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Re: [Phys-l] Earth's axis shift--measured how?



On 03/12/2011 11:05 AM, Richard Tarara wrote:
http://www.montrealgazette.com/news/world/Japan+quake+shifts+earth+axis/4428645/story.html

Every time there's an earthquake we get stories like this.

The question has been asked--measured where?

Doesn't matter. You get the same answer no matter where you
measure it. Adding a constant offset to the position (x)
doesn't change the velocity (d/dx). Hint: Chain rule.

In practice, lots of labs have equipment that can measure the
rotation rate of the earth to high accuracy. I suspect all
you need is a research-grade GPS receiver and a little bit of
patience, but undoubtedly there are other methods as well.

The interesting story here is how amazingly accurately we can
measure time. We can measure changes in the earth's rotation
rate that are far too small to have any practical consequences.

To say it the other way, whenever you're doing an experiment,
if you can convert it to a measurement of time or frequency,
you're in good shape. For example, you might use a Josephson
junction to convert a voltage to a frequency.

Measured relative to what exactly?

Doesn't matter.
-- The laws of physics are invariant w.r.t position, so you
are free to choose an origin.
-- The laws of physics are invariant w.r.t. velocity, so you
get to choose a velocity for your frame of reference.
-- The laws of physics are *not* invariant w.r.t. acceleration.
There is such a thing as absolute acceleration. Also absolute
rotation.

In the short run you can use a gyroscope to measure absolute
rotation.

As for the "fixed stars" : They are not entirely fixed. We can
measure the rotation rate of the arms of spiral galaxies (including
our own) by looking at Doppler shifts. Such rotations are very
small, and can be subtracted out to give a decent reference for
absolute rotation. This does *not* mean that any notion of "fixed
stars" is necessary for defining absolute rotation.

I am aware of "Mach's principle" but I don't buy it. AFAICT
it is just philosophy, without any physical basis or physical
consequences.