angle from centre point of an ellipse to an orbiting body.

gistfile1.txt

private float GetStartArcAngle()
{
    _focalDistance = (float)_orbitDB.Eccentricity * _orbitElipseWidth /2;
    //since the _focalDistance is only an X component we dont' bother calculating the Y part of the matrix
    double focalX = (_focalDistance * Math.Cos(_rotation * Math.PI / 180)); // - 0 * Math.Sin(_rotation * Math.PI / 180)); 
    double focalY = (_focalDistance * Math.Sin(_rotation * Math.PI / 180)); // + 0 * Math.Cos(_rotation * Math.PI / 180));
    //addt the body posistion
    Vector4 offsetPoint = new Vector4(focalX, focalY, 0, 0) + _bodyPositionDB.RelativePosition;
    //find the angle to the offset point
    float angle = (float)(Math.Atan2(offsetPoint.Y, offsetPoint.X) * 180 / Math.PI);
    //subtract the _rotation, since this angle needs to be ralitive to the elipse, and the elipse gets _rotated
    angle -= _rotation;
    //and finaly, normalise it useing modulo arrithmatic.
    angle = angle % 360;
    if (angle < 0)
        angle += 360;
    return angle;
}