Coordinates of the center and the radius of the circle on the image What is the simplest possible way to calculate the coordinates of the center and the radius of the circle on the image (for example BMP and I'm interested in image of the Sun included below)?
If you want to automate the process I would use Hough Circle Transform.  It is an algorithm used to detect circles in an image. This Wiki article will describe Hough Transform better than I can.  

You can manually find the center of a circle using these methods.