# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Japan earthquake speeds earth's rotation
From: David Fleming
Date: 2011 Mar 15, 10:21 -0400
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The whole earth is rotating to the east with constant angular momentum.

For Japan to move east there had to be a brief period of time in which it rotated faster than it does on average.  During that time the rest of the earth rotated more slowly than it does on average.  It is that brief slower rotation that accumulated to a different angular position of the earth relative to the universe that results in the 1.8 microsecond time shift.

This is independent of changes in moment of inertia which is what the spinning ice skater refers to.  Changes in moment of inertia lead to continuing changes in the rate of rotation, while a shift in position not involving a change relative to the axis or rotation are transient changes which change time but not rates of rotation from before and after the event.

Dave Fleming
----- Original Message -----
From: Frank Reed
Sent: Tuesday, March 15, 2011 7:51 AM
Subject: [NavList] Re: Japan earthquake speeds earth's rotation

Gary, you wrote:

"I think this is an error.
"The closer the mass
> shift during an earthquake is to the equator, the more it will speed up
> the spinning Earth."
moving the mass closer to the center of rotation will cause a speed up due to the "pirouette effect" which is actually a manifestation of the conservation of angular momentum."

Well, it's not an error, but it's rather poorly described. If you move a mass ten feet away from or towards the axis of rotation, the effect on total angular momentum is greatest at the equator because the speed is greatest there. This can be confusing because away from/towards the axis of rotation are two very different things near the poles and near the equator. Moving a rock ten feet away from the axis when you're near the north pole means rolling it south. Moving a rock ten feet away from the axis when you're near the equator means lifting it up into the air. If you roll a boulder north or south along the ground and assume that its altitude above sea level doesn't change, the biggest changes in total angular momentum will occur at 45 degrees latitude.

Here's an interesting puzzle: if global sea levels are rising at a rate of about one foot per century (which is what the tide gauges show), how much should the rate of the Earth's rotation be changing?

-FER

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