Toeing the line
If you visit the Royal Greenwich Observatory you'll often see groups of tourists jostle for position to have their photograph taken "on the prime meridian". The notion that you can stand with one foot in the western hemisphere and one foot in the eastern hemisphere is pretty cool. What seems to elude many of them however is that the prime meridian isn't a place, it's a line. It's a line of longitude runing north/south from pole to pole. If you were so inclined, there's a petrol station on Greenwich High Road where you fill up your car with one foot in the western hemisphere and one foot... well, you get the idea.

The other thing to remember about meridians is that they are the astronomical equivalent of an "imaginary friend". We talk about them as if they are real, but they are only consensual human constructs that mark nothing more than where an astronomer placed his telescope. The prime meridian is in reality the Airy meridian - marking the position where the astronomer royal George Airy made his observations.

This becomes more obvious with a visit to the RGO where you'll also find a Bradley meridian, a Flamsteed meridian and a Halley meridian, each within a few yards of each other; and each marking the position where the eponymous astronomer (that's hard to say) fixed their observing apparatus. Just to confuse matters further if you take a GPS receiver with you, you'll discover the GPS meridian is somewhere else again. It doesn't even run through the observatory; but several yards east near the statue of General Woolfe.

125 years of the Prime Meridian
In October this year the RGO are commemorating the 125th anniversary of the selection of Greenwich as the prime meridian of the world. I've mentioned the 1884 International Meridian Conference here before but if you are interested in the full story behind the prime meridian I'd recommend getting hold of a ticket for Graham Dolan's lecture, The Greenwich Meridian and its significance to the World.

Graham is an entertaining and humorous speaker and an expert on the meridian. A few years ago I attended a seminar day at the RGO entitled 'On the line' - not to be confused with the RGO podcast of the same name. Graham showed a high-speed photo slideshow of markers and monuments along the prime meridian. I've always thought this collection would make for a great Google Maps mashup!