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John Harrison: Inventor and Longitude Hero
szczys writes: Here's an interesting fact: when at sea you can't establish your longitude without a reliable clock. You can figure out latitude with a sextant, but not longitude. Early clocks used pendulums that don't work on a rocking boat. So in the 1700s the British government offered up £20,000 for a reliable clock that would work at sea. John Harrison designed a really accurate ocean-worthy clock after 31 years of effort and was snubbed for the prize which would be £2.8 Million at today's value. After fighting for the payout for another 36 years he did finally get it at the ripe old age of 80. The methods he used to build this maritime chronometer were core to every wrist and pocket watch through the first third of the 20th Century. One of his timepieces, designated Clock B, was declared by Guinness to be the world's most accurate mechanical clock with a pendulum swinging in free air' more than 250 years after it was designed.
Please, read Dava Sobel's book, Longitude, about the trials and travails of Harrison -- it's a tremendous read. And if you ever get to the Royal Observatory in Greenwich (England), look at the Harrison models, they are amazing.
This is a guy who was a Maker -- self taught and more.
And of course GPS is nothing more than very accurate clocks in orbit. So you're still using a clock to get your longitude (4 of them, in fact!).
You do not need a clock to determine longitude. In fact, a sextant can be used, as long as you have the appropriate tables that map various celestial angles to the correct date and time. These tables were originally overseen by Nevil Maskelyne, one of Harrison's rivals to the longitude prize.
I was going to bring up myself that technically, you don't need a clock, because of the lunar distance method. However, that's only a "technically"; the lunar distance method was never really practical for use at sea.
The two methods are an early instance of the closed-tech vs open-tech argument we're so used to now.
To call it open vs closed is a little bit of an overstatement. Harrison disclosed how his clocks worked and their method of manufacture. He did have patents on some of the techniques, but for the speed technology moved at the time, the length of the patents were quite reasonable. (Also, IIRC the admiralty was allowed to licence it out to others for a fixed rate).
The big thing is that longitude is hard. To this day a mechanical clock which can keep time well enough for accurate navigation is an expensive and specialized thing. Irrespective of patents, such clocks were simply expensive to build. However, once you bought one, they were easy to use. The lunar distance method required little in the way of equipment (that is, it had low capital outlay). However, it required highly accurate relative measures of many astrological features in a short time. From the deck of a rolling ship. With finicky table lookups. At night. With a bunch of finicky calculations afterwards. And of course, if it was partly cloudy and you couldn't make all of your measurements, well, you'd better hope your sand watch (that is, hourglass) had good holdover. That is, even with the lunar distance method, you still had to have good timekeeping to figure out your in between positions.
The insurance companies (that is, Lloyd's and their subgroups) eventually forced mercantile adoption of Harrison's clocks. And of course, today we just use clocks. Atomic clocks moving in relativistic conditions, but still easier than the lunar distance method.
An easier way to use the moon is by measuring it's position relative to the stars. Then you don't have to wait for a particular phase. In fact, that's the method that was in competition with Harrison.