2. Constructing a Point on a Cassini Oval

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This Demonstration shows another ruler-and-compass construction of a point on a Cassini oval.


Let and be two fixed points (the foci) a distance apart. A Cassini oval (or Cassini ellipse) is a quartic curve traced by a point such that the product of the distances is a constant .

Let and let be the circle with center and radius . Let be a point on and let be the midpoint of . Let be the orthogonal projection of on the perpendicular bisector of . Let be the circle with center and radius ; let be the circle with center and radius . Let be the intersection of the circles and . Let be the angle between and .

Then , . The difference of these two equations gives , so that satisfies the defining condition for the oval.


Contributed by: Marko Razpet and Izidor Hafner (April 2018)
Open content licensed under CC BY-NC-SA



This construction is described in [1].


[1] M. Razpet, "Ellipse and Cassini Oval" (in Slovenian), Presek, 36(1), 2008–2009, pp. 14–15.

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