Eric,
Unfortunately, the image fails to show the top and bottom.
Are you sure that the blue and green outer boundaries form part of a/one and the same circle?
You can (also) establish the total height, then create the white circle with that W = H, move it behind, and Align it with the existing artwork; and if it fails to have its boudary flush with the blue/green parts, you can adapt the boundaries of the latter (or the boundaries of the white circle if you must).
Edit: Hi Ton. Some of the birds believe it is spring here, most other creatures are unconvinced.