If ever you find yourself in London, I would very much recommend a visit to Greenwich. It’s a great day out – and includes attractions such as the Maritime museum, the old Naval College, and what remains of the tea clipper Cutty Sark after a recent devastating fire. But for a physicist like me, Greenwich really only means one thing, which is the Royal Observatory, home to the Greenwich Meridian.

Now, I grew up in Sussex, pretty well due south of Greenwich, and must have crossed the meridian about a zillion times in my life, but still there is something special about seeing it in marked on the ground in Greenwich itself. What I hadn’t appreciated before my visit last week was why the meridian is located exactly where it is. It’s simply the line of longitude that ran through the Astronomer Royal’s telescope. Where he put the telescope, that was where the prime meridian of the world was. In fact, there have been several Greenwich meridians in the past – each time a new telescope was built at the observatory, they simply relocated the meridian (and therefore by default the longitude of everything on the surface of the earth) to make it pass through this new equipment. In fact, the Ordnance Survey (the map-makers in the UK) still use an old version of the meridian that is slightly displaced from the current ‘official’ Greenwich meridian.  In the era of GPS, I’m surprised they can still get away with this discrepancy.

The meridian of course is to longitude what the equator is to latitude: the ‘reference point’. Only in the case of latitude, there is an obvious place to put the zero point (namely the equator), but in the case of longitude, one meridian is much like another. Greenwich might think it’s a special place, but, in reality, it just happens to be where the observatory was built – there is no reason it had to be there. This leads to the problem of longitude – which I’ll talk about later – how can you find your longitude when you are in the middle of the ocean? (Latitude is easy – the elevation of the sun at midday, amongst other things, will tell you that – but longitude is nasty.) The solution lies on display in the observatory – namely John Harrison’s clocks – but more of that next time.

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