# Waaay OT- Lunar physics question

I apologize for this being way off topic, but there appears to be some smart folks here and I thought I would ask a few questions.
If a regular pendulum clock, which keeps quite accurate time, were taken from the Earth to the Moon and the correct time set into the clock and the clock was restarted, would the clock keep as accurate time as it did on Earth?
Would the clock's three weights have to be pulled up every 7 days as mine does?
Would the lower Lunar gravity make any difference in time keeping?
If so, which way and by how much?
If not, why not?
Hoyt W.
p.s. The Sun will rise in the East regardless of the above.
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smart folks

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Looks like your question is how gravity affects a pendulum. A pendulum's movement is caused by gravity. Since a pendulum is just another falling object it should fall with the same acceleration as a free falling object (neglecting friction). An object on Earth will fall at 32 ft/sec^2. An object on the moon will fall with (I think) with 1/6 that acceleration, and 1/6 as fast. (An object in space, without gravity, will not fall at all). So a pendulum on the moon will keep time more slowly than on Earth. It seems like it should be 1/6 as fast, but I don't know exactly how the clock works, so maybe 1/6 isn't right.
dwhite <--- at least that's how it looks to me
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A pendulum clock will run slower on the moon.
http://scienceworld.wolfram.com/physics/Pendulum.html
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Just a little searching would get you this:
http://online.cctt.org/physicslab/content/Phy1HON/lessonnotes/pendulums/period.asp
Period goes as 1/sqrt g, so the clock would run slow by 1/sqrt 6. I'm not sure of the six...look up lunar gravity. The clock would need different gears!
If the clock is running more slowly, then the weights must be falling more slowly.
Here are clock pages:
http://home.howstuffworks.com/clock.htm
http://www.britannica.com/clockworks/t_pendulum.html
Obviously one can make a gravimeter (device to measure the strength of gravity) using a pendulum, given the equation.
Wilson

smart folks

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does?
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smart folks

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ABSOLUTELY NOT... The clocks TRUNNION would not survive the trip to the moon.. ;~)
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wrote:

Pendulum swing time= 2 * pi * sqrt (l) / g
So the clock time will slow down in direct proportion to reduced gravity. A pendulum about 1/36 (or so) of the length would set it right though, as the time is also proportional to the square root of the length.
It'll also stop working pretty quickly. The moon's atmosphere is a hard vacuum, and a dusty vacuum at that. Vacuum system lubrication is _difficult_ (many satellites lost because of it). Even low vapour pressure oils boil away in the vacuum and you're left with a sticky goo that jams mechanisms. Run the bearing clean and dry and you may get metallic welding instead. Then if you do find a bearing that works, the lunar dust is reputedly a real hazard for any long-term mechanism. With no air or moisture, there's not much to lay the dust and fine dust gets everywhere (Apollo suits started to show damage in just a few days)

They'd descend at exactly the same rate (in clock hand-movement terms) as on Earth. They may also need to be made heavier, as their mass would stay the same but they'd only have 1/6th of the weight - is this still enough to drive the clock ?
--
Smert' spamionam

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On Mon, 17 May 2004 01:14:32 +0100, Andy Dingley

Doh ! My memory must be going - that should of course be
2 * pi * sqrt (l / g)
(which considering the dimensions would have told me anyway)
So you'll run slow at about 1/2.5, or a 1/6 pendulum rod would fix it.
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Andy Dingley wrote:

Jeweled bearings, sealed case. It should run *more* accurately in a vacuum. Remove air and you remove a whole set of variables.

24 hours unwinds the same amount of cord.

How complex is the mechanism? Does it drive three hands, or one?
Or is it just a pendulum, which can by itself be considered a clock. You can read time simply by counting the beats. Hands add nothing more than convenience.
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Father Haskell, you raise several interesting points. I understand the dry bearing wear problem in vacuum since I worked for Dr. von Braun for about 20 years before he was forced to leave Huntsville.
With the lower gravity on the Moon, dust would settle in the near area, but more slowly than on Earth, and not be blown about at all since there is no Lunar atmosphere of any significance. There are no winds on the Moon. [well, the Solar Wind, but that is another topic entirely] AFAIK, moisture has nothing to do with time keeping per se.
"24 hours unwinds the same amount of cord."
I assume you mean on the Moon as well as on the Earth. On the Moon, I now think the clock would unwind 1/6 of the cable per a real 24 hours. so we seem not to agree on that point.
IMHO, you have missed the point. I would agree with you IF the clock kept the same time on the Moon as on Earth, but I now do not think that it would and other reply posters seem to say that as well. I believe that it would take about 6 x 7 days for the clock to run down, assuming that it now runs down in 7 days here on Earth - or in that neck of the woods. In other words, the pendulum would move more slowly and there would be fewer pendulum cycles, or ticks, per real minute on the Moon. Ergo, the clock would be inaccurate and run slower and the hour and minute hands would correspondingly also move more slowly.
I think it matters not how complex the mechanism is or how many hands the clock has. The atmosphere and moisture are not factors because I had assumed the clock would remain inside a manned Lunar Lander of some kind. I did not state that assumption and that is my error. So forget about atmosphere and moisture - or lack thereof outside of the Lander. The clock would not be exposed to the Lunar environment - other than the lower gravity.
My Howard-Miller GF clock has 2 hands and 3 weights actually.
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Hoyt Weathers wrote:

"'Vonce ze rockets go up, who cares vere zey come down, dat's not my department,' says Werner Von Braun." (Lehrer)
The tower shot of the Apollo 11 liftoff still leaves me speechless, regardless. My favorite arg to the moon hoaxers is, well, where did this monster end up?

Related point -- check out record producer George Martin's autobiography. Seargent Pepper was cut with a pendulum-regulated turntable. Martin knew this was something special, and wanted every detail done perfectly.
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<snip, snip>

Reminds me of The Pit and the Pendulum movie, or close enough to the actual title.
Hoyt W.
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There was a movie?
_Edgar Allan Poe_ <http://bau2.uibk.ac.at/sg/poe/works/pit_pend.html
scott
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Scott Lurndal wrote:

Oh yes - and now there is a DVD of it. The ending was different from Poe's novel IIRC. Vincent Price starred in it.
Here is one URL for the DVD:
Hoyt W.
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<snip>

<snip>
Hoyt,
Minor quibble having little or no effect on the discussion.
Although Moon gravity is appx. 1/6 Earth gravity, the Earth's atmosphere causes dust to remain in suspension and settle quite slowly. No atmosphere means dust on the Moon literally drops like a rock. So, in actual effect, dust on the Moon falls much more rapidly than on the Earth.
Tom Veatch Wichita, KS USA
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Tom Veatch wrote:
<snip, snip>

Quite correct Tom. IIRC, there was one Apollo flight where an astronaut, on video, dropped a feather and a more massive and compact object at the same time. Both the feather and the other object fell at the same rate and hit the Lunar surface at the same time. The reason for that, of course, is that there is no atmosphere on the Moon. I think we fully agree on this. You just stated it differently than I did and quite well I think. TKX Hoyt W.
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No. The weighs do not fall continually, they fall a fixed amount at each "tick" as the paws engage and disengage.

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Most likely two of the weights drive the pendulum and one the chimes. The pendulum is driven at the end of swing going both directions. In many clocks this is redundant and as long as one or the other weight is would it will run just fine.
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Having messed up the night before and not getting this message sent to the news group as I intended, I'm reposting now (slightly modified).

the Sun

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weaker
Umm, no on the "tides" influence. The moon does not spin in relation to the earth and thus the tidal forces are fixed: if the seas on the moon were full of sea water there would be no tides coming in and going out. If the moon was covered with water it wouldn't be round, it would bulge out on the sides facing and turned away from the earth. In regards to the sun: it's too far away to have near the effect the earth would have (assuming the moon spun). Also, it's the different distances from two points on a body to the other body that creates tidal forces, the moon is smaller so the distances are nearer the same as thus tidal forces are smaller. A gas planet will experience greater tidal forces than a rock the same mass in the same orbit.

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It would be just as accurate, just ticking at a slower rate. Well, I guess that depends on what you mean by "accurate": the clock would mark time at a stable rate but the rate would be much slower than here on earth. Nobody ever set a fixed swing rate for pendulums.
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wrote:

it's all a bit fuzzy now but I seem to remember reading sometime in the last few years an article about the moon and it's orbital relationship with Terra. yes, one side always faces the earth, sorta. the moon wobbles quite a bit. the "back" side does have a bit of area that has never been visible to earth, but it's a lot smaller than you might think.

yup.
but in relation to the sun the moon *does* spin. the effects of gravity obey the square of the distance bit, and the sun is a long way away, but it's also freakin' huge. nothing compared to the grav effect of Terra, but non-trivial.

and the moon is basically a rock, though there is some recent data indicating that it may have a squishier core than previously thought.
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OK, but _who's_ homework assignment is this?

The _stability_ of the time tick will be unchanged.
The _frequency_ will be lower. By a factor of about 2.42.

The weights have to be pulled up after the same number of cycles of the pendulum. Which will take about 17 days on the moon.

Yes.
The frequency of a pendulum is proportional to the square root of the local gravitational constant.
Thus, since Lunar gravity is _less_ than that of Earth, the frequency will be lower. And the clock will run slower.
"How much?" The factor is the square root of the ratio of the gravitational field strengths. A quantitative answer is left as an exercise for the student. <Big sh*t-eating grin>

not applicable.

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Actually, it will be much less accurate.
Think tides. They would be much greater on the moon, due to the greater gravitational attraction of the Earth. The gravitational attraction of the Sun would have a greater effect as well - while it would be the same on the Moon as on the Earth, it would be greater proportionally to the Moon's own weaker gravitational force.
Other than those "outside influences", the pendulum should be just as accurate on the Moon as on the Earth. Slower, but just as accurate.
John Martin
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