The Sagnac effect pertains to the relative phase shift between two beams of light that travel on an identical path in opposite directions within a rotating frame. Modern fiber-optic gyroscopes (Sagnac interferometers) used for navigation are based on this effect, allowing highly accurate measurements of rotation rates down to about 10−4−10−5 degrees per hour. Georges Sagnac (1869–1926) was the first to perform a ring interferometry experiment in 1913 aimed at observing the correlation of angular velocity and optical phase-shift. (An experiment conducted in 1911 by Francis Harress, attempting to measure the Fresnel drag of light propagating through rotating glass, was later recognized as actually constituting a Sagnac experiment; Harress had ascribed the observed “unexpected bias” to some other factor.) An ambitious ring interferometry experiment was set up by Albert Michelson and Henry Gale in 1926 to determine whether the Earth's rotation has an effect on the propagation of light in its vicinity. The Michelson–Gale interferometer with a 1.9 km perimeter was large enough to detect the rotation of the Earth, confirming its known value of angular velocity (obtained from astronomical observations). The Michelson–Gale ring interferometer was not calibrated by comparison with an outside reference, an impossible task given that the setup was fixed to the Earth.

Figure 14.1 shows the general design of a triangular Sagnac interferometer consisting of a light source, a beam-splitter S, mirrors M1, M2, and an observation plane, mounted on a base that rotates at a constant angular velocity Ω around a fixed axis.