# Earths Axis has moved!

Reports being aired in the press about the earths axis having moved.

Well I think its a load of codswallop. but the press is reporting this as if its real. For such an event to occur would mean that all the stars would have moved in relation to the earth by a small amount, which means every astronomer would need to recalibrate their equipment.

I have not heard of this happening.

It seems the actual explanation is the earth's "figure axis" has moved. As I understand it this is an axis drawn through the earths center of gravity, and not the real axis. so in other words the earth has not moved but over time may re adjust to the new alignment should no other events cause another re alignment.

And what is a figure axis really?

CC

ideasrule
Homework Helper
Where did you hear this? The Earth's axis is indeed constantly and somewhat unpredictably fluctuating, an effect called nutation. A more long-term and predictable effect is precession, which astronomers do have to adjust for. On geological time scales, there's also true polar wander due to the movement of Earth's tectonic plates.

Sorry should have said it was in respect to the chile earthquake.

I am not talking about geological time scales I am talking about when the earthquake occured.

But did it really?

... shortened an Earth day by 1.26 millionths of a second ...

The Chilean quake was a so-called thrust earthquake, which occurs when a large section of the Earth's surface—in this case, the Nasca tectonic plate—dives beneath an adjacent plate. This process, called subduction, can cause earthquakes and volcanic eruptions

So one plate goes below another which means the net result should be approximately zero. eg: one plate falls the nearby plate rises to accommodate it.

Gross also estimates that the Chile earthquake shifted Earth's figure axis by about three inches (eight centimeters).

Deviating roughly 33 feet (10 meters) from the north-south axis around which Earth revolves, the figure axis is the imaginary line around which the world's unevenly distributed mass is balanced.

This is the disturbing part, most media stories skip the word 'figure' so they discuss the actual axis which is incorrect. from what I can determine the earth's center of gravity may have changed therefore its figure axis which in turn may cause actual movement of the real axis (that which the earth rotates about) over a period of time

but my conclusion above about the negating effect of the two plates should also apply.

CC

Damn you mother nature!

This has happened before:
By speeding up Earth's rotation, the magnitude 8.8 earthquake—the fifth strongest ever recorded, according to the USGS—should have shortened an Earth day by 1.26 millionths of a second, according to new computer-model calculations by geophysicist Richard Gross of NASA's Jet Propulsion Laboratory in California.

For comparison, the same model estimated that the magnitude 9 Sumatra earthquake in December 2004 shortened the length of a day by 6.8 millionths of a second.

And I don't remember if there was a huge hubub about it, But we are still floating around generally well.

Nothing to see here.

Matterwave
Gold Member
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?

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?

Yes, the moon causes an increase in the length of the day by about 15 $\mu s$ each year by pulling on the earth. So this decrease of about 1 $\mu s$ will be quickly absorbed.

Any change in the distribution of mass of a rotating body has the potential to change its rotating speed i.e. you are changing the moment of intertia. Think of an ice skater pulling in their arms and thus spinning faster, similarly the change in the shape of the earth due to the Chilean earthquake (which must have reduced the oblateness of the earth slightly) has caused the earth to spin slightly faster, thus shortening the day.

See http://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake under "Energy released by the earthquake"

Where did you hear this? The Earth's axis is indeed constantly and somewhat unpredictably fluctuating, an effect called nutation. A more long-term and predictable effect is precession, which astronomers do have to adjust for. On geological time scales, there's also true polar wander due to the movement of Earth's tectonic plates.
on news-

http://uk.news.yahoo.com/5/20100303/twl-powerful-chile-quake-shifted-earth-s-3fd0ae9.html [Broken]

Last edited by a moderator:
Simply, when we measure force of gravity ( earth - moon ), speed in orbit around Sun, rotation over Earth's axis... And add strength of earthquake on piece of paper with all other data... Earthquake force is negligible.

Can you provide some numbers for this? It's not obvious to me that it is negligible at all.

http://www.nature.com/nature/journal/v262/n5566/abs/262259a0.html

Well I think its a load of codswallop. but the press is reporting this as if its real. For such an event to occur would mean that all the stars would have moved in relation to the earth by a small amount, which means every astronomer would need to recalibrate their equipment.

Which happens all the time. When you are doing microsecond measurements for things like interferometry, you have to take into account a *lot* of stuff.

For most things you don't have to worry about microsecond variations. For the things that you do have worry about, then yes. If you want the gory details about how time works in astronomy pick up The Explanatory Supplement to the Astronomical Almanac

mgb_phys
Homework Helper
Not sure quite what your point is.
We can measure changes in the rotation speed of neutrons stars due to star-quakes on their surface, since NS are so dense the movement of the crust needed to make a measurable change in rotation rate is tiny. Ironically we can measure a movement in the surface of a NS with an accuracy less than mm, while only knowing it's distance to an accuracy of 10s of lyr.

The Earth's rotation is changing constantly, there is along term slowdown due to tidal friction with the moon, there are annual variations due to rain and snowfall changing the distribution of the mass and there are slower effects such as the northern hemisphere rebounding from the last ice age. These effects add up to almost a second/year which is why we add leap seconds.
The effect of a large earth quake is measurable but not significant compared to this.

Borek
Mentor
So one plate goes below another which means the net result should be approximately zero. eg: one plate falls the nearby plate rises to accommodate it.

Quite unlikely that both effects cancel out EXACTLY.

Borek
Mentor
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 don't remember exact numbers, but I recall information from one of the http://de.wikipedia.org/wiki/Hoimar_von_Ditfurth books that change in a number of days per year can be observed in fossils (diatoms?). Main idea is that the frustule (or shell, or some other solid support) grows in layers and thicknes of these layers changes on a daily basis (day/night cycle) and on a yearly basis (seasons). This allows calculation of number of days per year in the past.

This book is somewhere here, but I have no idea where :grumpy:

mgb_phys
Homework Helper

The Chilean earthquake changed the length of a day by something like 0.001 ms, so negligible on the annual scale of changes

Why not discuss earth things in the earth forums. Then you have alot more chance that questions are answered and errors are corrected.

berkeman
Mentor
Very cool plot, mgb.

Quite unlikely that both effects cancel out EXACTLY.

hence I used the word approximately. in other words the actual redistribution of weight should be negligible.

Borek
Mentor
Obviously if the effect is measurable, redistribution of weight is not negligible.

The trouble is the effect is not measurable. its only an approximaguesstimatation. the minimum that can be measured is 20 milliseconds so yeah its negligible. still that's not the point the main point was about the change in the earths axis. 8cm is negligible on the global scale but on the cosmic scale I am sure its not negligible.

All I wanted to ask was did the earth really move or was it a figment of scientists imagination.

And I am not sure that a sudden change in the earths axis of only 8cm is negligible. Imagine alls the oceans of the world asked to move 8 cm at once. I am pretty certain thats not negligible.

CC

mgb_phys
Homework Helper
You can measure rotations changes in the usec range with VLBI, of course the effect of the earthquake is swamped by other earth effects - as the graph shows.

You can even more easily measure figure changes on this scale from RTK-GPS base stations and VLBI baseline corrections.

D H
Staff Emeritus
The trouble is the effect is not measurable. its only an approximaguesstimatation. the minimum that can be measured is 20 milliseconds so yeah its negligible.
Baloney. The residual in dUT1 and LOD are typically on the order of 0.01 milliseconds. See IERS Bulletins A and Bulletin B. A 20 millisecond residual would make a lot of high-precision VLBI applications useless.

All I wanted to ask was did the earth really move or was it a figment of scientists imagination.
Yes, that 8.8 magnitude earthquake was just a figment of scientist's imagination. :uhh:

That 8 cm change is relatively small potatoes compared to the 2 meter or so motion that can occur over the course of about a 435 days to Chandler wobble and a year due to seasonal changes. For example, the amount of snow that falls on Siberia measurably affects the Earth's rotation.

http://news.nationalgeographic.com/news/2010/03/100302-chile-earthquake-earth-axis-shortened-day/
Currently, scientists can measure the length of an Earth day with an accuracy of only about 20 millionths of a second, so the shortened day caused by the Chile earthquake can be estimated but not measured.

I should have said 20 microseconds, not 20 milliseconds.

My concern is a 8cm 'jump' as opposed to a gradual shift would cause massive objects on the earths surface to become unreasonable in their opposition to the change. you can't ask the oceans to reposition themselves by even 8cm in the short period of time that the earthquake occurred in. Try and move a bowl of water suddenly. it sloshes everywhere.

So and I say it again the problem is the report has been misquoted. yes the earths principal axis may have moved. and yes the earth may eventually catch up with that move but no the 'Actual Axis' didn't move by any appreciable amount during the earthquake.

I am glad to see others more reputable than me are now starting to voice their opposition to this claim.

http://www.bild.de/BILD/news/bild-e...cientists-put-planet-earth-back-on-track.html

As I asked in the very beginning one way to verify such a change is to observe the stars, (assuming the earths change in position didn't alter space time in such a way that it dragged the universe around with it [tic])

So again the question is simple did any astronomers have to recalibrate their equipment over and above the recalibration which would be required for the 'known' nutations to account for this supposed shift.

BTW The chandler wobble is impressive. thanks for the heads up on that :)

Last edited by a moderator:
D H
Staff Emeritus
My concern is a 8cm 'jump' as opposed to a gradual shift would cause massive objects on the earths surface to become unreasonable in their opposition to the change. you can't ask the oceans to reposition themselves by even 8cm in the short period of time that the earthquake occurred in. Try and move a bowl of water suddenly. it sloshes everywhere.
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.

The conserved quantity of interest is angular momentum. Over the short timespan of an earthquake the Earth's angular momentum will be very, very close to constant. The influence of the Moon and Sun discussed earlier is far too small to have any influence over a shortish period of time. Any short-term changes in the Earth's inertia tensor would necessarily result in a change in Earth's angular velocity.

Any change in the Earth's angular velocity can be broken down into two components, one parallel to and the other normal to the angular velocity vector. In the issue at hand, the 1.26 microsecond change in the Earth's rotation rate is the parallel component and the 8 cm change in the Earth's axis is the normal component. Both of these changes would have a small (immeasurably small) effect. Moreover, while change in the Earth's rotation rate would have the greatest impact at the equator and none at the poles, the change in the orientation would have the greatest impact at the poles and none at the equator.

How big (or rather, how small)? That 8 cm change in the Earth's pole corresponds to 2.6 milliarcseconds. The quake lasted about 3 minutes. The average angular acceleration is 2.6 mas/3 minutes * 360°/day or 1 nanoarcsecond/second2. The apparent force at the South (or North Pole) would be about 3.3×10-9 g or 3.2 microgals. Very tiny!

I am glad to see others more reputable than me are now starting to voice their opposition to this claim.

http://www.bild.de/BILD/news/bild-e...cientists-put-planet-earth-back-on-track.html
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.

The apparent force at the South (or North Pole) would be about 3.3×10-9 g or 3.2 microgals. Very tiny!

I am sorry but I don't know what this relates to. no one one claimed the gravity at the poles suddenly became greater or lesser. although of course the shortening of the day is in relationship to the redistribution of mass, which would have moved the earths center of gravity which would have altered the relative gravity at every point on the surface. I think we can agree the amount was negligible, and less so if the subduction was balanced as I contend it should have been (as one plate pushes below, the other plate rises to accommodate it, net effect close to zero. This assumes there was not some void below that allowed the tectonic plate to subduct [slide under] without displacing the other plate).

Also if the axis shifts its a worldwide thingy. You can't say the axis shifting would have the greatest impact at the poles and very little at the equator. If the shift was instantaneous then it would be different at different points on the equator depending on their relative position to the plane of the shift (sorry not sure what else to call it). but that also assumes that the shift rotated around the center of the earth and not some other point.

Still my question remains and I assume its because no one who has responded in this forum is an astronomer (which is why I picked this forum) or no one who is an astronomer is prepared to say (caught in groupthink?).

Another way to think of it. an observer watching the sun would have noticed during that three minute period that the sun had moved relative to him. it may have slowed down or sped up or shifted to another position relative to the horizon with respect to his position on the earth.

Another example, a photographer taking a long exposure of the stars in such a way that the stars smear into lines due to the rotation of the earth, would have noticed an aberration in his photograph. Dependent on his position on the earth, the lines drawn by the movement of the stars would suddenly change direction for a few minutes and then resumed their original path. The path of the stars would be kinked.

Just because someone from NASA or any other scientific organisation says its so doesn't make it so. Thats not science thats religion.

CC

I am aware a milliarcsecond is too small for a human observer to notice. my examples are exaggerations. only there to make a point

Last edited: