Re: The GPS zero meridian club

[rmallett]

On 30/04/2014 20:19, Patrick Powers wrote: Hi Ian,   Exactly so, and it would be interesting to see what dials there were on such a whole number list.  But the snag in all this though is that the definitions have all been changed. The arrival of satellite technology brought with it a global redefinition of the whole earth's ellipsoid and in 1999 the International Reference Meridian (IRM) was decided.  This is fixed but not fixed relative to a point on the earth!  It currently passes something like 5.31 arcseconds east of Airy's meridian or 102.5 metres (336.3 feet) at the latitude of the Royal Observatory, Greenwich. But since 1999 and as a consequence of continental drift and plate tectonics, the IRM has shifted a few centimetres West back towards the Airy meridian. Airy will eventually be right (again!). That must surely be a time for REAL celebration?   In the UK WGS84 latitudes and longitudes are changing at about 2.5 cm per year in a north-easterly direction. In 1989, the International Reference Meridian passed an estimated 102.478 m to the east of the Airy Transit Circle at Greenwich.  There’s more about this here:   http://www.thegreenwichmeridian.org/tgm/articles.php?article=7   So, members of the ‘Zero Meridian Club’ are chasers after ephemera.  They will need to keep coming back every few years simply to catch up...   Patrick A reason to continue ! -- -- Richard Mallett Eaton Bray, Dunstable South Beds. UK

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