http://tomscott.com - http://twitter.com/tomscott - If you travel to Greenwich, stand on the famous Prime Meridian Line -- which is marked with a physical line and a sculpture at the Royal Observatory -- and look at your GPS, it won't read 0° longitude. It'll be slightly out. Who's right? And why? Many thanks to my lovely cameraman Tom Morris (http://tommorris.org), who dealt with the cold wind admirably!