Longitude was written by Dava Sobel. This is a true story. It tells of an English carpenter and clock maker who, beginning in the early 18th Century, set out to solve the greatest scientific mystery of the time which was how to determine the longitude of a sailing ship at sea. His work was challenged by the scientific elite who disdained his efforts at perfecting the perfect clock, instead favoring an astronomical solution. After forty years of effort John Harrison succeeded and his work was recognized for its brilliance and utility. The book was first published in 1995 and was nominated for the 1997 Royal Society Book Prize. Dava Sobel is a well-known science writer whose work appears in publications such as The New Yorker and Harvard Magazine.


In the 18th century success at both commerce and war depended on dominance at sea. Sailing vessels plied the oceans with some success but also with significant loss. Regardless of the competence of those manning the ships or the skill of the builders a huge problem persisted on the high seas. The boats were not sure of where they were when out of sight of land and that means most of the time.

Knowing the exact time of day, usually noon, the mariner could determine latitude, how far north or south he was with confidence, but this was only half of what was needed. The question that remained was exactly, how far east or west was the ship’s location. That is longitude. Lack of precise knowledge of longitude and therefore location cost money, time, and more importantly, lives.

Finding a way to determine the exact longitude was the most important unsolved scientific problem of the 18th century. The King, recognizing this, offered, what was in present time millions for the person who found the answer. A contest prevailed between astronomers who would use the clock in the sky and a humble English carpenter and clock maker who believed the answer was in establishing a dependable way to maintain exact time using a clock on a ship in turbulent waters and varying temperature.

The author tells the story of a simple man who solved the problem over a period of forty years of near isolation by means of hard work, dedication, and singular genius. He built a clock that told time under the harshest of circumstances while maintaining accuracy to within seconds over a prolonged period.

The tale is not one of “Eureka I did it” followed by adulation but instead one of controversy, claims and counter claims, and delays for verification. The principal competition was between astronomy and horology with the common denominator being knowing the exact time. This book reads like a novel and teaches like a textbook while maintaining the best qualities of both.

Study Questions

1.  How did the author become interested in this subject?

2.  Was the difference between longitude and latitude explained adequately?

3.  Why did the process take so long? Who was to blame?

4.  How was the Government involved?

5.  Can you think of other examples of perseverance like this?

6.  Is the chronometer as necessary for navigation now compared to the 18th century?

7.  Did this book tell you something new about the travails of the inventor?

8.  Can you think of a similarly significant breakthrough in modern science?

9.  Why was John Harrison so modest and self-effacing? Can you think of a modern example?

10.  Where would you place Harrison in the pantheon of inventors?


Longitude as a technical non-fiction for the most part was enjoyed by the group. One comment that spoke for most said, “it was pretty technical at first then the book started to read like a novel which I enjoyed.” When it was said, “I was surprised the author was a woman,” the reply from one was “why not”.

The concept of longitude and latitude and the importance of the former was a revelation to all. This prompted the question, “I wonder how many other really important things have happened that I know nothing about.” The discussion continued with comment about two unsung heroes. One used Boolean algebra to launch the mathematical basis for digitization and the other was an amateur physicist who revolutionized radar in WWII.

To enhance the group’s understanding of longitude and latitude the following explanation was handed out and discussed.

Finding where you are in the world is revealed by coordinates: Latitude – North/South and Longitude – East/West
Indianapolis: Latitude 39.7602 degrees North and Longitude 86.1633 degrees North

Noon can be determined anywhere in the world where you can see the sun by determining the moment its shadow is shortest. This is done using a simple sighting instrument that measures the angle (declination) of the sun and points to the degrees you are from the equator. This number of degrees is the latitude, your distance north or south of the equator.

At the same noon reading you can look at an exquisitely accurate clock (chronometer) and see the time which was set precisely at noon Greenwich time. This clock will tell you how many hours, minutes and seconds you are away from Greenwich. The earth travels one degree in four minutes so you calculate: Local time is noon. Chronometer says it is 8:23 in Greenwich. This equals 503 minutes. It takes four minutes for the sun to travel one degree, so you are 503 divided by 4 degrees from Greenwich or at 123.7500 degrees west longitude.

The book was not graded, but the sense of the discussion was that those who read it were pleased. However, about a third of the group found the book a combination of too challenging and not that interesting to them.



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