Precision Pendulum Clocks, Gravity and Tides

16-Nov-2009, updated 08-Dec-2021

Below is a series of articles I wrote while exploring the relationship between pendulum clocks and gravity, specifically what happens to the timekeeping performance of high-precision pendulum clocks as gravity changes due to tides. We all know about ocean tides and how they are somehow caused by the motion of the earth, sun, and moon. We also all know that pendulum clocks swing back and forth based on the length of the pendulum rod and earth's acceleration of gravity.

But it is less well known, but perhaps now obvious, that the period of a pendulum clock varies ever so slightly as the earth's acceleration of gravity itself changes due to the periodic influence of lunar/solar tides. This effect is so small, however, that only the very best, laboratory-grade, vacuum-sealed pendulum clocks can "see" the effect. All other mechanical clocks are too inaccurate for tides to make any practical difference.

So in the long history of precision pendulum clocks, "seeing tides" has become a sort of prize. If your home-made precision pendulum clock can detect tides, it must be exceedingly accurate. This series of articles explores topics of tides, gravity, pendulum clocks, accuracy, stability, comparisons of the very best pendulum clocks ever made (such as Shortt, Fedchenko, Littlemore), and a search for why some pendulum clocks that should have detected tides didn't.

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