[LEAPSECS] Schedule for success

[Tony Finch]

On Wed, 31 Dec 2008, Daniel R. Tobias wrote:

> On 31 Dec 2008 at 8:08, Poul-Henning Kamp wrote:

>

> > Notice the "near": the 0° meridian no longe passes through the

> > transit instrument there.

>

> They moved it?  (The meridian, or the transit instrument?)


The meridian moved several times. About a year ago I wrote the
following...

When GMT was chosen to be UT at the Washington conference in 1884, there
was a requirement that the prime meridian passed through the transit
instrument of a good observatory which could thereby keep the time on
which longitude depended. By the start of WWI, people were developing
long-distance precision time transfer using radio telegraphy, so the
dependence on a single observatory and master clock was no longer
necessary or desirable. The French established the Bureau International de
l'Heure (BIH) which defined and maintained time and longitude based on
observations from multiple observatories, and thereby took back what they
had lost three decades previously at the Washington meridian conference.

This should have made no difference, but owing to longstanding
inaccuracies in the official longitudes of the various observatories, the
zero longitude of the BIH's geodetic model no longer matched the Airy
transit circle at Greenwich. Further errors (of about 10m) were introduced
when the Royal Observatory moved from Greenwich to Herstmonceaux after
WWII. The biggest change, however, happened when the standard geodetic
model was fixed to be geocentric, i.e. to have a common centre of
revolution with the Earth. Although the new model matched the old fairly
well at the equator, it ended up being off by about 100m at Greenwich.
This discrepancy is still preserved by WGS84, the world geodetic system
used by GPS. If you go to Greenwich you'll find that not only does your
GPS receiver disagree with the markings on the ground, but also your
Ordnance Survey maps - because the OS uses a datum established before the
construction of the Airy transit circle.

Tony.
-- 
f.anthony.n.finch  <dot at dotat.at>  http://dotat.at/
FAIR ISLE: NORTHEASTERLY 4 OR 5, BECOMING VARIABLE 3 OR LESS FOR A TIME, THEN
WESTERLY OR SOUTHWESTERLY 4 OR 5. MODERATE, BUT ROUGH AT FIRST IN NORTH.
MAINLY FAIR. GOOD.