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Frank wrote-

When you visit the old Royal Greenwich  Observatory, you can stand in line
with the transit instrument's  building and put one foot on either side of a
stripe in the paving stones  and get your photo taken "with one foot in East
longitude and one foot in West".  My question is, with various revisions and
refinements in the geodetic system,  is that precisely true today? That is,
does
zero degrees longitude still pass  right through that transit instrument's
original mounting point by  definition?

For fans of Google Earth (and if you're not a fan, you will  become one when
you try it out!), the longitude displayed for this location is  0d 0' 05.36"
W. Pretty close to zero, and I assume that the difference is  nothing more
than
the usual difficulty of registering aerial photography exactly  in lat/lon,
but maybe the old RGO is drifting...

==========================

from George-

I may be able to contribute here, with a quote from Derek Howse, "Greenwich
Time and the Longitude", 1997 edn., page 178.

He writes- "Indeed, even the Greenwich meridian itself is not quite what it
used to be- defined by the 'centre of thetransit instrument at the
Observatory at Greenwich'. Although that instrument still survives in
working order, it is no longer in use and now the meridian of origin of the
world's longitude and time is not strictly detfined in material form but
from a statistical solutionresulting from observations of all
time-determining stations which the BIPM takes into account when
co-ordinating the world's time signals. Nevertheless, the line in the old
observatory's courtyard today differs no more than a few metres from  that
imaginary line which is now the Prime Meridian of the world."

By the way, BIPM refers to the Bureau International de Poids et Mesures, in
Sevres, near Paris. I suspect it's all a dastardly French plot, to devalue
the primacy of Greenwich. Can anyone put a number to the amount of that
shift of the zero-reference from the longitude of the Greenwich transit
telescope?

However, the unspecific "few metres" that Howse mentions doesn't seem enough
to correspond well to Frank's reported 05.36"W discrepancy in the Google
Earth position of Greenwich. That would require an Eastward displacement of
the meridian line by some 104 metres. It seems more likely that Frank has
uncovered an error in translating computed Google-Earth coordinates to
actual position on the ground, as he has suggested. That may merit closer
investigation.

=========================

And Frank continued, responding to Peter Fogg-

> Peter, you wrote:
> "In the local paper  recently came across an assertion
> that the northern orbital axis of the earth  (ie; the North Pole) had been
> found to have moved some small (less than the  five and a half nautical
> miles
> cited by Frank) distance 'to the  east'."
>
> Actually the Greenwich "discrepancy" was only 5.4 arc SECONDS of
> longitude.
> But even that is unlikely and probably related to registering the
> imagery. I
> would still like to know if that transit instrument at RGO is still  the
> exact
> zero of longitude. Has anyone on the list turned on a GPS receiver  while
> standing there?
>
> But as for the assertion in the article you read,  the rotational axis
> surely
> could not move five nautical miles without noticeable  effects! That's a
> very
> large change. Speculating, could it be that the article  was talking about
> recent ice movements up there? --not a change in the axis of  rotation,
> but only
> a change in the location where the axis pierces the ice. That  does move,
> and
> changes in the Arctic ice are big news these  days.

From George-

No, ice-drift is certainly not the answer. Remember, the polar ice is
free-floating on the surface of the Arctic Ocean, and drifting with the
surface current, which can shift it several miles PER DAY.

That's what Nansen relied on when, in 1893, he set off in Fram, deliberately
intending to get trapped in the ice North of Siberia. The idea was that over
a period of a few years, the ice would take him over the North Pole (for
what that was worth). After two winters in the ice, when Fram had reached
about 84 deg N, he and a companion "jumped ship" to set off for the pole.
With sledges, kayaks, and dogs they reached just over 86 deg . The ship,
drifting on, eventually reached  about 85 1/2. Because of the unpredictable
drift, there was no hope, or intention, of their navigating back to Fram.
Instead Nansen and Johansen had to find their own way back South, spending
another winter in Franz Josef Land, until they were rescued the next year.
Fram, after another winter in the ice, finally drifted clear near
Spitzbergen, and returned to Norway at nearly the same time as Nansen did.
It's all in "Furthest North", by Nansen (2 volumes).

Peter Fogg's comment is less than specific, in stating-

"In the local paper  recently came across an assertion
> that the northern orbital axis of the earth  (ie; the North Pole) had been
> found to have moved some small (less than the  five and a half nautical
> miles
> cited by Frank) distance 'to the  east'."

He doesn't  state what "small distance" implies. But it's NOTHING LIKE the
"five and a half nautical miles", nor even the 5.4 arc seconds that Frank
actually quoted. Polar wander, over the short term (century or so) is
confined within a circle of about 15 metres diameter.

No, I think that the mistake is more likely this-  Someone (I hope, not
Peter Fogg) has taken the movement of the Earth's pole, which doesn't shift
by more than a few metres, and misconstrued the abbreviation "m" for those
metres, to mean miles instead.

Perhaps the lesson is not to take too seriously what one reads in the local
paper.

Peter asked-

 "Shouldn't any movement away from the North Pole be necessarily to the
south?"

If that's a serious question, then yes, it should. But you can go South,
from the North Pole, along any meridian, and presumably an easterly wander
of the pole involves moving along the 90deg E meridian. Reports of polar
wander use x,y coordinates, which presumably correspond in some way to the 0
to 180 meridian, and the 90 to 270 meridian, directions from the Pole.

"Anyhow, I would imagine that this, if true, would mean that all the lines
of
longitude and latitude would need adjustment."

Not really. The motion of the Pole is mostly in terms of the "Chandler
wobble", with a period of a bit more than a year in which the pole moves in
a small circle of varying diameter, about a mean position that changes
little. The pole, for lat/long perposes, is defined to be the mean position
at the centre of that wobble. Over long periods, no doubt, the Earth's crust
will move with respect to its spin axis (or vice versa), and continents will
move with respect to each other, calling for a remapping of the Earth in
terms of the coordinates that its axis defines.

George.

.

   

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