Sagnac, gclock effect and gravitomagnetism

New techniques to evaluate the clock effect are described. They are based on the flatness of the cylindrical surface containing the world lines of the rays constrained to move on circular trajectories about a spinning mass. The effect of the angular momentum of the source is manifested in the fact that inertial observers must be replaced by local non rotating observers. Starting from this, exact formulas for circular trajectories are found. Numerical estimates for the Earth environment show that light would be a better probe than actual clocks to evidence the angular momentum influence. The advantages of light in connection with some principle experiments are shortly reviewed and a couple of specific solutions are suggested.