Tag Archives: Greenwich Observatory

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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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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Lunar distancing

Okay, let’s say you’re in a rowboat at night with some friends—and you haven’t seen land for a while. You’re LOST. Nobody’s getting a signal on their cellphones, so you don’t know where you are. The strange old lady at the boat rental place left nothing but a weird navigational device; a map; and a book of star charts in the boat’s locker. Your friends are getting panicky and start blubbering. What do you do?

Because you’re a devoted reader of The Western Civ User’s Guide to Time and Space, you know exactly what to do. You tell your pals to stop their noise so you can concentrate. It’s a clear moonlit night, so you can see the moon, stars, and the horizon. You pick up the lovely brass sextant and set its sights on the moon—and a star, how about Regulus, just there to the left? You measure the altitude (how high above the horizon) of the moon; the altitude of Regulus; and the distance between them. You figure the angle of the 2 lines from you to the moon and you to Regulus. You do this measuring not in feet or miles but in degrees.

From the moon’s altitude you know what time it is (http://www.astrotulsa.com/page.aspx?pageid=27, scroll down)—and your latitude, too (http://www.lewis-clark.org/article/1268). Knowing the distance from the moon to Regulus, you pick up the book of star charts and find that lunar distance for your local time. Run your finger down the chart to find what time it is in Greenwich, England where it’s zero degrees longitude. The difference in time will tell you your longitude (15° for every hour, 1° for every 4 minutes). Find your latitude and longitude on the map and start rowing home. You don’t even need a compass—you keep Polaris, the North Star, above your right knee as you row.

You get safely back to land! Your friends can’t believe you saved the day with that stupid book. The lady at the boat rental gives you a wink and you all go home to bed.

This is how Nevil Maskelyne proposed finding your position while at sea.

I haven’t read these, but here’s a short list of books about ocean-going girls: https://books.google.com/books/about/From_Cabin_Boys_to_Captains.html?id=wBDWSAAACAAJ

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The astronomer

Gerard van der Puyl’s portrait of Nevil Maskelyne—just gorgeous. What a painter.

Nevil Maskelyne was the fifth Astronomer Royal at Greenwich Observatory. He proposed using the positions of the stars, planets and their moons as a method of calculating your position on Earth, just as Galileo had proposed using the moons of Jupiter as a universal clock. Maskelyne was a hard worker and determined to win that Longitude Prize. He believed that with accurate charts of stars’ positions, you could find longitude anywhere on Earth. At the Observatory, Maskelyne and a team of astronomers ‘worked feverishly through the year 1766, preparing tables for the new Nautical Almanac and Astronomical Ephemeris. Published first with data for the year 1767, it included daily tables of the positions of the Sun, Moon, and planets and other astronomical data, as well as tables of lunar distances giving the distance of the Moon from the Sun and nine stars suitable for lunar observations.’

Here’s Maskelyne in a nutshell:

Oops! Okay here’s Maskelyne in brief:

Sorry! Sorry! Here it is: Maskelyne’s idea was that you have a point zero of longitude—the Prime Meridian—as a reference point for time. Longitude is time measured in degrees. Each hour is 15° of longitude. When you’re at sea you take 2 measurements: the Sun’s position and the moon’s position. The Sun’s position tells you what your local time is; you find the moon’s position (the distance from the moon to one of the 9 suitable stars) in your almanac to tell what time it is at the Prime Meridian. The difference between your time and Prime Meridian time can be converted into degrees, which gives you your longitude.

You’re probably thinking: ‘Okay, Manders, how exactly do you measure the Sun and the moon? Usually when the moon’s out it’s nighttime.’ That’s an excellent point! You measure the Sun during the day and adjust the ship’s clock to the local time, maybe?






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Find longitude, win valuable prizes!

In 1714 the British government established the Board of Longitude and held a contest: Who can come up with a system to accurately tell where you are at sea? The Royal Navy disaster on the Isles of Scilly had caused a reaction from the British public—they’d seen their own sailors drowned on the coast of their own country. Whether that disaster was the motive for creating the Board of Longitude may be disputed, but there’s no doubt a better, more accurate system of finding your location at sea needed to be found.

The Longitude Act, ‘An Act for providing a Publick Reward for such Person or Persons as shall discover the Longitude at Sea,’ was passed. The government offered up to £20,000 for a method of finding longitude at sea to within half a degree. That’s a lot of clams!

The Board of Longitude included scientists and astronomers who would judge the quality of any longitude systems that were submitted. One was the Astronomer Royal who worked at the Greenwich (GREN-itch) Observatory. I think if I ever got my career to the point where my job-title was Astronomer Royal I would consider myself to have arrived.

Astronomer Royal John Flamsteed, who also made the first recorded observations of Uranus.


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