We start with a given circle. |
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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

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