Geography
The brass line marking zero longitude at Greenwich is about 102 metres off from true zero
The historic prime meridian line at the Royal Observatory Greenwich sits roughly 102 metres west of zero longitude as defined by GPS and the modern IERS reference meridian. The gap comes from 'deflection of the vertical': 19th-century astronomers aimed telescopes straight down using gravity and mercury pools, but local variations in Earth's gravity field tilted their 'straight down' slightly off from the planet's true geometric centre.