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After 1884 all was (more or less) harmony until 1954 when positional astronomy in Britain moved from Greenwich to Herstmonceux and the Airy Transit Circle was finally retired.   By the 1950s international timekeeping was being regulated by the BIH (Bureau International de l’Heure).  The world standard was then based on an average of observations from several observatories and the average reference meridian had probably already wandered around 8 meters from Airy.  The move to Herstmonceux required ‘recalibration’ to be done and this added another 10 to 20 meters.  Continental drift is also responsible for a bit of movement since 1884.


With the advent of the GPS satellite system in the 1980s, accurate navigation could be done by any child using a hand-held computer with more compute power than existed in the entire world in 1954, and with access to 25 satellites in Earth orbit, each carrying two caesium atomic clocks.  The system calculates position by receiving and comparing time signals from any three GPS satellites.  It needs an internal “map” of the world in the form of a computer program.  It wasn’t easy to create the computer map because the Earth isn’t a simple sphere.  It has a complicated shape which required a technique called ‘best fit’ to develop the map based on an Earth geodetic model -- WGS84.   This doesn’t fit the Earth’s surface exactly everywhere but juggles the map shape to find the position where it fits best at the most places it can.   Try as they might, the best fit they could get at Greenwich put the WGS84 meridian to the east of Airy by a tad under 102.5 meters.


That’s the scientific explanation (sort of).  I prefer to tell people that in the best tradition of Flamsteed, Halley, Bradley, and Airy, Dr GPS has built a new (metaphorical) meridian wall which just happens to be another 102 meters to the east.  If it was good enough for Airy...


Why isn’t the WGS84 meridian marked at Greenwich?  I’ll tell you—I don’t know.   Maybe we just like to show off Airy and get people in to see the Transit Circle and the Harrisons!


Mike D.

Flamsteed Astronomy Society

I wonder ...

...why the world’s Prime Meridian at Greenwich isn’t in the same place as the “GPS Meridian” used by today’s satellite navigation systems?                 continued from page 1