Tag Archives: clock

Hot- and cold-running time

After breaking his heart trying to perfect a clock to keep accurate time on the high seas, Harrison refused to give up. He turned instead to perfecting a watch.

No more worrying about pendulums!* Harrison got straight to work on a ship’s timepiece that uses a metal spring and balance wheel. That good ol’ metal spring and balance wheel would do the trick. No problems with a metal spring and balance wheel, no sir.

Well, maybe one small problem. When metal is warm, it expands slightly. When it cools, it contracts. This spring-powered timepiece was expected to be used in both tropical and arctic conditions. The temperature would change the character of the metal, which would make it less reliable, which would make the timepiece less accurate.

Now what?

* Okay, okay, pendula for you Latin nerds.

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Watch works

You remember an escapement is a way of slowly releasing energy that powers a clock. A watch didn’t use a pendulum for its escapement—it used a coiled metal mainspring and balance wheel. You wind the spring tightly and the spring wants to unwind. As it unwinds, its energy is released to oscillate the balance wheel back and forth. As the balance wheel oscillates, it swings a little fork side-to-side which stops and releases a gear. This is the watch’s escapement. No matter how the watch is bounced around, the spring keeps on releasing energy at a steady, reliable pace.

The wound-up spring wants to uncoil, to expand. As it expands, it pushes and turns the balance wheel. But the balance wheel is weighted so it only turns so far and then it swings back. When the balance wheel swings back it tightens the spring again. The wound-up spring wants to uncoil, to expand. As it expands, it pushes and turns the balance wheel. But the balance wheel is weighted so it only turns so far and then it swings back. When the balance wheel swings back it tightens the spring again. (Repeat over and over and over and…)

This beautiful video has French narration but the visuals are self-explanatory: The escapement animation starts at 3:30.

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Keep on trying

With John Harrison’s innovations, his clocks were more precise than any clock had ever been. The bad news was: his clocks still weren’t precise enough to win the Longitude Prize. “The amount awarded under the Act was commensurate with the accuracy of the invention in determining longitude: 10,000 pounds for 1 degree, £15,000 for 2/3 of a degree, and £20,000 for 1/2 of a degree.”

Rather than give up, Harrison tried something different. Instead of designing a precision clock, he turned to designing a precision watch. A watch is an analogue or mechanical (not digital) timekeeping device small enough to carry around with you. You can hold one in your hand. People attached an end of a chain to their watch, attached the other end to a belt loop or button-hole and kept the watch in a pocket.

Random side-note: A pocket-watch and chain play a part in the O. Henry short story, The Gift Of The Magi. https://www.enotes.com/topics/gift-magi Spoiler alert! DON’T unlock the summary until you’ve had the pleasure of reading the story itself.

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The clockmaker

In our last post you saw how to find your location by observing the moon and stars to calculate lunar distance. The object is to know both your local time and prime meridian time, or Greenwich Mean Time. A navigator needs to be an astronomer and a math whiz to use this method.

You may have asked yourself, “Wouldn’t it be easier to keep 2 clocks aboard the ship—one showing Greenwich Mean Time and the other kept to local time?” That’s an excellent question and I’m glad you asked it. In fact, that’s the question John Harrison asked.

John Harrison, English inventor and horologist, 1767.

John Harrison was a cabinet-maker with a side business building and repairing clocks. To win the Longitude Prize, he went for a straightforward solution: build an accurate clock that always, ALWAYS showed precisely the correct time in Greenwich.

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Let me see what moons are like on Jupiter

The moons of Jupiter travel around her at a regular rate, like the hands of a clock. Galileo thought that you could use the moons as a universal clock. With that clock as a reference point, you could use local time to figure out where you are on Earth.

This sounds like a great idea, but how does it work? I’m guessing that you look at Jupiter, see where her moons are, and calculate where you are on Earth based on which moons you can see. For instance, on Wednesday, May 25, if you’re in North America and you have a telescope you can watch Io and Europa pass in front of Jupiter. If you live on the east coast you’ll see them only starting out; on the west coast you’ll see them only at the end. If you live in the middle of North America you’ll see most of the passage.

Since they know exactly when those moons will be zipping across the face of Jupiter and how long it will take, astronomers are able to make charts of the moons’ progress showing local times everywhere on Earth.

This strikes me as a huge amount of work to figure out where you are on Earth. Then again, I’m holding a cell phone with a GPS (Global Positioning System) so it’s pretty easy for me to know exactly where I am. If I were floating around in the ocean in the 1600s, with no GPS, I imagine I’d be pretty desperate to know exactly where I were and would consider breaking out the old telescope to have a squint at Jupiter and her moons.


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Like a pendulum do

One time while sitting in church, Galileo noticed a lamp suspended from the ceiling that was swinging back and forth. That motion is known as a pendulum. As it swung, he observed the lamp kept the same rate of speed. It occurred to Galileo that you could use a pendulum’s regular rate of speed to regulate a clock.

We learned that in Galileo’s time a clock was powered by a weight that slowly released its energy as it was pulled to Earth by gravity. The mechanism that slowed down—regulated—the weight’s energy is called an escapement. Galileo thought to replace the verge and foliot escapement with a pendulum escapement.

Just like the verge and foliot, as the pendulum swings back and forth it allows a gear to move forward a little bit just before a pawl stops it—until the pendulum swings to the other side. The pendulum escapement releases-stops-releases-stops the gears as they move the hands of the clock. Here is an excellent animation of Galileo’s escapement. Notice how when the gear turns it gives the pendulum a teensy little push.


Watch this guy make a wooden pendulum clock: https://www.youtube.com/watch?v=rvU37Aho4FA

Here’s some terrible music:

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The amazing fantastic clock of Piazza San Marco

In 1493, the Venetian Republic commissioned the clockmaker Giovan Paolo Rainieri, from the town of Reggio Emilia, to design and build a clock. This clock would be big and beautiful and expensive—a tower would be designed and built on Saint Mark’s Plaza to house it. It would face the lagoon and the sea beyond, so the whole world could see how prosperous was Venice.

If you visit Venice you can see the Rainieri clock. Its face is decorated in gold and lapis lazuli (a mineral you make blue out of—blue paint ain’t cheap); the hand tells what hour it is and the current zodiac sign; above the clock is a statue of the Virgin Mary and Baby Jesus (made of gilded copper); twice a year a mechanical angel and three wise men parade in front of Mary and tip their crowns to her; above Mary is the lion of Saint Mark with his paw on the Gospel (the statue of the praying doge isn’t there anymore); and at the top, every hour two bronze giants ring an enormous bell with their hammers.

The entire contraption from top to bottom used a verge and foliot escapement to regulate the gears.



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How to slow down a clock

How did they do it?

Those medieval clock-designers came up with a system to slow down the unwinding. First, they attached a gear around the drive-shaft that meshed with a couple of other gears. As you saw with Archimedes’ odometer, the ratio of gear sizes and number of teeth-per-gear can control how fast one gear turns another gear.

That still wasn’t slow enough, though. You want a clock to operate for at least 24 hours before you have to wind it again. How can you make that unwinding even slower?

The answer: an invention called an escapement. An escapement is a mechanical device that interferes with the gear. It actually stops the gear’s movement for a fraction of a second, then lets go for a fraction of a second, stops it, lets go, stops it, lets go, stops it, lets go. The first escapement was called the verge and foliot. The verge is a second shaft (not the drive-shaft) with two paddles, or pallets, set at 90 degrees to each other. These pallets interact with a saw-toothed gear which is powered by the drive shaft. As the drive-shaft turns the saw-toothed gear, one pallet stops the gear for a moment until the other pallet is pushed aside.

This stop-and-let-go motion is controlled even further by a bar at the top of the verge shaft, called the foliot. The foliot has a weight hung on each end so that inertia (the weights’ unwillingness to move) slows down oscillation of the verge-shaft. You can control how fast the foliot swings back and forth by moving the weights closer or farther from the center.



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Hey, what time is it?

We’ve talked about telling time with sundials and water clocks and hourglasses. Those things are a headache to carry around. Mechanical clocks, like a pendulum clock, wouldn’t be invented until 1637. What if you’re traveling around in ad 800—how do you know what time it is?

One way to tell time was this fantastic little device called an astrolabe.

This sketch is based on a beautiful antique brass astrolabe.

Wherever you happen to be, if you can see the Sun or the stars, you can tell the time if you’re carrying an astrolabe with you. The main feature of an astrolabe is a flat map of the sky—with the stars and planets on a grid. The grid—called a climate—shows the sky as it appears in your part of the world. It’s circular and fits into a circular frame, called the mater (Latin for ‘mother’). On top of the climate is the rete (Latin for ‘net’), an openwork circular plate with pointers that you can line up to point at the Sun or a specific star on the climate. On top of that is a sighter—a straight arrow kind of piece. All these spin on the same axis. You pick a star, adjust the rete to point at your star on the climate, and hold up the astrolabe and sight the actual star along the sighter. When the sighter lines up with the star, you can read the time with remarkable accuracy. Here’s a video showing how it’s done. This guy even made his own astrolabe. And here’s more.

Here’s a website that explains how to use an astrolabe and even gives you pdfs you can download and print to make your own.

Read this now! Time is running out!

Hourglasses are good timers that are easy to use, so they’ve been part of people’s lives for centuries. My mom used to have a little one that measured 3 minutes—just right for boiling an egg.

The hourglass became a symbol for time itself, and just how quickly it seems to pass. Go into an old cemetery and you might see one carved into a tombstone—eeek!

On some computers a little icon of an hourglass shows up to tell you that your program is still loading.

By golly, I would have committed murder to own that Wizard of Oz hourglass when I was an art student, How about the lucky prop artist who got to design it!

If you ever saw the old Wizard of Oz movie with Judy Garland, you’ll remember that terrifying big hourglass that belonged to the Wicked Witch of the West. Here’s a website that tells how hourglasses are made. This one’s good, too.

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