner rediscovered the formula. In 1842, it was proven independently by Philip Beecroft, an English amateur mathematician. Frederick Soddy's contribution appeared in 1936.

Thus, the Soddy circles have a long history, in the course of which they were under close attention of several exceptional people. This is then so much more remarkable that one interesting property of the configuration has been discovered as late as 2001. In 2001, D. Eppstein, of the Geometry Junkyard fame, published the following observation (see also a partial online version):

(1) Four touching circles, when taken two by two, define two points of tangency and, therefore, a straight line. There are three such lines. The three lines are concurrent.

This is true for both inner and outer configuration, so there are two points of note, known now as Eppstein's points.

Eppstein derives his result as a particular case of a 3D configuration of four spheres. Any four mutually tangent spheres determine six points of tangency. The six points are naturally divided into three pairs of opposite tangencies, i.e. tangencies, in which one is defined by two spheres distinct from the pair of the spheres defining the other.


[Altshiller-Court, p. 231].

  The three lines through the opposite points of tangency of any four mutually tangent spheres in R3 are concurrent.


Let the four given spheres Si, i = 1, 2, 3, 4, have centers Oi and radii Ri. Let εi = ±1/Ri, where the sign minus is taken if Si contains the other three spheres, and "+" otherwise. Then the point of tangency tij between Si and Sj can be expressed as

  tij = (εiOi + εjOj)/(εi + εj).

This is a weighted average of the centers of the two spheres. A similar average for the centers of all four spheres

(2) M = (ε1O1 + ε2O2 + ε3O3 + ε4O4)/(ε1 + ε2 + ε3 + ε4)

could also be represented as

  M = ((ε1O1 + ε2O2)t12 + (ε3O3 + ε4O4)t34)/(ε1 + ε2 + ε3 + ε4),

which means that M lies on the line joining the opposite tangencies t12 and t34. By the same argument, it lies on the other two lines.

(1) follows from Lemma when the centers of the four spheres happen to be coplanar.

If one starts with a triangle ABC, then there exists a unique triplet CA, CB, CC of pairwise tangent circles centered at the vertices of the triangle. Let a, b, c be the side lengths of ΔABC (a = BC, etc.) Then the radii of the circles CA, CB, CC are given by

  RA = s - a, RB = s - b, RC = s - c,

where s is the semiperimeter of ΔABC, s = (a + b + c)/2. The points of tangency tAB, tAC, and tBC are exactly the points where the incircle of ΔABC touches its sides. The three lines AtBC, BtAC, and CtAB are concurrent at the Gergonne point Ge.

(3) Ge = (εAA + εBB + εCC)/(εA + εB + εC)

because, for example,

 AA + εBB + εCC)/(εA + εB + εC) = (εAA + (εB + εC)tBC)/(εA + εB + εC)

so that the point in (2) lies on the line joining A and tBC, and similarly for the other two lines. (2) suggests that a relationship between M and Ge should not be unexpected. Indeed, among other things, Eppstein proves the following


  Let S be the center of the inner Soddy circle. Then S, M, and Ge are collinear.


The proof follows by combining (2) and (3):

M = (εAA + εBB + εCC + εSS)/(εA + εB + εC + εS)
  = ((εA + εB + εC)Ge + εSS)/(εA + εB + εC + εS).

It's known that S, Ge, and I, the incenter of ΔABC are collinear. M therefore lies on the same line. By symmetry, the same is true of the outer Eppstein point.


  1. N. Altshiller-Court, Modern Pure Solid Geometry, Chelsea, 2nd ed, 1964
  2. H. S. M. Coxeter, Introduction to Geometry, John Wiley & Sons, 1961
  3. H. S. M. Coxeter, The Problem of Apollonius, Am Math Monthly, 75 (1968), pp. 5-15
  4. D. Eppstein, Tangent Spheres and Triangle Centers, Amer Math Monthly 108 (2001), pp. 63-66
  5. A. Oldknow, The Euler-Gergonne-Soddy triangle of a triangle, Amer Math Monthly 103 (1996), pp. 319-329
  6. D. Pedoe, Geometry: A Comprehensive Course, Dover, 1970

2D Problems That Benefit from a 3D Outlook

  1. Four Travellers, Solution
  2. Desargues' Theorem
  3. Soddy Circles and Eppstein's Points
  4. Symmetries in a Triangle
  5. Three Circles and Common Chords
  6. Three Circles and Common Tangents
  7. Three Equal Circles
  8. Menelaus from 3D
  9. Stereographic Projection and Inversion
  10. Stereographic Projection and Radical Axes
  11. Sum of Squares in Equilateral Triangle

Desargues' Theorem

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Copyright © 1996-2018 Alexander Bogomolny


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