Tag Archives: longitude

Mister Midshipman Hornblower

C.S. Forester’s character, the 19th-century British Royal Navy officer Horatio Hornblower excelled at navigation because he was a math whiz. This got him in trouble as a young midshipman when he showed up a big dumb bully who couldn’t figure out how to work a sextant. https://hornblower.fandom.com/wiki/Sextant
A must-read—https://www.goodreads.com/book/show/84748.Mr_Midshipman_Hornblower

The sextant

The sextant is an instrument used to determine latitude and longitude. It was invented by Edmond Halley or John Hadley or Thomas Godfrey based on Isaac Newton’s ideas. You use a sextant to measure how far above the horizon the Sun is at noon (or how high Polaris is at night).

I’ve talked about finding your latitude or longitude by measuring the angle between a line from you to the moon and a line from you to a star. How do you do that?

Sailors have been using a sextant for centuries. Isaac Newton dreamed up the idea, then Edmond Halley built one in 1692. Several refinements were made around 1730, until we finally got the good old sextant you see people using in the movies with wooden ships and guys wearing wigs. A sextant is like an astrolabe—you sight something familiar in the sky, like a star, by lining it up along a sighter or pointer or alidade. Then you mark the alidade’s position on the frame and use that information to find your location.

Instead of an alidade the sextant has a telescope you look through. Then you find both the object (usually the Sun) and its reflection in the mirror. There are 2 mirrors facing each other and smoked lenses so you don’t fry your eyeballs. One mirror is half mirror/half glass so you can see both images at the same time. You bring the image of the Sun down to the horizon by moving the arm on the sextant and—oh, who am I kidding? Do you think I know what I’m talking about? What I need is a video to show how it’s done. Luckily, there are a bunch of them on the ol’ internet. Listen to these guys.

Here’s an in-depth 4-part tutorial covering everything you ever wanted to know about how to use a sextant.: https://www.youtube.com/watch?v=00ZEIZsl5xk

And a guy who doesn’t own a comb but knows what he’s talking about: https://www.youtube.com/watch?v=DrAkrgZRb9Y

The thing you have to know is: the sextant will give you a precise angle between 2 objects that you can transfer to a chart or map to get your position on Earth.


Look! Look! Here’s a cardboard sextant you can build yourself! https://www.landfallnavigation.com/cardboard-sextant-kit.html?gclid=CjwKCAjwq832BRA5EiwACvCWsWqv38smCfmUdVE5SqxRVkVJI_R0B9aN9jSb-3oaweER7b4KTm4iJhoCUbMQAvD_BwE

Here’s a plastic sextant—https://www.google.com/shopping/product/15301038968540324177?q=navigation+sextant&prds=epd:1143652539522712674,prmr:3,tpim:CKyp-dn168W22QEQ1N-msv7m0OotGMCbhR4iA1VTRCjg3sL8BTCNyIhA,pdprs:6&utm_medium=tu_image&utm_content=eid-lsjeuxoeqt&utm_campaign=134358029&gclid=CjwKCAjwztL2BRATEiwAvnALcqb-eUHLE4Y_XEM7QpPWFcXOcl2YbghLFCIcLIG1ItYYMc__vQ3_jBoCA14QAvD_BwE

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So long, and thanks for all the longitude

After 19 years of tinkering John Harrison finally worked all the bugs out of his time-piece, the marine chronometer. To prove its reliability, he took it on a voyage from England to Jamaica.

“Harrison conducted a round-trip test at sea from Britain to Jamaica through the Caribbean via the Atlantic from 1761 to 1762. The watch lost only 5.1 seconds in 81 days, reaching the level of accuracy required to receive a £20,000 reward.

But just then a rival named Nevil Maskelyne, the head of the Greenwich Observatory and a member of the Commissioners for the Discovery of the Longitude, was vying to receive the same award for his astronomical theory. Maskelyne refused to recognize Harrison’s success. To Harrison’s chagrin, he was granted only a few thousand pounds.”

Well, how do you like that? This is why Maskelyne is always cast as the villain in this story. Seems like a conflict of interest to be on the Longitude Board if you’re also a competitor for the prize. It would be quite a while before Harrison finally got his reward—and that was only because King George III* stepped in to make the Board pay up.

My pal Kathryn Lasky wrote an award-winning book about John Harrison’s story, The Man Who Made Time Travel:

Kathryn and I worked on Two Bad Pilgrims together. https://www.kathrynlasky.com/books/book/two-bad-pilgrims

* George III ruled Great Britain during the American Revolution. He was an enthusiastic friend to the Royal Navy. It was George who decreed that his sailors could toast the king’s health sitting down, because the deck-beams in the wardroom (officers’ dining room) were so low you’d likely crack your head if you stood.

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It’s 2 metals in 1!

This next part is crazy. I don’t know how Harrison dreamed up this idea. I don’t know how it’s even possible. Maybe a reader with experience in welding can help me out.*

Because metal springs are susceptible to temperature changes, which make them less reliable and accurate, Harrison invented the bimetallic strip. It is 2 metals, like steel and brass, welded together.

Did you get that? It’s possible to weld, or fuse, or something, steel and brass now. The watch’s mainspring needs always to be springy. When it’s cold, the spring becomes too stiff. When it’s hot, the spring becomes too loose. Harrison fused two metals together—steel and brass—in a spring. If the steel were too tight, the brass would keep it loose. If the brass were too loose, the steel would tighten it up. This way the spring would keep the same springiness no matter the temperature.

* This weekend I had the pleasure of consulting 2 engineers, my sister’s boyfriend Dave and my nephew Andrew. They tell me with enough heat, two different metals can be fused together. The two metals would be hammered together many times under heat until they were one. Andrew added that it’s only air molecules that keep metals from fusing together in the first place.





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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 Grasshopper

Since John Harrison was a cabinet-maker, he knew all about constructing things from wood. In fact, the first clocks he built as a young man had all-wood mechanisms. When designing a ship’s clock, he replaced many metal parts with wooden ones. Harrison used an oily wood named Lignum Vitae which didn’t need to be lubricated. Then, he designed a brand-new kind of escapement: the Grasshopper. The Grasshopper escapement worked with way less contact with the clock’s gears, which meant less lubrication was needed.

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A dumbbell idea

The problem with clocks in the 1700s: the ship’s rocking messed up a clock’s pendulum movement; the salty sea air corroded the metal gears; changes in temperature and humidity made metal clock parts expand & contract. All these things made a clock inaccurate—it was too slow or too fast.

Harrison came up with some innovative ideas to counter-act these problems. The first one was a dumbbell-style of pendulum. Instead of a rod with a weight at the bottom swinging from an axis, Harrison put the axis in the middle of 2 rods with weights at top and bottom—then he connected them with springs so they would keep moving back and forth no matter how the ship bounced around.

Standard-issue pendulum at the left; Harrison’s dumbbell movement at the right.

Start at 1:00—

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