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I am trying to detect tidal forces influence on a pendulum. I use a laser interferometer that is able to detect very tiny space changes. The pendulum never stops even after long times quiet and protected from wind by a kind of plastic box. The pendulum is 2,60 m, long, 10Kg and one of the mirrors of the interferometer is placed in the Iron cable near to the pendulum hanging point. I can see that the top energy of the pendulum changes but I am not sure of the causes.

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  • $\begingroup$ Take a look at these articles leapsecond.com/hsn2006 $\endgroup$
    – PM 2Ring
    Nov 9 '18 at 0:26
  • $\begingroup$ accurate pendulum experiments are challenging but that's what makes them so interesting! Tracking down all of the systematic errors is hard. There will be tiny gravity changes due to tidal effects (Sun & Moon) and the cable will change length dramatically with temperature, For steel it's almost 1E-05 per degree C! If you would like to read more about the actual acceleration at your location, see this answer and check out this handy calculator at Wolfram Alpha $\endgroup$
    – uhoh
    Nov 9 '18 at 1:49
  • $\begingroup$ PM 2Ring I read three or 4 articles you sugested. They were useful but I prefer try search something by energy point of view. I dont want to restart from zero. Not yet. $\endgroup$ Nov 9 '18 at 12:40
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There are a series of resources on the leapsecond.com site which specifically talk about detecting tides with pendulum clocks. It has been done by e.g. the Shortt pendulum clocks which had a master pendulum in a vacuum to remove disturbances synchronized to a slave pendulum. From the first link in the resources at 1, the order of magnitude of the change in pendulum rate caused by the change in gravity is ~1e-7 so your pendulum clock needs to be stable to a few milliseconds per day (few x10^-8) if you hope to see any tidal effects. You will need a more stable source of time such as a GPS Disciplined Oscillator (GPSDO) and normally a time interval counter to measure the stability with.

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  • $\begingroup$ This is a helpful and practical answer for the OP's project. I'm curious if you have any thoughts on the title itself "Does Earth changes angular velocity in 24 hours evaluation?" I'm guessing its a half-dozen or more orders of magnitude lower than the 1e-07 figure, but it still may be reassuring to the OP to have a number for this, $\endgroup$
    – uhoh
    Nov 9 '18 at 1:12
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    $\begingroup$ That is measured by the UT1-UTC Earth Orientation Parameter or expressed a different way, by the change in the Length Of Day e.g. hpiers.obspm.fr/eop-pc/index.php?index=UT1LOD&lang=en and hpiers.obspm.fr/eop-pc/products/combined/… for 2017. Day to day change is about 1 millisecond $\endgroup$ Nov 9 '18 at 2:34
  • $\begingroup$ oh I see. I hadn't eve thought about short term variations, wow those are huge! $\endgroup$
    – uhoh
    Nov 9 '18 at 3:08
  • $\begingroup$ PM 2Ring, uhoh and astrosnapper thank you. I am checking your suggestions. $\endgroup$ Nov 13 '18 at 12:32

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