Gravity Measurement Changes Over Time

In addition to altitude, surface composition, crust make up, mantle contents and our position in the cosmos there is the molten contents of our planet shaking up the measurements.

I expect the amount of change to be minimal.. as in non detectable by Hollywood. Otherwise there would be better times and places to buy and sell precious metals?

Depending on the scale used...

OK I have a very accurate pendulum clock, it's calibrated by slightly varying the length of the pendulum to speed up or slow down the back and forth time variance...It would seem that once you got it set correctly that it would stay that way, but it doesn't...The force of the gravity shifts from time to time and speeds it up, or slows it down and it needs to be tweaked again...Is there any way of calculating forces at work to anticipate changes in gravity so that a control system could be designed to counter the changes automatically?

http://curious.astro.cornell.edu/about-us/42-our-solar-system/the-earth/gravity/93-does-gravity-vary-across-the-surface-of-the-earth-intermediate

..."Earth’s gravitational field is constantly shifting because its dynamic surface processes cause local changes in mass that occur on shorter time scales; these changes in the local gravity field can be tracked as the “time-variable” gravity field over periods of years, including the following:

• Redistribution of mass from tectonic events like volcanic eruptions and earthquakes.
• Changing mass of polar ice sheets and distribution of ocean currents.
• Shifting masses of groundwater and even soil moisture."...

https://www.earthdate.org/earths-changing-gravity

http://www.kylesconverter.com/acceleration/milligals-to-standard-gravity

Do you keep this clock in a temperature controlled vacuum?

There may be other forces at play.

..counterweights.. sprung weight.. thermal expansion etc..

How accurate is very accurate?

It's not in a vacuum, but it is climate controlled with about a 2°F swing and humidity is kept at 50% +/- 2% that is fairly regular, so it should cancel out...I don't know what you mean by how accurate, when it's on, it's on..This pendulum is a brass rod about 6" long with a weight at the bottom...the height of the weight is adjustable...

Time also changes do to gravity change, so no. All the computing power available is not enough to predict the adjustments.

Horse feathers, jmnjrpa.

There is sufficient computing power to provide accurate GPS. Those clocks see far greater changes to gravity as well as to relative time than a clock on a mantle.

....not that GPS clocks are pendulum based, but changes in gravity still have an effect and that hasn't presented an insurmountable obstacle.

Some Easy Tweaks but Probably Still No Joy

Six inches is a very short pendulum for accuracy purposes. A fireplace mantle is a very unstable site for an instrument which you expect to perform consistently. That you keep your overall house to the conditions you quote for temperature and humidity does not mean that your fireplace mantle matches those numbers. Do you operate your fireplace? Is the damper tightly closed ? What is your spring winding procedure? Does the wind oscillate in your chimney ? These questions and more cause me to believe that you are expecting far too much repeatability for the conditions affecting your instrument. If you have a basement(seems unlikely in FL so just use an interior closet), put your clock in an insulated box with a bunch of mass(washed river gravel you have oven dried), establish a stable winding regimen, do not move the overall clock when you wind it, and collect some more data and compare the data consistency with your current data corpus. Do not be fooling with the pendulum length setting until your comparison is completed. I suspect you will see a marked improvement in your data consistency but due to the short pendulum and spring drive it will still not be very good. Note that the approximations for pendulum period involve short travel of long arms and that the most accurate pendulum clocks had multi-floor lengths, huge mass, short travel and usually were powered by giant clock driver weights which provided very uniform force on the escapement.

I am curious. Does your clock have gimbals for its pendulum ? If it does not then the sideways forces could affect its period. Leveling your clock would then be very important and if you move it when you wind it you could disrupt everything. Even with perfect leveling you would need enough sideways give to accommodate Foucault pendulum effects.

______________

thewildotter

This is Florida, the fireplace exists ironically...it is sealed with both damper and glass doors...The wind oscillations may occur though...and you may be right that I'm expecting too much with this design....however it doesn't hurt to poke it around a little bit to see if something surfaces...Ideas can come from the humblest of sources...

The temperature variance design is interesting with the dissimilar metals, but I'm more interested if I can come up with a simple mechanical design that would compensate the swing rate of the pendulum for gravity variance...I see in the link that barometric pressure may come into play, so that's something else to consider...

..."4.6 Changes in gravity So far, looking at the expression for period T = 2π r L g � 1 + A 2 16� we have considered the effect of changes in amplitude A and length L. The remaining variation to consider is changes in gravity g. There are three main influences on this: the Moon and the Sun’s varying gravitational pull, which actually changes g; and buoyancy forces and the added inertia of entrained air, which effectively decrease g."...

This seems like it should cancel out as well...

http://trin-hosts.trin.cam.ac.uk/clock/theory/pendulum.pdf

https://physics.stackexchange.com/questions/143844/how-does-varying-temperature-affect-the-accuracy-of-grandmas-pendulum-clock

..."4.6 Changes in gravity So far, looking at the expression for period

we have considered the effect of changes in amplitude A and length L. The remaining variation to consider is changes in gravity g. There are three main influences on this: the Moon and the Sun’s varying gravitational pull, which actually changes g; and buoyancy forces and the added inertia of entrained air, which effectively decrease g."...

"but I'm more interested if I can come up with a simple mechanical design that would compensate the swing rate of the pendulum for gravity variance"

Well, if your pendulum rod were a perfect spring, the pendulum length would be proportional to g, and the factor (L/g) would be constant. However, now you have to worry about variation in the spring constant (and bouncing up and down), so that would open up a whole new kettle of worms.

I guess the point is that a timing device has to be based on something very constant, and g is almost perfect. If you try to get around minute changes in g, you are basically throwing away your standard. You would have to have a better standard to compare it to.

I've thought about having a pendulum bob with a quartz timing device inside and an accelerometer to sense the pendulum motion. A feedback circuit would move a weight up and down to keep the pendulum in time with the quartz timer. This is a hack, but it would replace "g" as the time standard with the quartz crystal.

Yes this seems to be the crux of the problem, a reference that is more consistent, but still mechanical...The crystal is an interesting hack, but then introduces the need for a power source...now this problem could probably also be hacked, but we have already become too complicated...thanks for fleshing this out...Maybe this is as good as it gets for the amount of effort available...

You might be interested...

Accurate Clock Pendulums

Robert J. Matthys

ABSTRACT

The Shortt clock, made in the 1920s, is the most famous accurate clock pendulum ever known, having an accuracy of one second per year when kept at nearly constant temperature. Almost all of a pendulum clock's accuracy resides in its pendulum. If the pendulum is accurate, the clock will be accurate. This book describes many scientific aspects of pendulum design and operation in simple terms with experimental data, and little mathematics. It has been written, looking at all the different parts and aspects of the pendulum in great detail, chapter by chapter, reflecting the degree of attention necessary for making a pendulum run accurately. The topics covered include the dimensional stability of different pendulum materials, good and poor suspension spring designs, the design of mechanical joints and clamps, effect of quartz on accuracy, temperature compensation, air drag of different bob shapes and making a sinusoidal electromagnetic drive. One whole chapter is devoted to simple ways of improving the accuracy of ordinary low-cost pendulum clocks, which have a different construction compared to the more expensive designs of substantially well-made ones. This book will prove invaluable to anyone who wants to know how to make a more accurate pendulum or pendulum clock."

https://www.oxfordscholarship.com/view/10.1093/acprof:oso/9780198529712.001.0001/acprof-9780198529712

I don't think it's gravity changes that you are seeing. Temperature, humidity and friction from gears and pivots are more likely. There were better designs for mechanical clocks for ships and laboratories than you probably have before quartz oscillators came about. It's too late to look for a link now.

Temperature, humidity and friction are all fairly constant...It's other variables I'm concerned with...

This does seem to be the most plausible explanation, although the clock sits on the fireplace mantle and seems rigid at first take, there's no stopping vibration like you said from large vehicles, thunder, loud stereos and other sources...Would a rubber pad placed under the clock dampen the effect, or is there some other way?

Use a quartz movement and just use the pendulum for show.

No...

I think putting the clock on a rubber pad might make it worse. The clock would be more free to tilt and move in other ways.

If you covered the entire mantle with soft rubber and covered the entire mantle with single a piece of steel (at least 1/4" or maybe 3/8" thick) cut to the same size, it could help noticeably Two layers of rubber (of different hardness, density and/or thickness) and two layers of steel (the top one should still be of decent thickness, probably at least 1/4") (clock, steel, rubber, steel, rubber, mantle) could provide better isolation.

Yes I was thinking of foam rubber between two rigid surfaces, this is how I have the subwoofer of my entertainment center setup to cut vibration transfer to the floor...

Putting something like this on the mantle (as a combo lossy pendulum/liquid column damper) might improve the situation by attenuating vibration.

I too have accurate pendulum clocks but I hypothesize that it a result of the temperature changes lengthening and shortening the pendulum length and thereby causing the time variation. Mind you my pendulum rods are not made of Invar or some exotic metal.

Maybe gravity is having an effect but the fact that I have to shorten the pendulum in summer and lengthen it in winter leads me to believe it is temperature related. With winter to summer variations in the unaircomditioned house being to blame.

The Shortt-Synchronome clock was a master slave pendulum clock that was accurate to one second in 12 years. This was the first clock to be more stable than the rotation of the earth which is affected by tidal effects of the sun and moon.

I remember reading a book about these clocks at school over 40 years ago (we had one of these clocks in our science library that had previously been used at the Greenwich Observatory).

There are a lot of factors that influence the swing of a pendulum, not least the fact that the period of a pendulum is not constant if the amplitude of the swing varies. It depends on the approximation sin x = x (in radians) when x is small. The Shortt-Synchronome clock goes to great lengths to maintain a constant amplitude and runs the master pendulum in a shock mounted vacuum chamber.

There is no way that a mantlepiece clock is going to be accurate enough for gravity variations to be significant.

see Wikipedia for an outline of the system. https://en.wikipedia.org/wiki/Shortt%E2%80%93Synchronome_clock

This is the clock we're talking about, but the principles discussed are not exclusive...

I have an old (1800s) Seth Thomas clock similar to that one.

Do you have to adjust the pendulum from time to time?

A bit of a long shot theory of mine, but as force is a change of velocity (acceleration) of a mass over a distance (in one direction), why not a force (something we call gravity) to move a mass simultaneously in all directions at once (thus the mass appearing stationary) from acceleration caused by a change in time itself ... .

.... don't know...

I'm now more concerned with resonant frequency from local vibration...An active vibration damping system might work...

I need a DIY version of one of these...

https://www.newport.com/n/active-vibration-damping

Perhaps consider building a tuned mass damper. You might have room to conceal it inside the ironic fireplace.

It won't eliminate motion, but it could significantly reduce amplitude and duration of motion.

You could start with some accelerometer readings to get an idea of the amplitude and frequency of the vibration.

This might give you some clues:

theclockdepot.com/clocks-blog/history-of-the-pendulum-clock

Here's another link you may like:

strangely-in-sync-scientists-solve-350-year-old-pendulum-clock-mystery