Gravitomagnetism, clocks and geometry

New techniques to evaluate the clock effect using light are described. Thes e are based on the flatness of the cylindrical surface containing the world lines of the rays constrained to move on circular trajectories about a spi nning 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, an exact formula for circular trajectories i s found. Numerical estimates for the Earth environment show that light woul d be abetter probe than actual clocks to evidence the influence of angular momentum. The advantages of light in connection with some principle experim ents are briefly reviewed.