Harrison’s Clocks

Harrison’s Clocks solved the problem of calculating longitude at sea. Harrison realized that one could could find one’s current longitude at sea, if one could accurately determine the difference in time between one’s home port and the current location. See the entry on Sextants for more on how. A mariner could use a sextant to calculate the precise arrival of noon at his given location on the sea, and by referring to an accurate clock set to his home port’s time, he could derive his longitude. Harrison spent 31 years before managing to build a clock which compensated for wave motion and changes in temperature and humidity, thus being accurate enough to produce a valid result.

While his experimental clocks and the final pocket watch chronometer are not allowed to be photographed, here are some images taken at the Greenwich Observatory:

Prime Meridian Globe 1Obs, wall with clock and measuresThe 24 hour clockObs, plaque for clock and time ballMeridian Height above sea levelMeasures: one foot

Instead of giving you some dry research, here again is an extract from an early draft of the book. Unless you’re interested in the story of how navigators solved the problem of calculating Longitude at sea, you will see why this draft never made it to the final form of the book. In this version, Josh and Ses have just consummated their affair (happened earlier in this draft), and they are now on their way to visit the Royal Observatory, in Greenwich.

We picked up coffees and croissants from a take-out counter on Romney Road, entered Greenwich Park and headed south, away from the river, up a slight incline.

“Okay sailor boy,” she said, looking around. “Where are we headed?”

“I thought we’d take a walk to the observatory. You can see the Prime Meridian.”

“I don’t get it, this thing with the Prime Meridian. It’s like the equator, only running vertically. What’s the big deal?”

“Yeah, I guess it’s because sailors knew how to calculate their latitude since the days of the astrolabe. But they only solved the longitude problem in the late eighteenth century. For hundreds of years, they knew where they were from north to south on the sea, but not where they were east to west, and they kept running aground. Charles II decided that as an island, it was important for England to rule the sea. In 1675 he appointed John Flamsteed the first Astronomer Royal, to solve the problem of calculating longitude. They built a house for him up there, and the Royal Observatory, there, you can see its dome on the hill. Flamsteed began by standardizing the method of numbering longitude on a chart, until then randomly assigned by the map-maker, by marking the Prime as running between his house and the Observatory. Poor old John… he spent the rest of his life taking moon and star sightings at night, and sleeping during the day. Forty-four years, him and his wife. They never had kids.”

We walked through the open wrought-iron gates of the observatory, into the garden and I said, “There’s the Prime Meridian. If you stand like this, you can have one foot in the east and one foot in the west.”

“Sounds like a split personality to me,” she said placing her feet on the panels of granite that border the aluminum and black-rubber line running across the garden. “If it took that long to figure it out, who did it? I mean Flamsteed didn’t live that long.”

“A Yorkshire clockmaker solved the problem. We’ll see his solution in a moment. It’s upstairs. We’re in the observatory.”

The room’s walls alternated floor to ceiling windows with half-height wooden panels topped by portraits of past Astronomer Royals. We climbed the narrow staircase to the Flamsteed’s living quarters. Their bedroom, with two dummies in the four-poster bed, asleep: the sound of a gentle snore from him.

I said, “You can see why they never had kids.”

I explain that the guy who solved the problem wasn’t even an astronomer. In 1714, the government offered a prize, worth about six million Pounds today, for a method of determining longitude. A while later, a clock maker called John Harrison realised that you could solve the problem with a very accurate watch. In 1773, 31 years and four versions of his clocks later, he proved that he could find longitude at sea within the required half a degree.

I stand, mesmerised by the brass machine in front of me, “This is Harrison 1, or H1 as it’s called.” A gracefully sculpted tower, a slightly oval face and two pendulums suspended about a foot apart, like vertical bridges. “The first clock proved that it was possible to make a pendulum that compensated for wave action at sea.” I said, remembering a movie I had seen, in which the clock was working. The bridges swung first their tops towards each other and then their bottoms, clicking the hands on the face in a delicate dance of the passage of time. (Harrison’s H1 clock was shown working in the A&E production of Longitude, based on Dava Sobel’s book of the same name.)

“But the clock didn’t work,” I continued. “The time-keeping mechanisms sped up or slowed down and the clock’s accuracy declined over time. So H2 and H3 here, were attempts to deal with that, but they failed too. And then this one, well this one worked.”

We stopped in front of H4: a large pocket-watch with a case of two silvered shell-shaped halves, almost five inches across and weighing about three pounds. A white face with elegant black roman numerals and three blue-tipped steel hands. The card underneath read: The most important watch in the world.

“How did he go from that huge piece of brass in that case,” said Ses, pointing at H3, “to this watch that could fit in a pocket?”

“The leap was in a technology he invented, called a bi-metallic strip. It compensated for changes in temperature. This watch maintained the time to within three seconds in twenty-four hours, at sea. An unheard of accuracy for most land-based clocks in those days, and he deserved every penny of the prize.”

I stood staring at the watch and, from behind me, Ses said, “But the dance has changed since old Harrison collected his prize. If you step over here, love, you can see the world’s most accurate clock today. It keeps time to within one second in fifteen million years.”

She stood mouthing the number, to make sure she had read it correctly. “But how can they measure that?” she said.

We were standing in front of the Caesium Fountain Atomic clock, watching the model’s flow of electrons. I said, reading the display, “I guess you use something that vibrates at nine billion times a second, for that’s what a Caesium atom appears to do.”

“Busy little bugger, ain’t it?” she said. I laughed and she said, “To borrow a line from a former tour attendee of mine, could we possibly take on some fuel. I’m starving.”

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