Abstract
Abstract. Tidal fluctuations in gravity will affect the period of a pendulum and hence the timekeeping of any such clock that uses one. Since pendulum clocks were, until the 1940s, the best timekeepers available, there has been interest in seeing if tidal effects could be observed in the best performing examples of these clocks. The first such observation was in 1929, before gravity tides were measured with spring gravimeters; at the time of the second (1940–1943), such gravimeters were still being developed. Subsequent observations, having been made after pendulum clocks had ceased to be the best available timekeepers and after reliable gravimeter measurements of tides, have been more of an indication of clock quality than a contribution to our knowledge of tides. This paper describes the different measurements and revisits them in terms of our current knowledge of Earth tides. Doing so shows that clock-based systems, though noisier than spring gravimeters, were an early form of an absolute gravimeter that could indeed observe Earth tides.
Highlights
The invention of the pendulum clock by Christiaan Huygens in 1657 tied precise time measurement and gravity together for almost the 3 centuries, until the development of quartz and atomic frequency standards
Did Huygens’ own research come from an investigation of how bodies fall (Yoder, 1989) and the first indication that gravitational acceleration differed from place to place on Earth came from the finding, by Jean Richer in 1672, that a pendulum beating seconds at Paris needed to be shortened by 0.3 % to do so at Cayenne, just north of the Equator (Olmsted, 1942)
The purpose of this note is to describe the few cases in which pendulum clocks have been used to measure temporal changes in gravity at a fixed location, the changes in the intensity of gravity g caused by the tidal effects created by the Moon and Sun (Agnew, 2015)
Summary
The invention of the pendulum clock by Christiaan Huygens in 1657 tied precise time measurement and gravity together for almost the 3 centuries, until the development of quartz and atomic frequency standards. He stated that while the long-period effects are the largest, their detection would require a level of long-term stability not seen in pendulum clocks and described the shorterterm changes as “within the limits of error of the most accurate time-measurements, but perhaps not so far within them as to be entirely devoid of interest”. Since the pendulum is a kind of falling-weight measurement, it is not surprising that it provides a measure of changes in g that can be tied back to standards of length and time This is not at all the case with spring gravimeters, the calibration of which was difficult until portable and highly precise free-fall absolute gravimeters were developed. A pendulum that can record the tides needs no calibration
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