We start with a given circle.


1. Place the rightangle corner of any object at any point on the circle. Any point will do. 

2. Make a mark where the two sides of the rightangle 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 rightangle 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 rightangle 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. 
