How a Clock Opened the World to Discovery

Do you know where you are, exactly? The question may sound foolish given that today’s sophisticated GPS systems can easily pinpoint a person or location anywhere on earth.

Nowadays, I always know where I am. I simply look at the GPS app on my iPhone. This device is constantly measuring my position. It utilizes signals from 24 geosynchronous satellites that stay above fixed points on the Earth. From 13,000 miles above the surface, they orbit the Earth twice a day. They can locate a position on the ground to within 16 feet, depending on weather conditions and structures such as bridges.

But it wasn’t always so.

Throughout much of history, people had little way of knowing precisely where they were, outside of their immediate surroundings. There were maps but they were woefully inaccurate, more akin to a child’s fantastical dreams than the actual topography they claimed to represent.

The problem was particularly acute for mariners. In the past, sailing on the open sea was a roll of the dice. The stars offered some aid to navigation, but if the sky was clouded, all bets were off. Which is why ships needed to stay within sighting distance of land less they become lost on the open sea.

Columbus is a prime example of the problem. Upon setting foot on the what is today the Bahamas, he thought he had actually reached India which was over 7,200 nautical miles away.

The challenge of accurately navigating the globe was not solved until the eighteenth century when an unassuming English clockmaker invented what became one of the world’s most important technological breakthroughs.

Latitude and Longitude

The earth is crisscrossed with an imaginary set of circles called “latitude” and “longitude.”

The latitude is the angular distance from the equator, which ranges from 0 degrees to 90 degrees, north or south. Latitude lines are called “parallels.” The latitude of Fort Lee, New Jersey where I live, is 40.8509 degrees N, which means that my town is about 40 degrees north of the equator.

The longitude is the angular distance measured in degrees from a fixed point located in England. Lines of longitude are called meridians. The meridian that runs through Greenwich, England, is internationally accepted as 0 degrees longitude. In 1851 Britain got to choose this point, because of its dominance on the seas.

The circumference of the Earth is divided into 360 degrees. Since it takes the Earth 24 hours to revolve 360 degrees, a degree of longitude corresponds to 12 minutes of time. An hour of time corresponds to 15 minutes of longitude. The longitude of Fort Lee is 73.9701° W. When it is noon in Greenwich, it is 6: 36 AM in Fort Lee.

The latitude is easily measured by “shooting the sun” at noon with an instrument called an astrolabe or sextant. The sextant is an essential tool for celestial navigation and is used to measure the angle between the horizon and a visible object, such as the sun or a star.

But using a sextant to determine the longitude by measuring the angles between certain stars and the moon, which are invisible during the day or in stormy weather, is cumbersome and time-consuming.

The Prime Meridian

Imagine an accurate timepiece that would work on the high seas. Now set the time on this clock to Greenwich time before leaving England. Such an instrument would enable sailors to calculate their longitude, the angular distance around the globe from a fixed point in Greenwich called the “prime meridian.” Each hour’s difference between “ship time” and Greenwich mean time represented 15 degrees of longitude.

To find the longitude at sea, a navigator on the ship could note the time in Greenwich England when it is noon aboard the ship. Each hour of time difference would correspond to 15° of longitude. Since knowing the longitude is important both for safety and efficient navigation, watchmakers in the 18th century focused on the problem of creating a timepiece that would remain accurate on a ship at sea, keeping “Greenwich mean time.”

Up to that time, pendulum clocks were the only accurate timekeepers. If you’ve lived with a grandfather clock that had a pendulum, you know that it requires winding every day. You may have noticed that as the day goes on, the pendulum’s angular excursion decreases. However, the period (time for one swing) remains exactly constant even as the clock spring winds down. This unusual feature makes the pendulum clock an extremely accurate timekeeper. But pendulum clocks, no matter how accurate they are on land, won’t work at sea. Such clocks are much too fragile to survive the rolling of a ship in high seas.

John Harrison and the Chronometer

In light of recurring disasters at sea resulting from errors in navigation, the British government created in 1714 a Board of Longitude empowered to award £20,000 to the first man who developed a timepiece that would work on a moving vessel.

The timepiece had to be able to calculate a ship’s longitude within half a degree at the end of a voyage to the West Indies. Such a timepiece did not yet exist, and many believed it would be impossible to build.

Around 1750, with the prize in mind, John Harrison, a Yorkshire clock maker, attempted to build a mechanism that could keep accurate time no matter how rough the voyage. His first three attempts were pendulum-based clocks and failed as seagoing timepieces.

On his fourth try, he abandoned the pendulums and instead built a clock that looked very much like a large pocket watch. He called his invention a “marine chronometer.” And it did the job. It kept Greenwich meantime to an amazing level of precision, sufficient to assure that mariners would know their location at sea with an accuracy of plus or minus a few miles. As a result, the world opened up and became much smaller and safer to explore.

The Legacy of the Chronometer

We were born to travel. We are explorers and wanderers. With the help of the chronometer, we learned to navigate the oceans. But now we have other oceans to explore. Surely in the next few decades we will establish colonies on our moon and perhaps other planets. What as yet imagined instruments and tools will we develop to do this? And what new perspectives will we gain from these immense journeys and new adventures?