The Naked Scientists

The Naked Scientists Forum

Author Topic: Do you weigh less when the moon is directly overhead?  (Read 19828 times)

Alandriel

  • Hero Member
  • *****
  • Posts: 523
    • View Profile
    • Some of my photography

Due to gravitational effects, you weigh slightly less when the moon is directly overhead


Yes? Really?
They must be kidding, no?



How much?  ;D


Can someone explain that in plain english please to a regular moon gazer like me who had no clue of the hidden benefits of this poplular sport?



« Last Edit: 29/04/2012 10:30:35 by chris »

JimBob

  • Neilep Level Member
  • ******
  • Posts: 6478
  • Moderator
    • View Profile
It is true. But I can't find my phyzics buk left now so will depend on the phyzikers to do the calculations. It ain't much so lay off the chocolate.

Alandriel

  • Hero Member
  • *****
  • Posts: 523
    • View Profile
    • Some of my photography
Lay off the chocolate ?!?!?!?  :o :o

What's the joy of living then?!?!?  V


N E V E R








 ;D






Marzipan half moon...........................

Andrew K Fletcher

  • Hero Member
  • *****
  • Posts: 2321
  • KIS Keep It Simple
    • View Profile
    • Operation OASIS
if we weigh less, then the earths surface we stand on must weigh less in relation to the location of the Earth.

Is it then a coincidence that following yet another unusual tide level here in Torbay that there is another major quake a few days later?

rosy

  • Hero Member
  • *****
  • Posts: 1028
  • Chemistry
    • View Profile
Weight differences (all data from wikipedia:
The earth-moon (centre-to-centre) distance is about 30*Earth's diameter, and so about 60* Earth's radius.
So standing on the Earth's surface you're about 59 Earth radii (R) from the centre of the moon.
The lunar mass is about 0.0123 Earths

Your weight on the earth's surface is given by Fearth = G * myou * mearth / R2
The force of the moon pulling up if it's right above you is Fmoon = G * myou * 0.0123 * mearth / (59R)2
Fmoon=0.0123 / 59R2 * Fearth = About three parts in one million of your "normal" weight

Andrew, no idea whether a 3 parts in one million change in the gravitational force would be enough to release geological strains. Not, I wouldn't have thought, unless they were pretty hair-trigger anyway. No doubt there is a huge amount of data available that would in theory allow it to be tracked, if anyone's got the computing power to trawl through it, after all the moon's path is pretty well understood (and indeed predictable) and a lot of data gets collected on geological events I believe.

rosy

  • Hero Member
  • *****
  • Posts: 1028
  • Chemistry
    • View Profile
Sorry, the above lost some units in the editing. F, the weight, is a force expressed in Newtons (N), m's are masses in kilograms (kg), R is a distance in metres (m). G is a constant with units (kg-1 m s3).

another_someone

  • Guest
I don't think the quake has anything to do with gravitational pull, but there is good evidence that water can lubricate rocks under stress, and so changes in the water table can trigger quakes.

Andrew K Fletcher

  • Hero Member
  • *****
  • Posts: 2321
  • KIS Keep It Simple
    • View Profile
    • Operation OASIS
Walking along the beach the other day my wife and I and friends noticed the tide was way out beyond normal level, we walked around rocks we had not been able to walk around previously, during which my wife and I discussed the possibility of a major earthquake following this event. We had been right on many other occasions. Sure enough a few days later there is a huge quake in the Caribbean and others around the World.

Earthquake rocks holiday islandsTimes Online, UK - 13 hours agoFORT-DE-FRANCE One person died and at least six people were injured after a 7.4 magnitude earthquake shook several Caribbean islands on Thursday afternoon. ...One dead as earthquake rocks the Caribbean Guardian UnlimitedPowerful Earthquake Hits Caribbean The Associated PressPowerful earthquake hits Caribbean MLive.comBloomberg - The Associated Pressall 369 news articles »
 PakTribune.com   Strong earthquake hits Indonesia's AcehReuters - 2 hours agoBANDA ACEH, Indonesia, Dec 1 (Reuters) - A strong earthquake measuring 6.3 on the Richter scale struck off Indonesia's Aceh province on Saturday, ...Earthquake off Indonesian coast Radio AustraliaStrong earthquake rocks Indonesia Sahara SamayStrong quake jolts western Indonesia Malaysia StarTimes of India - Bloombergall 67 news articles »
 WSBtv.com   2.6 Magnitude Earthquake Strikes Near DanvilleCBS 5, CA - 5 hours agoRead more in our Privacy Policy A 2.6 magnitude earthquake struck near Danville on Friday night, according to the US Geological Survey. ...Powerful Earthquake Hits Caribbean The Associated PressSmall quake centered near Danville San Jose Mercury NewsCaribbean Jolted By Magnitude-7.4 Earthquake [] RTT NewsWKYC-TV - Cruise Criticall 147 news articles »
Regional earthquake plan not up to mark - officialJamaica Gleaner, Jamaica - 6 hours agoA top regional disaster official says Thursday's powerful earthquake which jolted several Caribbean countries highlighted some deficiencies in the region's ...The Caribbean: Earthquake - Information Bulletin No.1 ReliefWeb (press release)all 3 news articles »
 Caribbean Net News   Estimated damage from earthquake set to trigger CCRIF policyCaribbean Net News, Cayman Islands - 4 minutes agoGEORGE TOWN, Cayman Islands: Caribbean Catastrophe Risk Insurance Facility (CCRIF), the Caribbean’s first joint reserve fund for earthquake and hurricane ...
Earthquake rocks DominicaJamaica Gleaner, Jamaica - 29 Nov 2007An earthquake measuring 5.2 on the Richter scale shook Dominica on Wednesday but caused no damage, the Disaster Management Office has said. ...
Quake forecast 'not on'The Nation Newspaper, Barbados - 1 hour agoInstead they advised people to be prepared and know what to do in the event of an earthquake. "What we do find is that people know what to do but they don't ...Lessons to be learnt from Thursday The Nation Newspaperall 2 news articles »
 KY3   KY3 News rewind to 1990: New Madrid endures earthquake predictionKY3, MO - 3 hours agoA magnitude 2.3 (that’s tiny) earthquake lightly shook an area near Cairo , Ill. That's right along the New Madrid Fault. The mention of the New Madrid ...
Second Matata earthquakeNZ City, New Zealand - 6 hours agoThere has been a second shake in earthquake-prone Matata in the Bay of Plenty. There has been a second shake in earthquake-prone Matata in the Bay of Plenty ...
Know what to do during an earthquake Antigua Sun, Antigua and Barbuda - 10 hours agoYesterday, I heard several people called in on the various radio shows and shared their experiences of the earthquake. It would appear that public needs to ...

JimBob

  • Neilep Level Member
  • ******
  • Posts: 6478
  • Moderator
    • View Profile
Movement of oceanic water is the result of, not a precursor of, earthquakes, There is no scientific evidence for any pre-earthquake -water movement correlation. And believe me, if there were a correlation the Japanese would have found it. They have more earthquake research than any one else and nothing in the way of ideas is off limits to them to investigate  for predicting 'quakes.

Considering that earthquakes, to register at any magnitude, must be of a very large crustal movement or an event, such as an explosion, causing seismic waves, the water table would not have influence on earthquakes. Yes, water does lubricate fault planes - at depth. Thus, it is under pressure as well.

When the water table influences earth movements, it is called a landslide.

wolfekeeper

  • Hero Member
  • *****
  • Posts: 849
    • View Profile


Due to gravitational effects, you weigh slightly less when the moon is directly overhead

Yes? Really?
Yes. The moon pulls on you slightly more than the center of the Earth because you are a bit closer than the center is. So you're lighter. And also, 12 hours later, when the Moon is the other side of the Earth from you, you're slightly lighter as well, because the center of the Earth is accelerated towards the moon slightly more than you, it turns out that this makes you slightly lighter as well, by the same amount.

These effects are called 'tidal forces'. They are also responsible for the lunar sea tides.

But the effect, as others have noted are very small, too small to notice on the scales, but the effects are more visible on the sea due to resonance effects.

Bass

  • Hero Member
  • *****
  • Posts: 1297
    • View Profile
You weigh less when the moon is directly overhead ...... yeah?!
« Reply #10 on: 02/01/2008 06:27:00 »
Walking along the beach the other day my wife and I and friends noticed the tide was way out beyond normal level, we walked around rocks we had not been able to walk around previously, during which my wife and I discussed the possibility of a major earthquake following this event. We had been right on many other occasions.
Quote from: JimBob
Movement of oceanic water is the result of, not a precursor of, earthquakes, There is no scientific evidence for any pre-earthquake -water movement correlation.

Not sure how to look up old threads, but Andrew and I discussed this once in the past. 

Years ago, just for fun, I wrote a short program that calculated the longitudinal position of the moon at any given time since the 1950's.  I then ran correlation coefficients between 80,000+ earthquakes (greater than M3) and position of the moon, full moon, new moon, aphelion and perihelion.  The earthquakes were broken down into categories for both size and depth.  There was absolutely no correlation (except for minor negative correlation for deep moderate earthquakes).

One would suspect that earthtides (land surfaces also rise and fall a minute amount in response to lunar tidal forces) might generate the last bit of stress needed to cause a fault under strain to fail and create an earthquake- i.e. the straw that breaks the camel's back.  Unfortunately, this notion is not supported by the data.  I suspect that the stresses on earthquake-causing faults are so great that the additional daily tidal forces are insignificant.  I have seen studies where the author "selects" a few, usually major earthquakes, and relates these to some lunar/solar phase.  If, as I suggest, the relationship is random, a few earthquakes will fall within the window of major tides, though the vast majority do not.

 

SMF 2.0 | SMF © 2011, Simple Machines