Imaginary Lines: A Meridian Primer


leadprimemeridian2What 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 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.

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.

Latitude can be defined as the angular distance, north or south, from the equator measured at 90 degrees.

Now to thoroughly confuse everyone, here is the Webster’s dictionary definition of longitude: It is the angular distance measured on a circle of reference from the intersection of the adopted 0 meridian with the reference circle to the similar intersection of the meridian passing through the object.

This is enough to make someone want to stay home.

Since the earth is basically round, imagine lines drawn from the north to south poles, all of them equal distance from each other at the equator. (Don’t ask about the equator, this is confusing enough.) These are longitudes. If you think of what a peeled orange looks like, you will get an idea of longitude lines.

Latitude lines run parallel to the equator where they are larger, to mere dots at the poles. In reality, these lines are about 69 miles or 110 km apart. This simple grid system divides the earth into equal parts supposedly allowing a traveler to know where he or she is in reference to each imaginary line.

So a meridian is just a place from which to measure distance and establish location. Once a prime or main meridian was established, it was also used to determine international time since it had to start somewhere regardless of what Albert Einstein said. When not confusing travelers, it also divided the earth into two equal hemispheres.

All of this was easier said than done. An ancient Greek astronomer named Hippachos was the first to introduce longitude after studying the movement of stars. He set up his own meridian line through Rhodes thinking Greece to be the center of the universe.

Later came the astronomer Ptolemy who likewise declared a meridian line to run through the Canary Islands. This was accepted at the time as the boundary of the known world. If you passed the Canaries, you fell off into total blackness.

Meridians came and went, piling up like opinions with everybody having one until 1851. A British astronomer named George Airy decided to run his meridian through the Royal Observatory at Greenwich England. Why not? No one had put one there before. Unfortunately for Mr. Airy, no one other than himself used it.

Nothing came of this until the first International Geographic Congress convened in Antwerp, Belgium, in 1871. They discussed Mr. Airy’s proposal and smiled over tea at it, but nothing was decided. The second Geographic Congress was held in Rome in 1875, and once again European bureaucracy carried the day and nothing was done to divide the earth with tiny little lines.

Finally in 1884, the Geographic Congress decided to meet in Washington, D.C. Perhaps it was the distance from former locations that got people off their behinds, but at this meeting, it was finally decided to adopt a single, “prime” meridian from which to measure distance and time. Mr. Airy’s meridian was as good as any, so they decided to adopt it as their own.

So the prime meridian was settled upon in Greenwich and people traveling the globe could now look at a map and know where they were.

I am still confused by the whole thing, and whenever I am in an airplane, I always look out the window hoping to spot one of those tiny lines.

If You Go

Greenwich is also home to the Royal Naval College, and the Cutty Sark, a famous old Tea Clipper ship, plus the Gypsy Moth, the first tiny sailboat to travel solo around the world. The London Marathon begins in Greenwich every spring.

Royal Observatory, Greenwich

World Heritage Site Maritime Greenwich