What is the prime meridian? It sounds like a nice cut of beef, but is quite a bit more.

In fact if you have ever traveled by ship or plane, the prime meridian has been an integral part of your journey. That’s an important role for something you can neither see nor touch, unless you count the facsimile of it created by the British Government at Greenwich, England.

Greenwich is a borough in southeast London and home to the Royal Observatory. It is also the location where the prime meridian passes through. The prime meridian is the one at which longitude is 0 degrees. Sometimes the zero meridian line is also called Greenwich Meridian.

It separates the eastern and western hemispheres. There is actually a line set in stainless steel at the Royal Observatory. Here, you can stand in both hemispheres at the same time by placing your feet on either side of the prime meridian.

The author stands on the prime meridian.

The Royal Observatory is latitude 51 degrees, 28 minutes and 38 seconds north of the equator and longitude 0 degrees, 0 minutes and 0 seconds (51º28'38" N – 0º0'0" O/W).

Founded by King Charles II in 1675, the Royal Observatory is set on the banks of the River Thames just four miles (6.4 km) downstream from Tower Bridge. As Great Britain grew into a powerful nation of seafarers, British mariners kept their timepieces on Greenwich Mean Time (GMT) in order to calculate their exact position.

GMT is mean solar time, theoretically defined as the moment when the sun crosses the Greenwich meridian. Astronomer Royal John Pond had a time ball installed at the Observatory in 1833 that was dropped daily to mark the exact moment of 1 p.m.

The ball is still used today although GMT was replaced as the international time reference by Coordinated Universal Time (UTC) in 1972, which is no longer based upon the somewhat irregular daily rotation of the earth but maintained by a set of much more accurate atomic clocks around the world.

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Today, the Observatory is part of the National Maritime Museum and one of the most famous features of “Maritime Greenwich” – a UNESCO World Heritage Site since 1997. Tourists spend the day straddling it for photographs and asking each other what does it mean? Inside the observatory there are galleries with charts and graphs, astronomical and navigational tools and tons of literature explaining the meridian and how it works.

When man first started to travel the oceans, he needed reference points to know where he was. This was a bit hard to do in thousands of miles of water where the only reference point was the horizon, so he devised latitude and longitude.