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 Frank, you wrote:
> Actually the Greenwich "discrepancy" was only 5.4 arc SECONDS of
> longitude.

So about 165.4 metres.

> But as for the assertion in the article you read,  the rotational axis
> surely
> could not move five nautical miles without noticeable  effects!

From memory, it was a matter of a few metres. Michael Dorl has contributed a
link to an image that shows movement of the pole against an x/y axis, the
units used seem to be seconds of arc. Perhaps this image relates to an
article that explains it, or the converse?

Then Richard Langley has given a link to an online article that would seem
to illuminate the discrepancy that Frank noticed. I would have thought that
as an arbitrarily chosen zero line of longitude Greenwich would have
remained frozen forever but such is not the case, it seems. The other
interesting nugget of information is that the Global Positioning System
takes all of this into account. Unfortunately this is not always helpful -
often enough the GPS cannot agree with local charts, whether because of
datum difference or chart inaccuracy is irrelevant when you are trying to
avoid the coral!

   

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