Butterflies in Ellipse: What is it about?
A Mathematical Droodle

27 October 2016, Created with GeoGebra


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Copyright © 1996-2017 Alexander Bogomolny

Butterflies in Ellipse

The applet below provides another perspective on the Two Butterflies theorem and the porism observed by Nathan Bowler. The theorem states this: given a cyclic quadrilateral inscribed in a circle, a line (not necessarily crossing the circle) and four points on the line in which the sides of the quadrilateral cross the line. For any other quadrilateral inscribed in the circle, if three of its sides pass through any three (of the above four) points on the line, then the fourth side passes through the fourth point. Number 4 in the statement can be replaced with any even number.

Furthermore, since the theorem only deals with incidences of points and line, it is of projective character and must hold for any non-degenerate conic. The applet below illustrates the theorem for ellipses.

We want two prove the following theorem:

Let there be two quadrilaterals ABCD and A'B'C'D' inscribed into a conic such that the sides DA, AB, BC, CD of the former pass through collinear points Fda, Fab, Fbc, Fcd. If the sides D'A', A'B', B'C' meet the same line L in Fda, Fab, Fbc then the fourth sides C'D' meets L in Fcd.

pascal in ellipse


Consider the cyclic hexagon ABCA'B'C'. Its opposite sides AB and A'B' meet in Fab and the sides BC and B'C' meet in Fbc. By Pascal's theorem the third pair of lines, AC and A'C' meet in a point P collinear with Fab and Fbc, i.e. lying on line L.

Now, consider the hexagon ADCA'D'C'. The sides AC and A'C' meet in P on L; the sides CD and C'D' meet in Fcd on L. Therefore, the sides AD and A'D' meet in a point on L. But AD passes through Fda on L. Hence, so does A'D' and we are done.

Given its projective character, it is not surprising that the theorem follows from Pascal's theorem. Perhaps surprisingly, the reverse is also true: we may derive Pascal's theorem from the statement of quadrilaterals.

Let the hexagon ABCDEF be inscribed in a conic, let AB meet DE in P, BC meet EF in Q, CD meet PQ in R and let AD meet PQ in T. Observe two quadrilaterals ABCD and DEFA. Their sides meet in pairs: AB and DE in P, BC and EF in Q, and their common side AD passes through T. By the definition of T, the three points P, Q, T are collinear. By the theorem just proved, line AF passes through the intersection of CD and PQ, i.e., point R. This proves Pascal's theorem.

The proof that the porism holds for a 2k-gon, k ≥ 2 is by induction, along the same lines as above.


  1. D. Jones, Quadrangles, Butterflies, Pascal's Hexagon, and Projective Fixed Points, Amer Math Monthly 87, no 3 (Mar., 1980), 197-200

Butterfly Theorem and Variants

  1. Butterfly theorem
  2. 2N-Wing Butterfly Theorem
  3. Better Butterfly Theorem
  4. Butterflies in Ellipse
  5. Butterflies in Hyperbola
  6. Butterflies in Quadrilaterals and Elsewhere
  7. Pinning Butterfly on Radical Axes
  8. Shearing Butterflies in Quadrilaterals
  9. The Plain Butterfly Theorem
  10. Two Butterflies Theorem
  11. Two Butterflies Theorem II
  12. Two Butterflies Theorem III
  13. Algebraic proof of the theorem of butterflies in quadrilaterals
  14. William Wallace's Proof of the Butterfly Theorem
  15. Butterfly theorem, a Projective Proof
  16. Areal Butterflies
  17. Butterflies in Similar Co-axial Conics
  18. Butterfly Trigonometry
  19. Butterfly in Kite
  20. Butterfly with Menelaus
  21. William Wallace's 1803 Statement of the Butterfly Theorem
  22. Butterfly in Inscriptible Quadrilateral
  23. Camouflaged Butterfly
  24. General Butterfly in Pictures
  25. Butterfly via Ceva
  26. Butterfly via the Scale Factor of the Wings
  27. Butterfly by Midline
  28. Stathis Koutras' Butterfly
  29. The Lepidoptera of the Circles
  30. The Lepidoptera of the Quadrilateral
  31. The Lepidoptera of the Quadrilateral II
  32. The Lepidoptera of the Triangle
  33. Two Butterflies Theorem as a Porism of Cyclic Quadrilaterals
  34. Two Butterfly Theorems by Sidney Kung

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Copyright © 1996-2017 Alexander Bogomolny


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