Tourists line up at Greenwich to straddle the brass strip that marks 0° longitude. Their phones disagree. The zero used by GPS runs through the park 102 metres to the east — and the reason is gravity, not error.
The brass strip follows the Airy Transit Circle; the zero your phone uses sits on open grass to the east.
In 1851 the Astronomer Royal, George Airy, installed a transit telescope at the Royal Observatory. The line through its eyepiece became the Prime Meridian, and later the brass strip in the courtyard.
Satellite navigation uses the IERS Reference Meridian, which became 0° when the Bureau International de l’Heure set the zero in 1984. It passes about 102 metres east of the telescope.
At Greenwich’s latitude, one arcsecond of longitude is about 19.3 m on the ground (computed), so the gap equals about 5.3″.
Each observatory counted longitude from itself, and each one also kept its own clock time.
France measured from Paris, and earlier from the island of Ferro (El Hierro), which a 1634 ordinance put exactly 20° west of Paris. Russia used Pulkovo, Italy used Rome’s Monte Mario, and the U.S. used its Naval Observatory. Ships crossing oceans had to convert longitudes between chart systems. Clock offsets above are computed at 4 minutes per degree.
Forty-one delegates from 25 nations met to pick one meridian for the world.
The result was decided in advance by practice: 72% of the world’s shipping already used charts based on Greenwich. The resolution passed 22 to 1. San Domingo voted against, and France and Brazil abstained.
The conference also passed resolutions for a universal day starting at Greenwich midnight. Britain had already made Greenwich time its legal time by law in 1880.
From Ferro to the satellite era: how 0° moved from islands to telescopes to a model of Earth.
France held out for 27 years. Its law of 9 March 1911 set legal time to “Paris mean time retarded by nine minutes twenty-one seconds”. That is Greenwich time, but the law never names Greenwich.
Airy’s telescope was levelled to the local vertical. That vertical tilts slightly from a smooth Earth model.
An astronomer’s “straight up” is set by a plumb line, which follows the local pull of gravity. Uneven masses in the crust tilt that line. A satellite system measures position against a smooth mathematical ellipsoid instead.
At Greenwich the east–west tilt is about 5.3 arcseconds. Malys and colleagues (2015) showed that this deflection of the vertical accounts for the entire offset. The ground never moved, and neither did the brass.
of world shipping already used Greenwich charts when the 1884 vote was held.
the offset France wrote into law in 1911 to adopt Greenwich time without saying “Greenwich”.
the gravity tilt at Greenwich that puts GPS zero 102 m from the brass line.
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