Derivation of the General Case Sagnac Result using Non-time-orthogonal   Analysis
  The experimentally determined Sagnac fringe shift dependency on angular velocity and enclosed area is derived from the rotating reference frame using non-time-orthogonal tensor analysis. The relationship for the most general case, in which the area enclosed is not circular and does not have the axis of rotation passing through its center, is determined. It is submitted that this quantitative result, along with a related thought experiment, can not be found using the conventional approach of local co-moving Lorentz frames. 
Robert D. Klauber