# Earths Axis has moved!

1. ### curiouschris

122
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

2. ### ideasrule

2,323
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.

3. ### curiouschris

122
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.

5. ### curiouschris

122
But did it really?

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.

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

6. ### MotoH

237
Damn you mother nature!

This has happened before:
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.

7. ### Matterwave

3,816
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?

8. ### BackEMF

53
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"

9. ### physixlover

87
on news-

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

10. ### twofish-quant

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

11. ### twofish-quant

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

12. ### mgb_phys

8,952
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.

### Staff: Mentor

Quite unlikely that both effects cancel out EXACTLY.

### Staff: Mentor

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:

15. ### mgb_phys

8,952

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

16. ### Andre

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

### Staff: Mentor

Very cool plot, mgb.

18. ### curiouschris

122
hence I used the word approximately. in other words the actual redistribution of weight should be negligible.

### Staff: Mentor

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

20. ### curiouschris

122
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