Warren S> Somebody in England actually did build what he claimed was the world's most accurate pendulum clock, using pendulum swinging low-amplitude in vacuum, photosensor "escapement" with electromagnet kicker, old-invar rod. Whole thing built inside a special hut with heavy concrete foundation, and active temperature regulation using "box inside a box" 2-level temperature control. Pointless exercise, really, but he enjoyed it and offered evidence from comparison with atomic clocks he'd indeed achieved greater accuracy than any previous pendulum clock, and also he was able using it to detect the motion of the moon, which might make a good sci-fi story -- you know, the alien race trapped on planet shrouded in perpetual fog, nevertheless detects the existence of moon via pendulum clocks. [...] ------- Without an atomic clock monitoring the pendulum clock? At any rate, I visited a lab in the 1960s testing a gyro on a concrete/steel pillar extending down to bedrock. It clearly recorded the tides. --rwg