[time-nuts] Gravity, solid Earth tides

[Steve Allen]

I have taken a much deeper dive through the history of geophysics
and upon coming up for air...

On Fri 2020-05-15T22:02:32-0700 Steve Allen hath writ:
> On Fri 2020-05-15T21:49:43-0700 Tom Van Baak hath writ:
> > Also, some of the very best pendulum clocks ever made were good enough that
> > their timekeeping was affected by lunar/solar tides. One is the English
> > Shortt-Synchronome and another is the Russian Fedchenko.
>
> Those are not measuring solid earth tides per se.  They are measuring
> the failure of the earth to deform as much as, and in phase with, the
> changing potential, so the local acceleration of gravity changes.
>
> I have not found any papers which say that two distant chronometers
> have yet been tied together with a stable enough optical network
> to measure the changing difference in their potentials.

Tom Van Baak is right, there is no question that pendulum clocks were
seeing the effects of solid earth tides by the 1940s.  They were
measuring (semi)diurnal changes in the local acceleration of gravity.
But clocks were not the first measurement of that effect, and not the
best.

The first measurements were from gravimeters which had been developed
and deployed to assist in explorations for oil.  See Truman, 1939
https://ui.adsabs.harvard.edu/abs/1939ApJ....89..445T/abstract
Before and after Truman there were other deep investigations of
(semi)diurnal variations in local gravity and their causes.
Those led to estimations of the Love numbers for the amplitude and
phase of the deformation of the solid earth in response to the
lunar and solar tides.

My reaction was prompted by the fact that all of those measurements
were struggling to distinguish the effects of ocean loading at sites
near a shoreline, deformations of the surface due to changes in the
weather, etc.  Getting better numbers needed tying together distant
sites with sufficient accuracy to see the differential ground motions.

I think it is true even now that two distant sites with atomic
chronometers have not yet been tied together with a sufficiently
stable optical fiber network to allow the clocks to measure the
differential changes in gravitational potential due to deformations.
But starting in the 1970s the deformations were measured by VLBI using
the incoming wavefronts from distant quasars as the tie to give the
distance between distant sites.

So Tom is right in answering Hal Murray's question, clocks, even
pendulum clocks, were good enough to *detect* solid earth tides.
But without other technologies they are not good enough to give clean
estimates of the magnitude and phase of those tides.

--
Steve Allen                    <sla at ucolick.org>              WGS-84 (GPS)
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