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Intersting posting ... I think the history of science is full of cases in 
which the detection of phenomena of which we were previously unaware follows 
someone paying attention to what at first seems a minor detail.

-----Original Message-----
From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM] On Behalf Of George Huxtable
Sent: Wednesday, July 28, 2004 1:29 AM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: no leap second coming in December


Thanks for information from Richard Langley and Andres Ruiz about this topic.

Perhaps I can summarise the situation as follows-

Right through most of the 20th century, up until 1994 or so, the difference 
between Ephemeris Time and Universal Time , or delta-T, increased each year 
by about 0.7 seconds. But over the last six years or so, this quantity has 
stopped increasing and at present remains roughly constant. This must be the 
result of the Earth's spin speed having increased, by about 1 part in 40 
million, rather suddenly, within a very few years.

I know that delta-T has fluctuated significantly in the past, but this time 
there seems to have been a rather sudden increase in spin speed, by quite a 
large amount. What could be the cause of this rather dramatic change? One 
part in 40 million may not seem a lot at first sight, but in view of the 
immense angular momentum of the Earth, some large torques must have been at 
work.

I would like to muse about some possibilities, aware that it's somewhat 
off-topic for navigation, but asking for a bit of indulgence on that score.

It can't possibly be due to tidal friction, which causes angular momentum to 
be transferred from the Earth to the Moon, can it? That is strictly a one-way 
process, which can only ever act to slow the Earth's rotation, and never to 
speed it up.

Could the Earth have shrunk slightly, perhaps? That would speed up the spin, 
as it does for an ice-skater doing a pirouette, as she draws in her 
outstretched arms. If the Earth radius were to shrink by one part in eighty 
million, with all its inner layers shrinking in proportion, that would be 
enough to do the trick. That needs no more than 3 inches or so shrinkage in 
radius. I suspect geodesists would be able to detect such an unlikely event 
using modern technology.

It couldn't possibly be due to interaction with the moving atmosphere, which 
doesn't carry nearly enough mass to have sufficient effect.

If the oceans were to acquire a change in East-to-West drift right around the 
Earth, including the Drake passage round the Horn, that would speed up the 
Earth's rotation, which is in the opposite direction. But to have sufficient 
effect, such a current drift would have to be sufficiently large that it 
would have been perceived already, according to my rough estimates.

We keep getting told that the polar ice caps in Antarctica and Greenland are 
melting. However, this displacement of mass, from being locked in the ice 
caps near the Earth's axis, where it has little effect on the Earth's 
rotational inertia, to disperse around the oceans where its effect on inertia 
would be greater, could only slow the Earth's spin, not speed it up.

The only other possibility that I'm aware of (and listed by Richard
Langley) is a change in the circulating currents within the Earth's liquid 
interior. We know that these exist, and change, because these motions in the 
conductive liquid result in the ever-changing magnetic field pattern of the 
Earth. It seems most likely, to me, that there has been a significant 
increase over recent years, and a rather sudden one, in the component of 
overall swirl in the opposite direction to the rotation of the Earth. I 
wonder whether any such changes may be detectable in terms of a sudden effect 
on the Earth's magnetic field over recent years. It's just a thought.

I admit to knowing little about geophysics, and hope that others, wiser about 
these matters, will correct any errors in my speculations.

George.

================================================================
contact George Huxtable by email at george@huxtable.u-net.com, by phone at 
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, 
Southmoor, Abingdon, Oxon OX13 5HX, UK. 
================================================================

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