The city of London is littered with grand remnants of empire. Museums display plunder from cultures around the world. Monuments commemorate the men who won battles against rival empires. Profits from the slave trade and enslaved labor paid for some of the grand homes and commercial buildings.
An even more compelling reminder of Britain’s empire, for me, sits east along the River Thames from the city’s center. A few years ago, a guide took my spouse and me there. We rode a boat up the river to Greenwich, passing the Docklands, where merchant and military ships once unloaded goods and took on supplies. From the ferry pier, we headed toward Greenwich Park, where broad green lawns sloped up a hill. At its top sits the Royal Observatory, built for the king in 1675.
From this spot, England claimed dominion over place and time.
The hill isn't really this steep, but the view along the Thames is great. An
unknown artist painted this around 1685, when the Observatory was new.
Royal Collection Trust / © Her Majesty Queen Elizabeth II 2021
As we walked up the hill, our very nerdy guide paved the way to the Observatory with background, covering a little history, a little astronomy, and a little (stay with me here) maritime navigation.
If Earth wasn’t moving, you’d see the same pattern of stars overhead every night. But the stars seem to drift, so they’re not reliable signposts by themselves. You need to know what time it is. Not just the time where you are, but the time in a second location, at the same moment. Then you can calculate your exact distance from that location.
Outside one of the Observatory’s buildings, we found a long, steel strip embedded in the ground, with lots of people posing around it. The strip marks the prime meridian, the imaginary line that runs from North to South poles through the Observatory. Starting in the 18th century, English sailors calculated their distance from this line as they traded and conquered their way around the planet.
Through a telescope lined up along the prime meridian, the Astronomer Royal watched the stars, including our Sun. When it reached its highest point overhead, he set the Observatory’s clocks to noon. The time at that exact spot became the official time by which all English sailors navigated.
© National Maritime Museum, Greenwich, London
Here’s the technical bit. Mapmakers drew meridians, also called lines of longitude, from pole to pole all the way around the globe. Starting with zero at the prime meridian through Greenwich, English mapmakers numbered these lines of longitude in degrees. They counted 180 degrees west and 180 degrees east from the Observatory. (Here in Boulder, Colorado, I’m sitting near 105 longitude west.)
Pearson Scott Foresman, CC BY-SA 3
So there are 360 degrees total around the globe and twenty four hours in a day. Just by dividing, you find that one hour’s difference between the time in two places equals 15 degrees of longitude in distance (360/ 24). If it’s noon on your sailing ship and eight hours later in Greenwich, then you’re at 120 degrees longitude, measured from the prime meridian (8 x 15).
TL;DR: Wherever Britain’s sailors went, as they calculated their location they affirmed that England was the center of place and time.
To share the correct time, the Observatory had a big sphere mounted on a pole on the roof, visible to ships up and down the Thames. Just before one o’clock every day, the sphere rose on the pole. Precisely at one, it dropped. Ships’ navigators set their chronometers accordingly. It’s still operating! But we weren’t on time to see it.
When the Shepherd company invented a synchronized clock system, the Observatory installed it. For years that system conveyed the correct time to Britain’s railways. It also ran the large clock at the Observatory’s entrance, by which passersby could set their watches.
Meanwhile, of course, other country’s navies and merchant fleets used their own standards, usually the time and the meridian in their capitol cities. With different systems for knowing when and where they were, diplomats, merchants, and scientists in different countries had a hard time dealing with each other. Finally, in 1884, representatives from a couple dozen countries met to settle on a standard. They chose the British time and meridian. To this day, the prime meridian for the whole world passes through Greenwich.
On our visit, I walked over the prime meridian purposefully, moving from Eastern to Western hemispheres with one step. Our guide pointed out all the historical gadgets with which Astronomers Royal kept track of time. I briefly understood all the technical things he said and then couldn’t explain any of it until I did a lot of reading. He mentioned that Greenwich Mean Time was replaced in 1972 with a more precise standard, Coordinated Universal Time (UTC), measured by the vibration of atoms. I told him that we live within a mile of one of those atomic clocks in Boulder, Colorado.
I kid you not: he dropped to his knees in the middle of the exhibit about time.
Peter, our fabulous guide from Context Travel, armed with enthusiasm, knowledge, and visual aids.
The National Institute of Standards and
Technology in Boulder, home of an atomic clock.
For more information:
PBS. (2008) Lost at sea: the search for longitude. Internet Archive. https://archive.org/details/LostatSeaTheSearchforLongitude
Rosenberg, M. (2018) What is the prime meridian? ThoughtCo. https://www.thoughtco.com/what-is-the-prime-meridian-1435653
Schuler, CJ . (2014) Ships, clocks and stars: the birth of navigation. Here360. https://360.here.com/2014/08/06/ships-clocks-stars-birth-navigation/
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