Re. circle around an ellipse

Hi! All,

I thing I had better explain the probblem more fully,

The centre of an ellipse is at (0,0) with it's major axis along the X axis, let the semi major axis be "a". long. At a point "o" along the positive X axis at say "2a" distance from (0,0) is the centre of a circle drawn tangent to the outside of the ellipse, this line along the positive X axis joining the two centres,is rotated in a counter clockwise direction carring with it the circle, but the circle must remain tangent to the outside of the ellipse. obviously to do this the distance between the two centres must be continuously changing, So herin is the problem, to find this length at any given rotation of the line joinging the two centres??

Regards Brian