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## Chilean Earthquake Shortened Earth's Day374

ailnlv writes "Days on Earth just got shorter. The recent earthquake in Chile shifted the planet's axis by about 8 cm and shortened days by 1.26 microseconds 'The changes can be modeled, though they're difficult to detect physically given their small size. ... Some changes may be more obvious, and islands may have shifted. ... Santa Maria Island off the coast near Concepcion, Chile’s second-largest city, may have been raised 2 meters (6 feet) as a result of the latest quake ...'"
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## Chilean Earthquake Shortened Earth's Day

• #### Re:How often do such quakes occur? (Score:5, Informative)

by Anonymous Coward on Tuesday March 02, 2010 @01:06AM (#31326592)

The reason the day length changes is because the quake caused a net motion of mass toward the center of the planet. This reduces the moment of inertia, and because of conservation of momentum, the planet's rotation must speed up.

If this happened repeatedly, it would mean that the density of the planet was increasing. That can't happen to any significant degree, because it would involve compression, which requires a source of energy (note -- I don't mean that the increased rotation is due to an energy input, just that it takes energy to compress a planet). Earthquakes just move energy around, they do not create it. So over long spans of time, earthquakes tend to increase the length of the day by about as much as they decrease it. It all depends on whether the net motion was toward the center of the earth or away from it.

This is based on my knowledge of physics, but I am not a geologist, so there may be complicating factors I don't know about. However, I'm pretty sure that the planet's density cannot increase arbitrarily.

• #### Re:How often do such quakes occur? (Score:3, Informative)

on Tuesday March 02, 2010 @01:10AM (#31326624)

Some quakes elevate land which would slow the spin of the earth, but some might lower it, and erosion is constantly lowering the land. After 5 billion years we are probably pretty much in a steady state as far as that goes (earthquakes push it up, and erosion tears it down). Of course other things like tidal forces between the earth and sun are having long term affects which will accumulate overtime (I believe slowing the earth's spin and moving it further from the sun).

• #### Re:How often do such quakes occur? (Score:4, Informative)

on Tuesday March 02, 2010 @01:31AM (#31326748) Homepage Journal
A quick back of the hand calculation tells me that tidal friction is only two orders of magnitude less effect than this. So about 100 days of tidal friction is equal to this event.
• #### Re:Great! (Score:3, Informative)

on Tuesday March 02, 2010 @01:43AM (#31326824) Homepage

While at it, also note that Earth rotation period is ~23h56m, not ~24h because the surface of the Earth facing the Sun moves in the opposite direction of the Earth moving around the Sun.

• #### Re:Great! (Score:5, Informative)

on Tuesday March 02, 2010 @02:03AM (#31326922) Journal
"We should also note that making a pseudo-sphere diameter smaller doesn't shorten the daylight/obscurity period on the said sphere. Even if Earth went to half its current diameter, days should remain the same length unless we change the rotation speed as they suggest in TFA."

Shrinking the sphere and keeping the mass the same will increase the rotation speed. This is why the nutron star left behind after a supernova spins so fast. It's also the reason an ice skater spins faster when they draw in their arms. - Please hand in your geek card on your way out. ;)
• #### Re:How often do such quakes occur? (Score:3, Informative)

on Tuesday March 02, 2010 @02:14AM (#31326972) Homepage

a billion years ain't that long if you're a planet...

Yes, it is [ucolick.org]. Our sun's lifespan is about 10 billion years, and it's half-way through. In other word, the solar system should be having its mid-life crisis now.

• #### Re:GPS affected? (Score:5, Informative)

on Tuesday March 02, 2010 @02:44AM (#31327112)

If a guided missile is launched to fly into a window of an enemy-occupied building, the offset can be enough to make a difference between hitting the window and hitting the wall.

GPS doesn't have the kind of precision to guide a shot like that regardless of whether the time is uncalibrated. If we need to launch a missile into a building and it is imperative that it enter the building through a small window, we would surely use laser or thermal guidance... not GPS.

• #### Re:Nit-Picking Science (Score:3, Informative)

<techkitsune@gmail.com> on Tuesday March 02, 2010 @03:42AM (#31327270) Homepage Journal

"Why this tsunami did not ended on Hawaii like the one in 1960?"

This one is easy - the 1960 earthquake was FAR more powerful than 2010. It was out of steam by the time it got close.

• #### Re:Did this affect climate (Score:5, Informative)

on Tuesday March 02, 2010 @05:30AM (#31327726)

As you probably already know, there are loads of other more mundane ways to instigate an earthquake. This wired article [wired.com] is quite interesting. To summarise:

Build a Dam
Inject Liquid Into the Ground
Mine a Lot of Coal
Drill a Gusher Dry
Create the World’s Biggest Building
• #### Concepcion, Chile: not the second largest City... (Score:2, Informative)

<lathama@latha[ ]com ['ma.' in gap]> on Tuesday March 02, 2010 @07:36AM (#31328250) Homepage Journal

While I am new to Chile I do know my facts. According to http://en.wikipedia.org/wiki/Cities_in_Chile#Largest_cities [wikipedia.org] Concepción is not even in the top 10 Cities. The Conurbation of Concepción is the second largest Conurbation.

Read more and learn: http://en.wikipedia.org/wiki/Conurbation [wikipedia.org]

• #### Re:GPS affected? (Score:1, Informative)

by Anonymous Coward on Tuesday March 02, 2010 @08:36AM (#31328498)

The quake does affect GPS, but not as you might expect.

GPS positioning is based on measuring the distance to GPS satellites, which is equivalent to measuring the clock difference between them. The result of this measurements gives you a position relative to the satellites you see.

Now, to map that satellite-relative position to an earth coordinate, you need to know the satellite orbits. Those are slowly fluctuating anyway. The earth isn't exactly round, the moon is also pulling on satellites, and tidal forces slow the earths rotation every day. Therefore you need periodic orbit updates. (The google terms for this are almanac and ephemeris). Therefore all GPS satellites always broadcasts the current set of orbits. As a result of the quake, these orbits indeed change a bit, relative to the ground, and this is a permanent change. But in the GPS architecture, it's an irrelevant change.

• #### Re:FFS! (Score:3, Informative)

on Tuesday March 02, 2010 @09:04AM (#31328670)

someone didn't get the joke

• #### Re:How often do such quakes occur? (Score:5, Informative)

on Tuesday March 02, 2010 @09:11AM (#31328710)

This is based on my knowledge of physics, but I am not a geologist, so there may be complicating factors I don't know about. However, I'm pretty sure that the planet's density cannot increase arbitrarily.

What goes up must come down...in geology, it's called istosasy. It's sort of like gravitational equilibrium. What sinks in one place is usually offset by a height increase elsewhere. Over years, the small geologic events (and yes, the Chile earthquake is small when measured in geologic units) balance themselves out. I would not worry too much about the lost microsecond. We'll gain it back next year.

• #### Re:Great! (Score:5, Informative)

on Tuesday March 02, 2010 @09:52AM (#31329072) Journal

Einstein said everything was relative and that your perception depends on your point of view.

Velocity is relative, but acceleration isn't relative. Rotation involves acceleration. So it isn't equivalent to say that X rotates around Y is the same as Y rotates around X. (Hypothetical example: consider a universe empty except for a single planet which is rotating. What does it mean to say it's rotating, without reference to background stars? Is it equivalent to a model where we say the planet doesn't rotate? No - we could see the difference in a centrifugal force causing the planet to bulge as it rotates.)

On a funny note Wikipedia says:

"is the astronomical theory that the Earth and planets revolve around the Sun and that the Sun is STATIONARY and at the center of the universe. "

I don't see what's funny? It's perfectly correct that this is what heliocentrism means. And yes, it still wasn't correct - but the point is it was a vast improvement over geocentrism. The Wikipedia article already covers this, at http://en.wikipedia.org/wiki/Heliocentrism#The_view_of_modern_science [wikipedia.org] .

• #### Re:Great! (Score:3, Informative)

on Tuesday March 02, 2010 @11:57AM (#31330672) Homepage

Indeed.

Remember calculus 2 and wheel problems? One approach to the equations was to put the center of rotation at the center of the wheel. Another approach is to consider the wheel as rotating around the point of contact with the surface. One seems non-intuitive, but can simplify a bunch of other equations. Or dealing with rotating CoM equations...

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