GREENWICH MERIDIAN

The Greenwich Meridian is an imaginary line that divides the world into East and West.
The path of the meridian is marked out on the ground here – if you stand one side of the line you are in the Western hemisphere; stand the other side and you're in the Eastern.
Wilson stepped across the line a couple of times, but complained that he didn't feel as if he was abroad!
Nërp then briefly explained the significance of the Standard Measures (although these have long been superseded by non-physical standards) and of the Greenwich Clock above them (which is apparently not actually a clock in the normal sense of the word):
❛As the United Kingdom developed into an advanced maritime nation' he began, 'British mariners kept at least one chronometer on GMT to calculate their longitude from the Greenwich meridian, which was considered to have longitude zero degrees, by a convention adopted in the International Meridian Conference of 1884.
❛It was the first clock dial to publicly display Greenwich Mean Time and was constructed by Charles Shepherd in August 1852 on the instructions of the Astronomer Royal, George Airy.
❛Airy had seen the system demonstrated at the Great Exhibition in Hyde Park the previous year. Despite labelling himself as the patentee, Shepherd was in constant battle with Alexander Bain, a Scot, who claimed an earlier patent.❜
Nërp then told me in a whisper (so as not to embarrass me in front of Byron) that the actual line of 0º Longitude was a few metres distant from the line marked on the ground, and perhaps that's why Wilson hadn't felt that he was 'abroad'.
I hadn't realised that the longitude line on the pavement was inaccurate, but I think the real reason Wilson didn't feel as though he was in a different hemisphere was because he was standing next to the rest of us in London – but never say this stuff isn't educational!
Byron was still paying close attention to Nërp's explanation, but Wilson had by now wandered off to a nearby snack concession to enquire whether they stocked Spicy Ant-burgers – and if not, why not…