Advent Calendar of Science Stories 12: Time Tables

Returning to our mostly-chronological ordering after yesterday's brief excursion, we come to one of the great problems of the 1700's, namely determining the longitude at sea. Latitude is easy to find, based on the height of the Sun at noon-- we told that story last week-- but longitude is much trickier. Thanks to the rotation of the Earth, the best way to measure longitude is by measuring time-- if you know what time it is where you are, and what time it is at some reference point (now established as Greenwich, UK), the difference between those times tells you the difference in longitude.

This is relatively easy to do on land, but making a system capable of tracking the time difference on a moving ship at sea was an extremely difficult problem. It was, of course, finally cracked by a lone genius working in...


And, OK, I skewed the presentation of that a little just to mess with Thony C. He wasn't really a lone genius, and he's not really famous. I'm talking about Tobias Mayer, a young German astronomer and mathematician whose observations made it possible to implement the "method of lunar distances" on a practical scale.

To modern people, accustomed to cheap and reliable timekeeping, the solution to the longitude problem seems obvious: just carry a clock with you. In the 1700's, though, that was an awfully hard thing to manage, because mechanical clocks were a new and not entirely reliable technology. And while the clockmaker John Harrison did eventually crack the problem, it required almost superhuman effort, and the necessary clocks remained expensive and difficult to maintain for many years to come.

There's another way to measure the time where you are, though, which is through astronomy. As the Moon moves across the sky, it's visible more or less everywhere on the dark side of the Earth at the same time. Most of the Moon's motion is just the rotation of the Earth, but as it moves in its orbit, it changes position relative to the background pattern of stars-- not by a huge amount, but by enough over the course of a single night to serve as a useful method of establishing time. If you know that the Moon as seen from London will pass directly in front of a particular bright star at precisely midnight in London, and you see it pass that star at 10pm where you are, well, you're two hours west of London, or about 30 degrees of longitude.

For this to be a useful method, of course, you need to know exactly where the Moon will be at various times in the future. Which is doable in principle, but actually pretty complicated, due to all the various forces that tug on the Moon in its orbit. Predicting the position of the Moon with sufficient accuracy to be useful for navigation defeated a lot of great scientists, due to the complexity of the calculations from Newton's law of gravitation.

Mayer finally cracked it not with any spectacular new technique, just a series of improved observations of the orbit of the Moon over a period of several years. This gave him an exceptional empirical model of the orbit, allowing prediction of the Moon's position well enough to find the time within a few minutes. Sadly, Mayer died young, of typhoid, but in recognition of his achievement his widow was granted three thousand pounds from the Board of Longitude, a very substantial sum of money.

Mayer's method was still fairly complex, requiring calculations by a trained astronomer, and making it practical for use by seamen took a further refinement. This was done by Neville Maskelyne at the Royal Oservatory, essentially doing a lot of the more complicated calculations in advance and producing a simplified table. These tables were printed and distributed to ships, and became the first large-scale practical method of determining longitude at sea. Comparable tables are still generated-- by the US Naval Observatory, for example-- and used for celestial navigation. Nowadays, we have reliable clocks and GPS, but the methods developed by Mayer and Maskelyne can provide a fallback option for times when the batteries run out.

John Harrison and his clocks (and his pissing contest with Maskelyne and others over whether he should get the Longitude Prize) make for a better story in that they involve dramatic new technological inventions-- bimetallic strips! novel escapements! diamond chips for reduced friction! Mayer's less glamorous story is important as well, though, because it's a reminder that science isn't always dramatic "Eureka!" moments and sudden breakthroughs. Sometimes, great advances are made simply through dogged persistence and careful, patient observations. That might not sell a lot of books two hundred years later, but it saved an awful lot of ships at sea, which is a pretty damn good legacy.


(Featured imaged from this astronomy page.)


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Great piece about the quest to find longitude, especially the stuff about Mayer who is often overlooked. Just one tiny note though: there was no such thing as the Longitude Prize-it was actually a series of rewards and the Board was more like a proto-science funding body.

By Katherine McAlpine (not verified) on 12 Dec 2014 #permalink