Why the Greenwich line shifts on a GPS map
The brass strip at the Royal Observatory marks the meridian established by the observatory’s nineteenth-century astronomers. They determined north–south direction with a transit instrument: a telescope aligned to the local vertical, the direction a plumb line points under gravity. Their measurements made Greenwich the reference for zero longitude on maps and charts.
GPS coordinates use a different foundation. Satellites and receivers calculate positions in a global reference frame centered on Earth’s mass, rather than aligning the whole planet to one observatory’s plumb line. Earth’s mass is unevenly distributed, so gravity’s local direction does not point exactly toward the planet’s center. That small mismatch matters when translating the old astronomical meridian into the modern global system.
A difference in reference frames, not a mistake
At Greenwich, the modern zero-longitude line falls roughly 100 meters east of the historic marker. The gap is often described as a few hundred feet. It does not mean the observatory’s marker was carelessly placed, or that GPS has misplaced the meridian: each line follows a different method of defining direction and position.
The distinction is easy to see on site. Stand over the famous brass line and a GPS device may show a longitude slightly greater than zero; walk a short distance east and it can reach zero. The marker records how astronomers first fixed the meridian, while the GPS reading reflects a coordinate system built to work across the entire globe.
