I have an image of a circle, I want to find the circle but not using hough circles.

I found a way, linked [here](https://math.stackexchange.com/questions/675203/calculating-centre-of-rotation-given-point-coordinates-at-different-positions/1414344#1414344). 

But I can't find the transition coordinates from white to black as I don't know the x and y coordinates in the circle. What other methods are there, or how can I make that approach work?

This is my test image:

[![enter image description here](https://i.stack.imgur.com/u62F7.jpg)](https://i.stack.imgur.com/u62F7.jpg)