The AM Forum

From Bloomberg News:

"Earthquakes can involve shifting hundreds of kilometers of rock by several meters, changing the distribution of mass on the planet. This affects the Earth’s rotation, said Richard Gross, a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, California, who uses a computer model to calculate the effects.

“The length of the day should have gotten shorter by 1.26 microseconds (millionths of a second),” Gross, said today in an e-mailed reply to questions. “The axis about which the Earth’s mass is balanced should have moved by 2.7 milliarcseconds (about 8 centimeters or 3 inches).”

Ya know Bill, I've felt a bit off center lately.


Guns cause crime so pencils cause misspelled words..... right Huz?

Theoretically, ANY mass movement, no matter how small, can shift the Earth's axis and change its rotational speed to some degree.  It's a matter of what overall movements cancel each other out and what remaining force has the net effect.

For example, it's proven scientific fact - if every person on Earth takes a dump at the same time, the day speeds up 0.000059 microseconds and the axis shifts by 0.0000599 microarcseconds, due to the bio-magnetic inertial turd mass concentration in Asia...   ;D

T

The mass of water accumulated behind the 3 gorges dam(n) in China has altered the earth's axis...I blame the global climate change on that.....

If that is true including turd mass ;-)  consider all the construction that involves moving large amounts of earth to support construction projects.  I wonder if it's like having a tire out of balance and maybe the Earth is "seeking" a natural balance through plate tectonics.

dunno, but I cant spell 'people' to save my life. it looks to me to be more natural to spell it poeple. I have to let the spell hacker fix it in every post.

So, the cutting down of all the forests that's going on should speed up the earth's rotation in the same way that an ice skater's rotation speed increases as she draws her arms in to her body, right?

ldb

Probably not. Look at the relative length of the trees, and the mass of the trees and the diameter of the earth and the mass of the earth.  All the trees together would probably not be a significant fraction of the planetary mass. Also, the mass of the trees will stay on the surface of the planet mostly, so there will not be a net change in rotational inertia.   
No change in rotational inertia, no change in rotation. 

I'd be willing to bet that erosion, which moves stuff closer to sea level, (and closer to the center of the earth) speeds up the rotation more than all the earthquakes slow it down in a given year.

Tiny amounts in either case.

Wonder if NIST will issue a correction for WWV/H?  ;D

I think they wait for longer RCH units before changing the tic-toc on the radio

fred

Back when the Earth was young and filled with volcanos and meteor hits, the day was once about 12 hours long. It's slowing down from normal gravitational drag and friction.

Today's 24 hours is a perfect number considering how easily it divides into 360 and works well with compasses, time etc. But, then again, that's exactly why they made an hour as long as it is when the Greeks? worked it all out.

BTW, I find it fasinating that we live in a moment in time where the rotation of the moon is so sync'd with the Earth's that we see only one side of the moon.  It was not always this way and it will change again in the future.  I'll bet present day astrologist doom and gloomers have a field day with that, though it is a very slow progression.  The moon will eventually move so far away that it will be lost, but the sun will take us out before that happens.  Can't wait.

T

The following from one of the physicists on a planetarium listserv I'm in:

Here's a link...

http://news.yahoo.com/s/space/20100302/sc_space/chileearthquakemayhaveshorteneddaysonearth (http://news.yahoo.com/s/space/20100302/sc_space/chileearthquakemayhaveshorteneddaysonearth)

to a Space.com article about how NASA/JPL scientist Richard Gross has calculated that the Chilean earthquake may have sped up the rotation of the Earth.

Unfortunately, this article seems to have several mistakes.  The worst one is that it says the quake made the Earth’s rotation period shorter by 1.26 milliseconds, but according to this article from National Geographic on the same subject:

http://news.nationalgeographic.com/news/2010/03/100302-chile-earthquake-earth-axis-shortened-day/ (http://news.nationalgeographic.com/news/2010/03/100302-chile-earthquake-earth-axis-shortened-day/)

the shortening was by 1.26 *millionths* of a second, or 1.26 microseconds – an effect that is 1,000 times smaller.  Considering that both articles say the 2004 Sumatran earthquake decreased the Earth’s rotational period by 6.8 millionths of a second, I would think that the similar 1.26 microsecond figure is the right one for the Chilean quake.

 

In my opinion, the National Geographic article also does a much better job of explaining several other points, such as the necessity of the quake being a *thrust* quake to cause the speed-up (as opposed to horizontal strike-slip quakes), the similarity of the Earth’s speed-up to an ice skater bringing in her arms, and the difference between the Earth’s *figure* axis (which was shifted slightly by the quake) and its *rotational* axis (which was not, whatever the 2012 woo-woos might want to claim).

 Finally, there is this quote from the Space.com article:

“It [the length of the Earth’s day] increases in the winter, when the Earth rotates more slowly, and decreases in the summer, Gross has said in the past.”

 First question: which winter and summer is the sentence talking about – the Northern Hemisphere’s or the Southern Hemisphere’s?  But the bigger problem with this sentence is that it says the Earth spins slower in the winter and faster in the summer – and that is not true as far as I know.

 What is true is that the length of the solar day – measured from one solar transit to the next – changes seasonally.  (Solar transit is when the Sun is highest in the sky, crossing the meridian.)  The average length of the solar day is 24 hours, but throughout the year it varies in a complicated way that is related to the analemma and the Equation of Time.  In particular, around December 21 the solar day is about 30 seconds longer than 24 hours; and around June 21 it’s about 12 seconds longer than 24 hours.  In both cases the day is longer than average because the Sun is far from the celestial equator.  (In March and September, when the Sun is near the celestial equator, the length of the day is shorter than 24 hours.)  But in December the Earth is near perihelion and moves faster in its orbit around the Sun, whereas the opposite is true in June when the Sun is near aphelion – and this additional effect causes the length of the day on December 21 to be about 18 seconds longer than the length of the day on June 21.

 So I think the quoted statement from the Space.com article should be corrected to read as follows:

“It [the length of the Earth’s day] increases in the (Northern Hemisphere’s) winter, when the Earth *moves faster in its orbit*, and decreases in the (Northern Hemisphere’s) summer, Gross has said in the past.”

Actually, that is not mere coincidence.  The moon's rotation is locked to its orbital period due to tidal forces.  There are other known examples of this in the solar system.  The one I can think of from the top of my head is mercury.  The same side always faces the sun.

Of course it is only speculation, but scientists have predicted that some of the exo-solar planets recently discovered are tidally locked to their star.

The moon will lose synch when it moves far enough away from earth that its orbital period slows down too much for its rotational momentum, to remain locked.  Kind of like a PLL'ed oscillator when the L-C tuning is adjusted too far away for it to remain locked on frequency.

What I find fascinating is the extreme coincidence that the moon is just the right distance from the earth that its disc appears to an observer standing on the ground to be almost precisely the same size as that of the sun. But I believe that, also, is due to tidal locking effects, but as it slows down in its orbit due to frictional drag (space isn't a perfect vacuum, but filled with scattered molecules of gas, plus dust and other junk, plus drag from the solar wind as well as loss of energy due to tidal effects), it will eventually make a visibly smaller disc in the sky.  By then it will have lost its rotational synch as well. And there may not be any humans on earth by then to observe this.