The Prime Meridian line is 100 metres away from where we thought, but at least the anomaly is only confined to Greenwich

A small crumb of comfort is that the anomaly is only at Greenwich and it does not mean that the entire lines of longitude elsewhere in the world all now have also be move 102 metres east

Greenwich really is a place where time and space meet. The Greenwich meridian does not just mark the man-made divide between east and west, it was the place where the clocks were set in the form of Greenwich Mean Time, now known as Universal Time.

The fact that the real meridian is actually 102 metres (334 feet) east of the Victorian telescope used to determine the meridian is a feature of the way the Greenwich meridian was calculated with the help of a basin filled with mercury to ascertain the precise vertical from the telescope to the “clock stars” passing overhead.

This method introduces errors because for it work exactly right, it assumes the Earth is perfectly round, which it is not, and that the gravitational pull of the surrounding terrain is uniformly the same as in other parts of the world, which is not the case.

The telescope at Greenwich, known as the Airy Transit Circle, named after a former Astronomer Royal, measures star positions for determining local time. To do this precisely it has to mark the exact moment a given clock star passes directly overhead, which of course means that you must be sure that you are pointing the telescope absolutely perpendicular to the Earth.

This is where the dish of mercury comes in, because it is assumed the surface of the liquid metal is exactly parallel to the surface of the Earth. Unfortunately, if the Earth is not exactly round and the local gravitational effects are not uniform, then this can give a wonky reading, which is the reason why the real meridian is not running through the Airy Transit Circle, but near a litter bin on a nearby footpath.

We know this because satellites can effectively ignore these surface anomalies and draw a line directly to the centre of the Earth at a perfect tangent to the ground. A small crumb of comfort is that the anomaly is only at Greenwich and it does not mean that the entire lines of longitude elsewhere in the world all now have also be move 102 metres east. Phew.