Tag Archives: Prime Meridian

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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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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Turn that sandglass, ye lubber!

If having a different Prime Meridian on every map weren’t enough of a headache, ocean-going navigators had to deal with another problem: clocks that didn’t keep accurate time on the bounding waves. Galileo’s clock was a bust on the ocean because the ship’s movement kept spoiling the swing of the pendulum—so the clock kept stopping. Ferdinand Magellan used sandglasses to keep time and made sure there were 18 of them on each of his ships. Sandglasses were still the most reliable timekeeper aboard a ship. Somebody had to be in charge of turning the sandglasses to keep Prime Meridian time. They probably made the littlest sailor do it—who was also in charge of taking out the trash and pulling weeds.

Why is time so important? Because longitude is time converted into degrees. To know your location you must know 2 times: what time it is where you are and what time it is at the Prime Meridian. If you don’t know both those times, you’re lost.

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Good news for canaries

Ptolemy published a book titled Geography which gave the latitude and longitude for over 8,000 places.

Ptolemy’s map* shows all of Earth he knew back then: Europe, the north coast of Africa, India and most of Asia. The Prime Meridian of Ptolemy’s map runs through the Canary Islands just west of Africa. Everything else is east.

As mapmaking took off as a business in the Age of Exploration, mapmakers in every little country, state and principality used a different Prime Meridian. You can see how this might get confusing.



* Maybe Ptolemy drew this map, maybe he didn’t. The locations on the map, though, are based on their coordinates given in Ptolemy’s book.

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Zero degrees in Alexandria

Eratosthanes figured out how to mark places on a map using latitude and longitude.

I know what you’re thinking: “Eratosthanes marked Benghazi 9 degrees west on a map. Nine degrees west from where?”

That’s a terrific question and I’m glad you asked it. When you measure longitude, you have to start at zero degrees. Everything else on that 360° circle is either east or west of 0°. Eratosthanes was the head librarian of the Great Library in Alexandria, so he used his home town as that starting point. It’s the Alexandrian Prime Meridian. The Prime Meridian is an imaginary line that extends from the North Pole to the South Pole. The other meridians—lines of longitude—are east or west of it.