A phenomenon encountered in interferometry that is elicited by rotation: a split beam follows a ring interferometer in opposite directions, and rotation produces a measurable difference on recombination.
It underlies ring-laser and fibre-optic gyroscopes; ring lasers suffer 'lock-in' at rotation rates below about 100°/h.
GPS signal processing must correct for the Sagnac effect; a 1984 verification used three ground stations and several satellites with eastward and westward relays.