The Problem of Zero Longitude

The precise point of zero longitude (the line from which all longitudes around the Earth are measured) was defined by an official scientific committee in 1884 as 51°28’38 North, 00°00’00 West. At the time these were the exact co-ordinates of the Airy Transit Circle eyepiece, (i.e. the cross-hairs in the observatory telescope) at Greenwich, London, UK. However, the Greenwich line is now out-of-true according to the modern WGS84 datum, which most present-day geographers (including this website) use as a standard.

The historical Airy Telescope at the old Greenwich Observatory is now at 51°28’40.4″ N, 0°00’05.4″ W

How can this happen?

The Earth is not actually a very stable object, in any sense. Thanks to continental drift, the UK is shifting at a rate of 2.5 cm (one inch) per year in a SW direction. Other significant factors such as changes to the horizon at Greenwich, variations in Earth’s gravitational field and changes in the way longitude is defined have all contributed to push the Greenwich Observatory right away from where it used to be in 1884!

So where is the originally-defined zero point now?

The zero-longitude point as originally defined in 1884 is now right by one of the footpaths in Greenwich Park. It can be found 141yds (127m) SE of the officially marked line at the old observatory (where people flock in their hundreds to have their photo’s taken supposedly straddling zero longitude!). The exact point is unmarked – it would have to be shifted every month to keep it accurate.

What are the implications for my place of birth?

It’s an odd thought, but yes, by the time you are 50 years old, your exact place of birth could be measured as 50 or more metres away from where it was when you were born, depending on exactly where in the world you were born. The birth-places of some historical characters may have shifted a mile or more, enough to make slight differences to their birth-chart if the modern co-ordinates of that place are used.