Monday, December 10, 2007

The "Three Gorges Dam" & the Earth's rotation

Don't bother reading this unless you just happen to be curious about these sorts of things. Seriously, just skip this one. It's probably self-indulgent and nonsensical.

So Here is an email I wrote to some people at a science radio show. I know it makes me sound crazy, but whatever that's why the title of this blog is a disclaimer. Besides, I think it's pretty interesting.

Hey guys -
So I recently heard that the partial filling of the Three Gorges Dam on June 1 2003 was theoretically supposed to alter the earth's rotation slightly by .06 microseconds (from
). This perked my interest, because theoretically you could use this event to "weigh the earth" without using gravity like a Cavendish type experiment does. I know that the numbers would almost certainly be too sensitive to get any meaningful results, but I still think it is an interesting thought experiment:

In a Cavendish experiment we use gravity to measure the earth's density and then derive values for the gravitational constant and the mass of the Earth. However, in this Three Gorges Dam thought experiment couldn't we could measure the mass of the earth without using gravity? We can measure the mass of the water, we know how the angular momentum of the earth is effected by holding that water back to a "unnatural" line of latitude. (for this angular momentum equation we also need a density map of the earth which I'm pretty sure we measure with seismic refraction.) And we can measure changes in the rotation of the earth with crazy accurate GPS.

Here's why I can see that this data wouldn't be valuable.
Assuming we can even measure the .06 microseconds accurately, can we filter that number out from the 2/1000 sec/day/century change in rotation caused by tidal friction (I'd bet so), and from the constant fluctuation in rotation (up to an order of 10^-6 seconds) caused by the erratic seismic activity in the earth. (I'm guessing no way) (numbers also from

Then even if we can filter out the .06 microseconds, is our seismic refraction model good enough to compare the change of the earth's rotation with the mass of the earth and the water's mass and position? (I'm guessing no again)

But nevertheless, it seems like an interesting thought experiment anyway, you think? If we wanted to bad enough, I could imagine doing a very long term experiment that could use this same concept to measure the Earth's Mass and therefore the Gravitational Constant without directly measuring the gravitational force.

I tried writing to NASA scientists Richard Gross and Benjamin Chao to see what kind of data they collected, but my email bounced back. Oh well, wanted to share the idea with someone and thought you might get a kick out of it.

Thanks guys - Eric

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