Earth's Axis: What is Figure Axis & Have Astronomers Had to Readjust?

[ViewsofMars]

[russ watters comment to curiouschris]

Also, from what I understand, the tsunami is what caused most of the deaths in Chile.
A tsunami is generated when a piece of the Earth's crust moves by many feet, so when the whole Earth moves by a few inches, it wouldn't cause a big, coherent, localized wave. Yes - you reacted to a non-specific crackpot claim as if it applied here. You were mistaken...though we probably shouldn't have allowed that post to remain. I think ViewsofMars posted it for the irony and assume that he knew what DH said: That was obvious to me too...a complete pole flip would be a total of a 180 degree change, so obviously, it wouldn't just be miliarcseconds that we'd be looking for.

Anyway...
You got that in post 11, but maybe it wasn't clear enough.

Hi Russ,

First off, I'm a hetrosexual woman. I'm getting very upset by men calling me a "he." This isn't the first time here on physics forums it has happened to me where I HAVE HAD TO CORRECT the person for mistaking my identity. My question is why is it continuing? The impression I'm beginning to think is that women don't usually partake in discussions about Astronomy, Cosmology, etc. I've posted here before with valuable information. Also, I'm here to say, "WOMEN are welcomed to participate!"

My second point is that everything I've posted was reflecting what I felt to be information pertainent to what has been going around the Internet on different forums and what has been presented in newspapers and Internet articles. It's been confusing for many people.

My last post (3 of 3) on the previous page reflects what was often omitted from on-line articles. "Gross calculates the quake should have moved Earth's figure axis (the axis about which Earth's mass is balanced) by 2.7 milliarcseconds (about 8 centimeters, or 3 inches). Earth’s figure axis is not the same as its north-south axis; they are offset by about 10 meters (about 33 feet)." Reports have been confusing for lack of mentioning "earth's figure axis." Instead article's have been written stating "earth's axis" without denoting it's not the north-south axis in mention. The lay person is confused for good reason. This is why I posted my first response on the previous page from David Morrison. Here is a snippet from it, "if the pole shifted even a small fraction of a degree, all astronomical telescopes would be unable to find or track their sources, and all GPS systems would cease to function." My second post was an merely an *example* for the confusion.

Thanks,
Mars

 

[D H]

The north-south axis is simply the line connecting the Earth's geographic north and south poles. This is not the Earth's rotation axis. Think about it this way: Just because the Earth's rotation axis moves by some amount (20 meters over the course of a year) does not mean that the extremely well-mapped latitude and longitude coordinates of a spot on the Earth need to change to suit. Were it not for continental drift, those well-mapped latitude/longitude coordinates would remain constant no matter what axis the Earth happens to be rotating about.

The north-south axis is what geographers thought was the Earth's figure axis (mean pole) at noon on January 1, 1903. The intersection between this axis and the Earth's surface are the north and south poles. The official name for the geographic North pole is the "Conventional International Origin" and was estimated via six International Latitude Observatories. (You can google those phrases). Our latitude/longitude system was frozen in 1903 with early 20th century tecnhnology.

The Earth's instantaneous rotation axis deviates from the CIO by a measurable amount by some period changes (435 day Chandler wobble and yearly variations) plus what appear to be a longer-term drift. A plot:

http://www.iers.org/SharedDocs/Bilder/EN/Polar__motion2001__2006__polhody,property=default.png

Another name for the figure axis is the mean pole. In other words, it is the location of the rotation axis after accounting for the quasi-periodic Chandler wobble and annual variations. Notice that the motion of the mean pole is considerably smaller in magnitude than is the motion of the instantaneous axis of rotation (polhody in the above plot). An 8cm shift might be observable after the fact. The difficulty lies is untangling all of known contributors to polar motion. The biggest are seasonal changes in the atmosphere and Chandler wobble, and both are a bit noisy.


A more recent release from NASA on this topic:
http://science.nasa.gov/headlines/y2010/11mar_figureaxis.htm

 

[ViewsofMars]

A more recent release from NASA on this topic:
http://science.nasa.gov/headlines/y2010/11mar_figureaxis.htm

Thanks D H,

From the url you presented dated March 11, 2010 plainly states, "On Feb. 27, 2010, the Chilean quake may have moved the figure axis as much in a matter of minutes as it normally moves in a whole year. It was a truly seismic shift—no pun intended.

So far, however, it's all calculation and speculation. "We haven't actually measured the shift," says Gross. "But I intend to give it a try."'


In the meantime I think I will stick with David Morrison, NAI Senior Scientist, has stated on March 11, 2010, which I presented earlier on page 3.

And thanks for being attentive and helpful. :)

 

[ViewsofMars]

But the point is moot the Earth didn't move the the ocean wasn't unsettled the sky didn't fall and apparently Viewsofmars took the mickey out of me.


CC

Hi Chris,

I don't know what a mickey is but whatever it might be, I'm glad it was removed by me. However, I never claimed 'the ocean wasn't unsettled'. And you are correct the Earth didn't move out of it's orbit around the Sun (if that is what you meant) but we did have an earthquake, and most definitely the sky didn't fall down upon us.

Keep those happy thoughts moving in the right direction.

 

[D H]

In the meantime I think I will stick with David Morrison, NAI Senior Scientist, has stated on March 11, 2010, which I presented earlier on page 3.

That quote by Morrison has nothing, absolutely nothing to do with the topic at hand. Nothing.

Look at the context of his response. He was talking to a lay person who asked "Could the fact that the poles are melting and there is unusually cold weather in other places be a sign of pole shifting?" Think about the huge magnitude of pole wander that would be needed to be the cause of recently observed losses in sea ice coverage. Hazarding a guess, I suspect a shift on the order of a degree (probably more) would be needed. This is the context of what Morrison meant by "if the pole shifted even a small fraction of a degree".

Moreover, the person was essentially asking if that shift might not have been detected. Not a chance! Very precise Earth orientation data are needed by many applications. Astronomy and GPS are a couple of examples. Knowing the Earth's orientation is such an important concept that the International Astronomical Union and the International Union of Geodesy and Geophysics jointly founded the International Earth Rotation and Reference Systems Service in 1987. Scientists know the Earth's orientation to an incredibly degree of accuracy.

It takes the US Naval Observatory a couple weeks to backtrack through all the data. After the fact, the error in the observed orientation of the Earth's rotation axis reduces to 0.03 milliarcseconds or less. In terms of "fractions of a degree", that is less than one 100 millionth of a degree. That is not the "tiny fraction of a degree" that Morrison was talking about.

Have you looked at the polar motion plots I have provided? (The link in post #50 and the polyhody plot in post #52.)

 

[ViewsofMars]

D H, be patient with me. I'm still working my way through the postings. I'm very intrigued.

[msg. 24]Where have those scientists been for the last fifty years? The short-term changes in the Earth's rotation rate and axis result from internal changes, not external. Google the term polar motion.

I hope you don't mind me back tracking a touch. It appears to me an example of the term "polar motion" would be the following, “Chao and Gross routinely calculate earthquakes' effects on Earth's shape and rotation. They also study changes in polar motion--that is, the shifting of the North Pole.” (How the Earthquake affected Earth, The Dec. 26th Indonesian megathrust earthquake quickened Earth's rotation and changed our planet's shape.) http://science.nasa.gov/headlines/y2005/10jan_earthquake.htm

So if you review my message 35 it seems to infer that David Morrison’s comment ‘if the pole shifted even a small fraction of a degree’ could be applicable to the above statement in quotes. Don't you think?

[msg. 24]You are looking at things wrong. It doesn't help that the lay articles imply that the Earth 'jumped' by 8 cm. That isn't what happened. It wasn't the Earth that moved; it was the Earth's axis.

So the Earth's axis as you apply in your first quote above has given me the impression I need to refer to polar motion. Don't you think?

Bare with me, I'm having a busy day, and hopefully will look further into your most recent comments.

 

[D H]

So if you review my message 35 it seems to infer that David Morrison’s comment ‘if the pole shifted even a small fraction of a degree’ could be applicable to the above statement in quotes. Don't you think?

Absolutely not. Once again, Morrison was talking in terms of the *huge* changes needed to change Earth's climate. A "fraction of a degree" would not cut it. On the other hand, a "fraction of a degree" (and remember, Morrison was talking to lay person) is something we can easily observe. In fact, we need to observe those changes. Modern instruments such as telescopes and GPS require incredibly precise knowledge of the Earth's orientation.

It might be worthwhile to review how incredibly small a milliarcsecond really is. A full circle is 360 degrees. A minute of arc, an arcminute, is 1/60 of a degree. An arcsecond is 1/60 of an arcminute. A milliarcsecond is 1/1000 of an arcsecond. What does that mean?

A dime held at arm's length is about 1 degrees wide. The bumpy part at the edge of a dime: that's about an arc minute. Now imagine dividing that bumpy edge another 60 times. That's an arc second. Now divide that another 1000 times, and you have a milliarcsecond.

Another way to look at how small the angles being discussed truly are: Stack one meter stick lengthwise atop another. Now put a vertical spacer so that at one end the meter sticks are still together but at the other they are separated by some measured amount. To get a one degree angle you will need to insert a 1.75 cm spacer between the two sticks. One arcminute: 0.29 millimeters. Use a standard sheet of paper as your spacer and you will have an angle of about 25 arcseconds. Use the very thinnest of tissue papers and the angle will still be over a second of arc. Now divide that tissue 1000 times, and you have a milliarcsecond.

 

[russ_watters]

Hi Russ,

First off, I'm a hetrosexual woman. I'm getting very upset by men calling me a "he." This isn't the first time here on physics forums it has happened to me where I HAVE HAD TO CORRECT the person for mistaking my identity. My question is why is it continuing? The impression I'm beginning to think is that women don't usually partake in discussions about Astronomy, Cosmology, etc. I've posted here before with valuable information. Also, I'm here to say, "WOMEN are welcomed to participate!"

I apologize. Yes, it is a combination of the fraction of women we get here, difficulty in writing in gender neutral language in English and my laziness that often has me assuming I'm addressing males.

 

[ViewsofMars]

D H, you did gave me the impression by stating “The short-term changes in the Earth's rotation rate and axis result from internal changes, not external. Google the term polar motion” that your statement did lead me to the conclusion that ‘the shifting of the North Pole’ was inclusive in your statement since you said ‘Google the term polar motion. Just being honest. Obviously, I mistook it wrongly. Ah well, we grow together.

The March 11, 2010 article from NASA that you presented also states at the bottom of the page, “In a few months Gross hopes to have the answer. Stay tuned.” Obviously, at this point in time Gross doesn’t have the answer. As he has said, "We haven't actually measured the shift.” I guess all we can do is be patient and wait since there isn’t any evidence or observation that has determined it to be a fact as of yet.

D H, it’s been a pleasure having this conversation with you. Thank you. I’ll be looking forward to seeing what Gross has to say in several months. I hope you will keep us informed about it.

I'm very interested in earthquakes. Lived through quite a few of them. We never know when they will hit. Not yet anyway. Hopefully, one day we will.

Have a wonderful day.

Mars

I apologize. Yes, it is a combination of the fraction of women we get here, difficulty in writing in gender neutral language in English and my laziness that often has me assuming I'm addressing males.

Hi Russ,

You are very thoughtful. I appreciate you taking the time to talk to me. Let's hope there will be more women in the future participating. I absolutely love SCIENCE! It's a valuable learning experience that enrichs my life in more ways than I can count.

Have a great day. You've made mine much brighter. :)

Mars

 

[D H]

D H, you did gave me the impression by stating “The short-term changes in the Earth's rotation rate and axis result from internal changes, not external. Google the term polar motion” that your statement did lead me to the conclusion that ‘the shifting of the North Pole’ was inclusive in your statement since you said ‘Google the term polar motion. Just being honest. Obviously, I mistook it wrongly. Ah well, we grow together.

You misunderstood.

There are essentially three ways the Earth's (or any rotating body's) orientation changes.
1. External torques. "External" means applied by some other body, "torque" is, in law terms , a twisting force. In the case of the not-quite-spherical Earth, the Moon and Sun apply torque to the Earth by grabbing the equatorially bulge gravitationally. The biggest change (and it is very big) in the Earth's orientation is also very slow. The Earth's rotational axis sweeps out a cone with a half angle of 23.4°, but it takes almost 26,000 years to complete a full cycle. This is the lunisolar precession. The Moon and Sun have other shorter term, but much, much smaller affects on the Earth's orientation. The 18.6 year nutation is the next largest term, 20 seconds of arc.

2. Inertial torque. This is a fictitious torque, akin to the centrifugal and coriolis force. Fictitious forces and torques arise whenever one does physics in a rotating frame of reference. Our Earth is a rotating frame of reference. This inertial torque is what is responsible for the Chandler wobble. It is not an external torque. It is instead a consequence of the Earth's angular momentum and angular velocity vectors not be collinear.

3. Changes in the inertia tensor. All kinds of things change the Earth's inertia tensor. Tides, seasons, continental drift, motion deep in the Earth's core ... and earthquakes. Any change in the Earth's inertia tensor has to be matched by a corresponding change in the Earth's angular velocity vector because angular momentum is a conserved quantity. (Angular velocity is not.)


The long-term variations in Earth's orientation most certainly arise from external causes. However, these long term variations are loooong term. 26,000 years long. The short-term variations are mostly internal (i.e., items numbers 2 and 3 above).

The March 11, 2010 article from NASA that you presented also states at the bottom of the page, “In a few months Gross hopes to have the answer. Stay tuned.” Obviously, at this point in time Gross doesn’t have the answer. As he has said, "We haven't actually measured the shift.” I guess all we can do is be patient and wait since there isn’t any evidence or observation that has determined it to be a fact as of yet.

It will take quite a while to see if there truly was an observable change. That 8 cm change is, over the short haul, just too small to see amidst the polar motion. As I noted earlier, the observed polar motion between Feb 25 and Mar 2 was 37.8 cm.

However, he is claiming an 8 cm change in the mean pole. Most of the polar motion averages out to nothing over the course of a year or so. The signal just might be visible in the mean motion. Gross faces several challenges in proving this: Noise (those seasonal changes are *noisy*), detection (that 8 cm change is *small*) and of course attribution. He will have to convince the scientific community that he has addressed all of these issues. So for now, it is wait and see.

 

[curiouschris]

For me this is my last word of the subject.

Nobody, including the press article you cited, said that the Earth shifted by 8 cm. The Earth's rotation axis moved, not the Earth as a whole. This continued misperception on your part

I never said the Earth shifted.

I said people are crazy if they think the Earth shifted. but that was the impression the articles (many of them) gave. Many of the contributors here concluded incorrectly that I thought the Earth shifted. No I didn't. read my first post.

I am again repeating myself here...

When someone says the Earth's axis has shifted most people, except the people of this forum will think that you mean the shaft about which the Earth rotates changed its alignment. The only reason I raised the point was because, hey! if the Earth supposedly did shift (and therefore I was wrong, in my belief that it DID NOT change its alignment). There would have been other ramifications (other than a few press articles misquoting a scientist) There may have been an alignment problem with telescopes, hence the choice of forum, and I am convinced other more dramatic problems like say the entire north pole shifting by 8cm and the consequences of that.

Now again because its been a few of lines since I wrote it last, I Do Not Believe nor ever have that the Earth shifted.

I think that's plain enough.

My apologies to ViewOfMars. I don't think I assigned gender but if my words bely that please forgive me. BTW to take the mickey out of someone is to call them a fool, in a roundabout sort of way. I never believed you did, but DH certainly inferred that.

 

[D H]

When someone says the Earth's axis has shifted most people, except the people of this forum will think that you mean the shaft about which the Earth rotates changed its alignment.

That is exactly what supposedly happened.

The only reason I raised the point was because, hey! if the Earth supposedly did shift

That is exactly what didn't happen. What supposedly did happen was that the Earth's rotation axis moved with respect to the Earth.

There would have been other ramifications (other than a few press articles misquoting a scientist) There may have been an alignment problem with telescopes, hence the choice of forum, ...

The operators of those telescopes, along with several other modern gizmos, need extremely precise knowledge of Earth's orientation -- and that is exactly why the International Earth Rotation and Reference Systems Service (IERS) was formed.

As has been pointed out, those telescopes are not so sensitive that a tiny 2.7 milliarcsecond shift will affect them. However, this change is cumulative. If that change truly did occur, the effects of it will build up over time.

 

[pixel01]

But the Earth's rotation is also slowly slowing down due to tidal friction...anyone have an estimate on that rate, and how the change in rotation speed due to this earthquake is comparable?

I am not sure about the rate of slowing down, but the process will end when our day is as long as a month (and the month then will be longer than it is now). We have only one semisphere who can see moon all the time just as stationary satellites.

 

[russ_watters]

A localised tsunami. but that's not a 5' tsunami that circled the globe. which is what you stated. If it did I didn't hear about that.

I said 3', but Hawaii is 6500 miles from Chile, which is a little more than 1/4 the circumference of the earth. Because of landmasses, it is tough for a tsunami to literally go all the way around the earth, but halfway would have the wave as spread out as it is going to get.

A wave is caused when I kick the the water at the beach. so what.

I don't see your point. Kicking the water doesn't cause a tsunami.

But take the entire globe and shift it by just 8 cm and I can't see that it wouldn't cause a massive disruption. You can't just pick up an ocean and move it.

The strength of a wave is a matter of amplitude and size. The way I understand tsunamis is that the amplitude is literally the amount the crust shifted. A 3' drop creates a 3' tsunami and a 3" drop would create a 3" tsunami...and direction matters: the ground moving laterally instead of up and down wouldn't cause a tsunami. I think that's why tsunamis are tough to predict.

 

[Philosophaie]

With reguards to the Sun, the Earth wobbles because of the Barycenter attraction of the Earth and the Moon and the not parallel axis of the Moon to the Ecliptic.