The Board of Longitude -
an amazing chapter of the history of navigation

 

The problem

The vikings were able navigators. They could determine their latitude on the open sea by sighting the sun or the North Star. A Danish viking who wanted to visit friends in Greenland could sail north along the coast of Norway until he reached the latitude of the Shetland Islands, and then due west, south of Iceland, until he reached the southern tip of Greenland. If he got off course during bad weather, he could get back to the right latitude, but he would need to guess whether to go east or west from there, because he didn't know the longitude.

The need to approach major destinations along a known latitude was well known to middle age pirates, who could intersect their prey in the middle of the ocean on the established routes across the sea.

To determine one's longitude on the open sea was a major challenge of navigation. In 1598, king Philip III of Spain offered a prize of 6,000 ducats to the one who could solve the problem. Similar prices were offered by several seafaring nations.

In 1707, a British fleet under Vice-Admiral Shovell grounded off the Scilly Isles, and 2,000 men were lost. The problem of determining the longitude became a concern to the whole British society, including the scientific community, headed by Isaac Newton. The Board of Longitude was established in 1714, and was empowered to offer prizes as follows:

  • 10,000 GBP for a method that could determine longitude within 60 nautical miles,
  • 15,000 GBP for a method that could determine longitude within 40 nautical miles, and
  • 20,000 GBP for a method that could determine longitude within 30 nautical miles.

The longitude of a location is directly related to the difference between the local time and Greenwich time. The local time can conveniently be fixed by a noon sighting of the sun, but the time at any other location requires a reliable clock, or some other way of distant time synchronisation. On land, that could be done by astronomical observations of when the moons of Jupiter passed into its shadow, or when the shade of the Earth passed a certain location on the surface of the Moon during a lunar eclipse. Obviously, however, such observations can be made only once in a while, and they are in any way not practical at sea.

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Within a small margin, the latitude equals the angle between the horizon and the North Star
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The longitude is directly related to the difference between true local time and Greenwich time:
1 hour time difference =
15 degrees longitude difference

One solution: Lunar sightings

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Relative to the stars, the Moon moves 13 degrees per 24 hours


As it is well known, the Moon makes the 360 degrees orbit around the Earth once per month, or about 13 degrees per day. This movement is so relatively fast that the Moon's exact position among the stars can serve as an indicator of the universal time.

For practical applications, two things are required:

  • A practical and accurate instrument for making the observations; and
  • accurate tables of the Moon's position among the stars, together with the required refraction and parallax corrections.

Refined instruments - octants, sextants and quadrants - suited for accurate observation of celestial angles from the deck of a ship were developed in the course of the 18th century. Hereby, a way was opened to using Moon sightings to establish the time at some other location than the one where the observations were made.

The required lunar tables were developed between 1751 and 1753 by Tobias Mayer, a German cartographer. Mayer's tables were accurate within around 1 arc-minute. Allowing for practical measurement errors, the method could determine the longitude to within around 60 nautical miles. Hereby, the tables represented a decisive step ahead with respect to positioning at sea. Mayer himself, by the way, had never even seen the sea.

The method was highly practical, to the extent that it required only a few simple observations with a good sextant (which was also at that time a standard implement on any seagoing vessel). The subsequent calculations were more intricate. It took a skilled navigator around 4 hours to perform the iterative corrections that were needed to obtain the result.

The Board of Longitude hesitated with their acknowledgement. Not until 1765, 3 years after his death, Mayer's heirs eventually received 3,000 GBP as a reward from the British Parliament.

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Tobias Mayer (1723-62) developed the lunar tables for navigation. The tables were published in the Nautical Almanac from 1767-1908

Another solution: The Sea-Clock

With today's technology, it is obvious that the direct way to know the time in Greenwich is simply to bring a good clock onboard. In the early 18th century, however, this was a far imagination. Clocks were based on pendulums, which do not at all perform well onboard a heaving and swaying ship, not to speak of the temperature shifts. This had clearly been confirmed during several attempts to use clocks as a means for navigation. 

John Harrison was a carpenter from Yorkshire. Already at the age of 20, he built a clock with a pendulum made of two metals that could compensate each other's temperature expansion. When he heard of the prize offered by the Board of Longitude, he spent several years developing his first chronometer (later named H1). It had 2 symmetrically connected pendulums that could compensate for each other's acceleration caused by the movement of the ship.

He built two more large chronometers, which were never tested at sea, and then a watch type chronometer, H4, which was thoroughly tested. On the first trial, when approaching Madeira from the North, Harrison's son William won a bet with the captain about the ship's position. His ability to point out the right course made him very popular on board, since the ship was critically low on beer. Later on, when the ship reached Jamaica, the longitude determined by the chronometer was accurate within 1 nautical mile. Hereby, Harrison earned his first 2,500 GBP from the Board of Latitude.

At a second trial, in 1764, the error was less than 10 nautical miles for a 6-weeks crossing to Barbados, three times more accurate than required for earning the maximum prize of the Board. Still, Harrison was paid additional 7,500 GBP only. He did not receive the balance until 1773, after a hectic dispute involving a large part of the scientific community, and finally settled by the king.

A copy of Harrison's watch was made in 1770. It was used by Captain Cook on his second and third voyages. Cook, perhaps the best navigator of all times, liked it well. At a time where the use of chronometers at sea was still a matter of professional debate, he was in no doubt about its value.

Initially, it took a master watchmaker several years to make a chronometer, so the price was high. Still, the new technology spread rapidly, and was in general use after a few decades. Whalers were quick to adapt chronometers, merchant ships more hesitant. Around 1825, chronometers were common in the Royal Navy.

The Board of Longitude was dissolved in 1828. Since 1833, the exact time has been indicated to mariners by a globe that is hoisted and dropped on a signal mast at Greenwich Observatory every day at 1 PM.


Home

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John Harrison (1693-1776) constructed the first usable marine chronometer around 1730
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Harrison's first Sea-Clock (H1) (built 1729-36). Height 76 cm, weight 36 kg. Most of the wheels are made of wood. It was accurate within a couple of seconds per day
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Harrison's Watch (H4) (completed in 1759). On two trials across the Atlantic, it determined the longitude within 10 nautical miles