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Marine Chronometers

I have just been looking back at some photos from a sailing trip. It is always a great way to spend some time, especially when the skies are blue, the seas calm and the wind favourable.

Maybe this Summer you found yourself onboard a boat at some point? Today, with satellite navigation systems, finding your position at sea and keeping to a route have become more straight forward but in the past sailors relied upon marine chronometers, precision timepieces, to determine longitude and therefore their exact location. As many of you will, I am sure know, it was John Harrison who built the first marine chronometer that kept accurate time at sea and which in doing so, allowed ships navigators to accurately fix their longitude, that is their east-west location.

It was in response to the prize offered by the British government of £20,000 in 1714 that John Harrison built his timepiece which kept accurate time at sea and so could reliably record the time at the place from which a ship had sailed. One degree of longitude equals 4 minutes of time and so a ship's position east or west from the place where it began its journey can be found by comparing the difference between the recorded time and local time. Local time and latitude can be found by observation of the sun and stars.

The ideas from Harrisons' timepiece were taken further by John Arnold and Thomas Earnshaw who developed the marine chronometers we are familiar with today. Marine chronometers cannot rely on pendulums due to the motion of a ship, instead they use springs and balances. The movement has to be able to compensate for changes in temperature which can affect the balance and therefore timekeeping. It is supported in a gimbal, to keep it level onboard a ship at sea and an outer wooden box provides protection.

Knowing their exact position kept ships safe, rocks off the coast could be avoided and ships could travel more directly and therefore more quickly to their destinations. Vital for the navy but of equal importance to shipping companies. Faster journeys meant economic advantages, being the first back with a cargo and commanding higher prices or fitting in more journeys over the course of a year. Faster journeys could also be the difference between life and death. Time spent at sea was dangerous, the longer a ship was at sea the more risk to which it was exposed. Storms could wreck ships, pirates could plunder them and disease could flourish on board in cramped conditions.

Today a sextant and accurate timepiece which could now be a quartz wristwatch, are still recommended as an emergency back up system for mariners because they do not rely on onboard electronics.

This month we are lucky to have an interesting collection of antique marine chronometers for sale . A fascinating addition to any clock collection or a good stand alone piece they are all incredibly high quality, precision objects that take up very little space, work well on a desk or small table and are good value for money. Take a look at the collection here. 

If you are interested in the subject of chronometers and navigation at sea and would like to find out more this BBC documentary by Adam Hart-Davies  is worth a watch, it's a few years old but covers the topic well. For a more in depth study of this fascinating subject I highly recommend the book Finding Longitude by Richard Dunn and Rebekah Higgitt.

Date: 18/10/2023 | Author: Tobias Birch