In Our Time podcast has an episode on Longitude


The BBC’s In Our Time program, and podcast, did a 50-minute chat about the longitude problem. That’s the question of how to find one’s position, east or west of some reference point. It’s an iconic story of pop science and, I’ll admit, I’d think anyone likely to read my blog already knows the rough outline of the story. But you never know what people don’t know. And even if you do know, it’s often enjoyable to hear the story told a different way.

The mathematics content of the longitude problem is real, although it’s not discussed more than in passing during the chat. The core insight Western mapmakers used is that the difference between local (sun) time and a reference point’s time tells you how far east or west you are of that reference point. So then the question becomes how you know what your reference point’s time is.

This story, as it’s often told in pop science treatments, tends to focus on the brilliant clockmaker John Harrison, and the podcast does a fair bit of this. Harrison spent his life building a series of ever-more-precise clocks. These could keep London time on ships sailing around the world. (Or at least to the Caribbean, where the most profitable, slavery-driven, British interests were.) But he also spent decades fighting with the authorities he expected to reward him for his work. It makes for an almost classic narrative of lone genius versus the establishment.

But, and I’m glad the podcast discussion comes around to this, the reality more ambiguous than this. (Actual history is always more ambiguous than whatever you think.) Part of the goal of the goal of the British (and other powers) was finding a practical way for any ship to find longitude. Granted Harrison could build an advanced, ingenious clock more accurate than anyone else could. Could he build the hundreds, or thousands, of those clocks that British shipping needed? Could anyone?

And the competing methods for finding longitude were based on astronomy and calculation. The moment when, say, the Moon passes in front of Jupiter is the same for everyone on Earth. (At least for the accuracy needed here.) It can, in principle, be forecast years, even decades ahead of time. So why not print up books listing astronomical events for the next five years and the formulas to turn observations into longitudes? Books are easy to print. You already train your navigators in astronomy so that they can find latitude. (This by how far above the horizon the pole star, or the sun, or another identifiable feature is.) And, incidentally, you gain a way of computing longitude that you don’t lose if your clock breaks. I appreciated having some of that perspective shown.

(The problem of longitude on land gets briefly addressed. The same principles that work at sea work on land. And land offers some secondary checks. For an unmentioned example there’s triangulation. It’s a great process, and a compelling use of trigonometry. I may do a piece about that myself sometime.)

Also a thing I somehow did not realize: British English pronounces “longitude” with a hard G sound. Huh.

Author: Joseph Nebus

I was born 198 years to the day after Johnny Appleseed. The differences between us do not end there. He/him.

3 thoughts on “In Our Time podcast has an episode on Longitude”

  1. Also a thing I somehow did not realize: British English pronounces “longitude” with a hard G sound. Huh. You mean you weirdos don’t???

    I use an example of longitude to explain about time zones to my students – on the equinox, the sun rises in Sydney at 6am but not in Melbourne until 6:30 am.

    (I’m also looking to buy a sextant to see if I can do readings and calculate them).

    I’m going to have to listen to this chat – I know how important the longitude is, but even with the charts and tables, if you don’t know the time, I’m not sure how to calculate where you are. I thought the whole point was that you needed the charts and tables of “Venus will rise at 4am and be at 10° inclination at 0° latitude at 4:30 am” and the time to then say “oh, it’s at 5° at 4:30am so I must be at X”.

    Liked by 1 person

    1. Always said ‘longitude’ with a soft ‘g’, as in ‘lonjitude’.

      For finding longitude by star sightings, yeah, you need to know what the local time is. But that’s something you don’t need a really good clock for. Something that keeps the time tolerably well for several days will do. (At least long enough that you can find noon, or sunrise, or sunset, through the weather.) To find longitude by timekeeping you need a clock that keeps your meridian’s time for the whole voyage, despite changes in temperature and humidity and everything else a ship endures.

      Liked by 2 people

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