Longitude, Harrison Clocks, Lunars and Captain Cook (Part 1)

One of the biggest problems facing early European navigators including Captain Cook, who was the first European to map New Zealand, was determining one’s longitude while at sea. As you may remember, lines of latitude are the lines that run east-west on a map or globe and determine how far north or south of the equator you are. Lines of longitude are the lines that run north-south and determine how far around the globe you are.


Sailors could readily determine their latitude, how far above or below the equator they were, by measuring the sun’s angle at its highest point above the horizon, or by measuring the angle of the North Star or other stars above the horizon.

Longitude was different and far harder to determine.

Celestial Navigation

Celestial navigation, the method sailors in Cook’s day used to find their way around the globe, refers to measuring the angle between a celestial body (the sun, moon or star) and the horizon and using this data to determine one’s position on the Earth’s surface.

Rather than the Earth rotating, think of the Earth as being fixed in position and the celestial bodies rotating across the sky above us. Celestial navigation is based on the fact that at any specific point in time, a given celestial body is located directly over one point on the Earth’s surface, i.e. a specific latitude and longitude.

Since one day is 24 hours in length and the Earth is a sphere of 360 degrees, it follows that the Earth rotates at a rate of 15 degrees every hour (360/24 = 15).

Again, if for practical purposes we think of the Earth as being fixed in position, then another way of saying the same thing is that the stars rotate above the Earth, or across the sky, at a rate of 15 degrees every hour.

Using meticulous observations and calculations, tables were created listing where and at what angles commonly observed celestial bodies should be seen at a given time and a given location—Greenwich, England. Why was Greenwich, England used? Because that was where the British Royal Observatory was and from where the initial observations were made.

These results could then be extrapolated to any location and any time around the Earth. Hence, one’s longitude could be determined by knowing the angle at which you are viewing a star, what your local time is (readily determined by the sun) and what time it is in Greenwich, England.

Time in Greenwich, England is called Greenwich Mean Time (GMT) or more commonly now Universal Time (UT). The longitudinal line which passes through Greenwich, England is given a longitude value of zero degrees and is termed the prime meridian, an arbitrary starting point for the longitude lines that run around the Earth.


Therefore, for example, if we know that a given star is at its highest point in the night sky at a certain time in Greenwich, England, and we are seeing that star at our location at its highest point in the sky four GMT hours later, then we know we are 60 degrees west of Greenwich, England (4 hours X 15 degrees/hour = 60 degrees).

Essentially, time differences can be converted into differences in degrees of longitude.

But without knowing the time in Greenwich England, if you just look at the sky and see a star at a certain angle at a certain local time, there is no way of knowing where you are around the globe. Indeed, at some point throughout the day, that same star will appear at that very angle at every longitude around the entire Earth.

Sextant, 1730

Sextant, 1730

So to determine one’s longitude, three things were needed.

Accurate measurements of the angles between celestial bodies. These could be determined particularly after the invention of the sextant in the 1730s. YES

Tables showing where a celestial body should be based on observations and calculations. YES

Time at your location and the time in Greenwich, England on which the tables were based. Time at one’s location can be determined each day by noting the highest point of the sun, but there was no way to consistently know the time in Greenwich, England. NO

Despite how inconceivable it might seem to us today, there was no clock accurate enough to be carried on a sailing ship that could accurately reproduce the time at Greenwich England, the location on which the observations and tables were based. If a clock keeping Greenwich, England time that you were using while traveling around the world was ten minutes off, that amounted to an error of 150 nautical miles. Even being one minute off, places you fifteen miles off course. And clocks during this time were often far worse than that, particularly at sea.

Also, clock errors tend to compound over time. An inaccurate clock over a voyage of several years would eventually become all but useless, particularly if, for example, one was trying to find small islands scattered across the Pacific, let alone finding one’s way back home.

Longitude Act of 1714

In 1707, a catastrophic British naval disaster brought the problem of the inability to determine longitude to the forefront. A number of naval ships and almost two thousand lives were lost due to the ships not being able to accurately determine their longitude. In 1714, the British Parliament passed the Longitude Act, which offered a prize of 20,000 pounds (equivalent to millions of dollars today) for anyone who could come up with an accurate and consistent method of determining longitude while at sea.

To show you how desperate England was for a solution to this vexing problem, the monetary awards were for a method providing accuracy for from one to one-half degree. This amounted to saying that they would be happy for a method accurate enough to place a ship within even 60 nautical miles of where it really was.

Powder of Sympathy

Powder of Sympathy Discourse

In the years leading up to the Longitude Act, many strange solutions to the problem of longitude had been proposed. Perhaps the most bizarre had to do with the so-called “Powder of Sympathy”. In 1687, a supposedly miraculous powder that could heal from a distance was discovered in France. If you took a bandage from a wound and traveled some distance away and then sprinkled the magic powder on the bandage, the healing of the wound would be hastened. However, the patient would feel the effect of this at the time of the sprinkling and would often cry out in pain. Hence, it was suggested that a dog with a wound be carried on board any ship bound for sea. At exactly noon back on the mainland (Greenwich time), a trusted individual would sprinkle the “Powder of Sympathy” on the dog’s bandage, which had been left behind. At that moment, the dog on the ship, now far out at sea, would yelp in response, and the ship’s members would know it was exactly noon back home.

Another scheme proposed in 1713 consisted of anchoring a series of ships at several hundred-mile intervals across the oceans. At a set time back on the mainland, a cannon would be fired. Similar cannons and exploding rockets would then be fired into the air in sequence down the length of ships stretching thousands of miles across the sea, and hence, nearby ships—with some calculations—could allegedly figure out what time it was back home.

Proposed solutions proliferated after the Longitude Act was passed, but very few had any merit. Indeed, “discovering the longitude” became synonymous with doing the impossible. Even the great minds of the age could make no headway in solving the problem.

Most of the luminaries of the day, including the famed Isaac Newton, thought that a solution would be found by studying the stars themselves, not by something as mundane as making a clock that could accurately keep the time at Greenwich while at sea.

John Harrison, Clockmaker

One man, John Harrison, a self-educated carpenter and clockmaker in Lincolnshire, England became intrigued with the challenge of building a nautical clock of sufficient accuracy to solve the problem. Harrison would spend the next forty years of his life on this quest.

The problems associated with building a nautical clock of sufficient accuracy were numerous. First, Harrison realized that any sea clock couldn’t have a pendulum like traditional clocks of the day, since a pendulum’s motion would be altered by the rocking of the ship. Instead he developed a series of oscillating springs to create the motion necessary to run the mechanism of the clock. Other difficulties which could affect accuracy of a mechanical device at sea were as follows: changes of gravity with travel, humidity changes, atmospheric pressure changes, temperature changes and the rocking motion and shocks of sailing itself. Each of these had to be addressed separately.

John Harrison

John Harrison

Harrison spent four years working on the plans for his first nautical clock, and brought them to London in 1730. By this time, although the Longitude Act with its awards had been in effect for sixteen years, no one had created a viable method of determining longitude. That’s right, nobody had solved the problem over the past sixteen years. In fact, no offices existed for the board, and the five commissioners of the Longitude Act had never met together over this period of time since none of the proposals had even evidenced enough merit to demand further investigation.

However, one of the board commissioners, Edmond Halley of Halley’s comet fame, met with Harrison and although impressed with Harrison’s ideas was reluctant to pass them on to the rest of the committee. The problem was that the members were made up of mathematicians and astronomers and were still set in the belief that the answer to the longitude problem would come from celestial observations and calculations rather than from a mechanical clock.

Harrison was referred to a famed clock-maker in London who loaned him money to continue his project. Harrison spent the next five years building his first sea clock termed H-1, which weighed 75 lbs. and was 4’ X 4’ X 4’ in size. In 1735, Harrison brought H-1 to London. This time the board did convene. H-1 showed much promise but it was primarily Harrison who wasn’t fully satisfied with the design. The board awarded him some funds to make an even better version.



Harrison spent the next two years building a better clock (H-2), which weighed in at 86 lbs. He presented it to the board in 1741. But it was still not accurate or reliable enough.



Harrison, who was now 48 years old, then spent the next twenty years working on his next clock, H-3, which had over 753 separate parts.



During the time he was working on H-3, Harrison in 1753 had a pocket watch made to his specifications. This changed his thinking. He now decided to build a smaller watch-size nautical clock, H-4, which he finished in 1759. H-4 was only five inches in diameter and weighed only three pounds. It was Harrison’s masterpiece.

“I think I may make bold to say, that there is neither any other Mechanical or Mathematical thing in the World that is more beautiful or curious in texture than this my watch or Timekeeper for the Longitude . . . and I heartily thank Almighty God that I have lived so long, as in some measure to complete it.”                                                  John Harrison




H-4 was tested on a sea voyage to Jamaica in 1762. William Harrison, John Harrison’s son, took part on the voyage. H-4’s performance and accuracy were impressive. H-4 lost only five seconds in 81 days and less two minutes for the entire journey. In a second test in 1764, H-4 gained 54 seconds in 156 days, equivalent to being only ten miles off in distance over that long period of time.

But despite this success, Harrison was not given the Longitude Act award, in great part due to an opponent and a competing idea.   (continued in Part 2)


3 thoughts on “Longitude, Harrison Clocks, Lunars and Captain Cook (Part 1)

  1. Truly fascinating! I read the book by Sobel and even enjoyed the creativeness used to make it into a movie.

  2. Pingback: Longitude, Harrison Clocks, Lunars and Captain Cook (Part 2) | movin2newzealand

  3. Pingback: London: Royal Observatory, Greenwich | movin2newzealand

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