Geoscience Reference
In-Depth Information
It would be surprising if, after all these changes of definition and improvements
in technology, the Prime Meridian remained aligned with the strip laid down at
Greenwich and the longitude of Paris remained unchanged from the measurements
by the historic French geodesists. The Prime Meridian of ETRS89 in fact differs by
100 meters from the line through the Airy Transit Circle telescope that defines the
original Greenwich Meridian at the Royal Greenwich Observatory. All those tourists
at Greenwich who get their friends to take photographs as they stand astride the
meridian strip have one foot in the western hemisphere and one in the eastern hemi-
sphere, according to the historic definition of the Prime Meridian, but if the tourists
used a satellite navigation device to make the journey to Greenwich Park, its GPS
receiver will read about 3 arc seconds west of zero. The Prime Meridian according
to the GPS system runs across the car park of the Pavilion Tea Rooms and close to
the park bench along the ridge footpath that lies beyond the statue of Wolfe. The
longitude of Paris has shifted by an equivalent amount. Both the Greenwich and
Paris meridians have become in science entirely abstract, and in their modern form
are not geographic locations at all.
