The pendulum prediction
Ideal Foucault precession is Ω sin(latitude). It vanishes at the equator, reverses sign across it and has a sidereal full-turn period at the poles. A short pendulum can acquire release bias, elliptical motion and support effects that hide the slow signal.
Gyro orientation selects a component
A sensor’s axis determines the measured component of Earth’s angular velocity. Bias, temperature response and scale calibration must be separated from that component. Rotating or reversing the instrument provides a useful check because the Earth signal changes with orientation while some biases do not.
Laser rings compare two directions
Counter-propagating light in a rotating loop has a travel-time difference. Large ring-laser experiments resolve Earth rotation through this Sagnac response. Pendulum and gyro observations measure rotation directly; combine them with distance and gravity observations when considering surface shape.