Longitude zero

This linePrime medirian marker at Greenwich

represents the “prime meridian,” our planet’s agreed line of zero longitude.  It’s a steel strip across one of the paths surrounding the famous old observatory at Greenwich.

King Charles II founded his Royal Observatory in 1675.  The idea had come, the year before, from Jonas Moore, chief of the Ordnance (artillery) Office; and Christopher Wren, architect of this and many other London buildings, chose the site.  It is at the crest of a surprisingly steep slope looking north toward the Thames.

Greenwich Observatory

The red ball drops at 1 PM so that ships on the river can set their clocks to Greenwich Time.  To the left of my bicycle are the 24-hour clock and the standard Imperial Yard and other measures; behind is the path leading to the prime meridian marker.  Someone is emerging from the steeper path.  Beyond the lower park is the National Maritime Museum; beyond the river are office towers of the modern business district called Canary Wharf.

The king appointed John Flamsteed as first Astronomer Royal.  Astronomy merited royal support because of its seniority among the leaping sciences of the seventeenth century (Galileo, Kepler, Newton, Halley, Huygens – and Robert Hooke, who probably helped Wren design the observatory), but even more vitally for the hope it offered “to find out the so much desired longitude of places for the perfecting of the art of navigation.”

If you read Dava Sobel’s 1995 best-seller Longitude (full title Longitude, the true story of a lone genius who solved the greatest scientific mystery of his time), you’ll vividly remember how ships lost their way and collided with capes, because they did not know how far west they were.  Latitude is easy to find from the stars, but longitude is far more difficult.  The established astronomers believed the solution would come from measurements and calculations of the Moon; the “lone genius” (John Harrison) found it by building a sea-worthy clock.

The world agreed to base its systems of longitude and time on this observatory, and the observatory worked for over two centuries to carry out its other mandated tasks, “the rectifying of the tables of the motions of the heavens, and the places of the fixed stars.”  But its operations had to be moved in 1948 to Herstmonceux Castle in Sussex,


and then in 1990 to Cambridge, leaving the Old Royal Observatory as a museum.

Actually the Herstmonceux move had been germinating since 1924 and took from 1948 to 1957.  It was – hard for me to believe at this moment – even more of a struggle than our move from Lyme Regis to Greenwich, which took us and five good-natured Polish movers from early in the morning to late in the night of April 16.  I spare you more about that strenuous day, except that it was a lovely spring day and I spent the middle of it riding in the truck beside driver Pyotr.  His GPS system to my surprise started by taking us along Dorset lanes that I had not seen since exploring them many months before and may never see again; and then (about half way between Lyme and London) close past StonehengeStonehRoadthough in the opposite direction to this sketch from Astronomical Calendar 2003.

7 thoughts on “Longitude zero”

  1. I loved visiting Greenwich in 1997. But I was surprised at the time, and remain so that the International Reference Meridian is a full 102.4779 m east of the Prime Meridian as marked by the Airy transit circle. At the time I neither found nor heard of any markers for the actual international ‘prime meridian’.

    I know the IRM was derived from an average of the observations at many observatories, and there are some other explanations for the discrepancy here:

    But it seemed to me, when I looked as closely as I could into this, that the observational errors during the time when the IRM was being refined were much less than 100 m, so I’m still puzzled. Another factor I tried to figure out was the differences caused by “deflections of the vertical”, since local land masses and other effects can cause the plumb lines they used for observations to cause them to be off when compared with other measures.

    Do you or any of your readers have a full accounting of where the over 100 m of difference comes from?

  2. Guy, your place of residence has been edging ever closer to astronomical “zero” for reference and ease of calculations. I believe that at one time you were living nearly 100 degrees west. It might make for an interesting personal space time map.

    1. Good idea, Larry. Each person could have a graph of longitude against time. Not counting places merely traveled in (such as Indonesia), I’ve lived at longitudes from Iran to California, that’s about 50 east to 120 west, so my graph would show a sort of oscillation narrowing to the center. Don’t much like the idea; I had imagined moving ever outward, not inward.

      1. I imagine that any oscillation, if followed long enough, will move outward, and then inward, and finally to rest.

  3. The evolution of humanity’s understanding of “time & place” is so fascinating. I visited Stonehenge only once, as part of a college group from the USA, back in 1972. What a grand place (although the British bus driver who gave us directions… teased us by saying that he didn’t understand why so many people were interested in “a bloody heap of stones”. Thank you for this article and many more you have posted.

    1. Hello, David. I think Stonehenge has moods, and can sometimes seem like “just a heap of old stones”. In the sketch I re-used here, the mood was glowering because of the cloud front and the roaring traffic on the road along which we were cycling to get there. When I passed by this month, the mood was more peaceful and I wish I had made a sketch of it – the skyline not far away to my left, with a few people wandering among the stones, under a pleasant spring sky. The silouetted people and stones seemed like friends.

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