Today, we will talk about how people discovered an important piece of information that was necessary for sailing safely on the oceans. It is called longitude.
One foggy night in October 1707, four English navy ships hit rocks in the Atlantic Ocean and sank. Two thousand men drowned. The ships had been sailing in thick fog for twelve days. There was no reliable way of knowing where they were.
The commander of the ships was worried that they might hit rocks if they were not careful. He asked his navigators for their opinion on their location. The navigators were not really sure. They told the commander that they thought they were west of a small island near the coast of north-western France.
They were wrong. Instead, the ships sailed onto rocks near a small group of islands southwest of the Atlantic coast of England. This lack of knowledge resulted in the loss of four ships and two thousand lives.
When people began sailing out of sight of land, sailors did not know how to determine their position on the open sea. Land travellers can look at a mountain or a river to show them where they are in relation to where they came from. On the ocean, however, there are no such signs to help sailors find their way.
The most important device for finding your way at sea is a compass. A compass contains a metal object that always points towards the magnetic north pole. This shows navigators the direction of north, and therefore also south, east and west. However, sailors need more information to sail safely on the open sea.
Most world maps show lines that do not exist on the Earth’s surface. One such line is the equator. This imaginary line encircles the widest part of the Earth.
Similar lines exist both north and south of the equator. These circles become smaller and smaller towards the North and South Poles.
These lines, or circles, are parallel, meaning they are equally distant from each other everywhere around the world. These lines represent what is known as latitude.
A navigator can determine the latitude of their ship by observing the location of the stars, where the sun rises and sets, and the time of year. Using this information, they can determine their position in relation to the North or South Pole and the Equator.
However, there is one more important piece of information necessary for safely sailing the oceans.
For many centuries, scientists, astronomers and inventors searched for a way to determine longitude. The lines of longitude run in the opposite direction to the latitude lines. They stretch from the North Pole to the South Pole and back again in great circles of the same size. All the lines of longitude meet at the top and bottom of the world.
In her book Longitude, writer Dava Sobel recounts the history of longitude and how the problem of determining it was solved.
For centuries, great scientists around the world struggled to develop a method of determining longitude. Knowing longitude at any given location requires knowledge of time. A navigator needs to know the time on their ship and the time at another place of known longitude at the same moment.
The Earth takes twenty-four hours to complete one full turn, or revolution, of 360 degrees. One hour marks one twenty-fourth of a turn, or fifteen degrees. Therefore, each hour’s time difference between the ship and the starting point indicates the ship’s progress of fifteen degrees of longitude east or west. These fifteen degrees of longitude represent the distance travelled.
At the equator, where the Earth is at its widest, fifteen degrees stretches to around one thousand six hundred kilometres. However, north or south of that line, the distance covered by each degree decreases. One degree of longitude is equivalent to four minutes of time everywhere in the world. However, when measuring distance, one degree shrinks from approximately 109 kilometres at the equator to zero at the north and south poles.
For many centuries, navigators hoped to find longitude by observing the movement of stars at night. During the day, the sun provided information about time and direction on a ship. However, it did not provide the necessary information about the time elsewhere.
In the sixteenth century, an astronomer suggested that navigators could use the position of the Moon in relation to known stars to determine longitude.
However, there was insufficient knowledge of the stars to make this method effective. Astronomers could not predict the moon’s position accurately from one night or day to the next.
Nevertheless, those seeking to solve the longitude problem believed that the solution lay in the moon and stars.
During the seventeenth century, English astronomers began a major effort to map the stars and their relationship to the Moon as it passed across the sky. Royal Astronomer John Flamsteed devoted forty years to this task. His successor, Edmund Halley, spent another forty years gathering information about the moon’s orbit.
After many years of gathering the necessary information, it became possible to determine longitude by observing the stars and the moon. In 1766, the Royal Astronomer Nevil Maskelyne published the Nautical Almanac and Astronomical Ephemeris.
This publication contained all the information that sailors needed to determine their longitude based on the positions of the moon and stars.
This new method was not simple. A navigator had to use complicated observation instruments to record the positions of the moon and stars. Then they had to find the relevant information about the moon and stars at that time of night or day in the Nautical Almanac. The final step was to apply the mathematical information from the book to the current situation and solve the resulting problem. On average, this took four hours.
While scientists were studying the stars and moon in an attempt to solve the longitude problem, a man named John Harrison was working on another project. He was trying to build a clock that would help sailors determine their longitude.
This task was also difficult and complex. Mr Harrison had to develop a clock that would not be affected by a ship’s movement on the ocean, or by changes in temperature or atmospheric pressure.
He began developing his clock in 1730. It took him five years to complete it. The complex device weighed thirty-four kilograms. Several years later, Mr Harrison built a second clock. Although it was smaller, it weighed more than the first. Unsatisfied, he began working on yet another device.
Twenty years later, he had completed a device that was smaller and lighter than the first two. However, Mister Harrison was still not satisfied.
Two years later, in 1757, he produced a small clock that he could hold in his hand. This clock could tell the correct time in two places, thus meeting the requirements for determining longitude at sea.
For many years after Harrison completed his work, the idea of using a clock to determine longitude was rejected. However, this changed when manufacturers learned to produce better, less costly versions of Harrison’s clocks.
These clocks became known as chronometers. By 1815, five thousand chronometers were in use on ships sailing the world’s oceans. Complex documents and mathematical calculations were no longer necessary. Almost any sailor could determine their longitude simply by looking at a clock. The world had changed.
John Harrison’s clocks can be seen today at the Old Royal Observatory in Greenwich, England. The first three are still operational and show the correct time.
Looking at them reveals the simple solution to a problem that had worried people for many centuries.
Nowadays, the solution is so commonplace that it’s hard to imagine there ever being an issue.
