Sagnac effect (Sagnac interference) is well described by the author Anderson. There is the original paper translated into English. The generally accepted interpretation is based on different trails (The difference in the length of roads caused by peripheral speed). Interference fringes are evenly lined with dark stripes, move with the changing angular velocity. Theoretical calculations given significant deviations from the experimental results. Experimental data suggest: Sensitivity (accuracy) of the device is proportional to the square of the length of optical fiber. The official interpretations (and others) argue: Sensitivity is proportional to the length of the optical fiber. Exposed is a mathematical analysis of the experimental data.