Greenwich prime meridian

Thursday, 29th November 2007 by

The equator, or latitude 0, separates the northern and southern hemispheres of the globe. Defined as the middle point of the two geographic poles, there's no way anyone could argue about its location.

However the meridian - longitude 0 - has no scientific definition so could basically be anywhere you like on the planet.

The most widely accepted meridian was defined at the Royal Observatory in Greenwich, London. However, even here the meridian has shifted about a bit.

18th century astronomer James Bradley's meridian is still shown on UK maps today, but has otherwise mostly been forgotten. The much more famous meridian, which is now used as the base point for all the world's time zones, was defined by George Airy.

Airy's prime meridian was chosen using the very scientific system of setting up his equipment in the observatory as close as possible to Bradley's equipment, but without getting in the way!

Two miles directly north of Greenwich, on the banks of the River Thames, Airy's meridian is marked out by a line of trees.

Also just to the north of Greenwich is a millennium sundial, which is a few metres off the meridian. Purportedly this was a construction error and the clock will always be wrong by about 8 minutes!

If you were to take your satnav device along to the observatory at Greenwich, or just set longitude to 0 in Google Earth, you'll find yourself outside the observatory at this seemingly insignificant point, 102.5 metres east of the prime meridian.

That's because Google Earth, and your satnav, use the International Reference Meridian, which was defined much later by observing stars from many different countries.

So readers, if you were defining a new meridian, where on the globe would you put it?

More information on the prime meridian, the world geodetic system, how it's used in Google Earth and the prime meridian at Wikipedia.

Thanks to LionelB, Reagan Blundell, Sly Golovanov, Frank Taylor & Roy G. Ovrebo.