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
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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