We start with a given circle. 1.  Place the right-angle corner of any object at any point on the circle. Any point will do. 2.  Make a mark where the two sides of the right-angle cross the circle. 3.  Draw a line between these two marks. Because of Thales Theorem, this is a diameter of the circle. 4.  Place the right-angle corner of the object at any other point on the circle. Any point will do, but for greatest accuracy, make it about a quarter the way round the circle from the first point. 5.  Make a mark where the two sides of the right-angle cross the circle. 6.  Connect these two points with a straight line. This is the second diameter. 7.  Done. The point where the two diameters intersect is the center of the circle. ## Why it works This method works as a result of Thales Theorem. The diameter of a circle subtends a right angle to any point on the circle. The converse is also true: A right angle on the circle must cut off a diameter. By finding two diameters, we find the center where they intersect

Visit Thales Theorem for an animated description of how this works.

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Nice right up Ted.  I can put this to use.
I knew I should have paid more attention in Geometry class!