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Chilean Earthquake Shortened Earth's Day
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 ...'"
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.
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.
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 .