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We’ve found the real 0° meridian!

And it turned out to be a that little path next to the Royal Observatory in Greenwich, not the Prime Meridian line. The 0° meridian is what the GPS uses for global navigation, the discrepancy results from the fact that the Prime Meridian was originally measured without taking it into consideration that the Earth isn’t a perfect smooth ball (if the measurements are made inside the UK, as it it was originally done, this does’t lead to as much discrepancy as when vaster areas are included).

Simon standing with one foot in the Western hemisphere and the other one in the Eastern hemisphere
The GPS determines the longitude of the Prime Meridian as 0.0015° W
Simon tried to use JS to program his exact coordinates, but that took a bit too long so we switched to the standard Google Maps instead
The Prime Meridian from inside the Royal Observatory building
Looking for the real 0° meridian: this is an open field next to the Royal Observatory. At this point, the SatNav reads 0.0004° W.
And we finally found the 0° meridian! Some 100 meters to the East of the Prime Meridian
The 0° meridian turned out to intersect the highest point on the path behind Simon’s back!
Simon and Neva running about in between the measurements of longitude
Astronomer Royal Edmond Halley’s scale at the Royal Observatory
Halley’s scale is inscribed by hand