This is a great read. The book is highly entertaining as well as being very informative. Dava Sobel makes a complex subject, the measurement of longitude, come alive. The story is not only that of a scientific and technical quest, but also of human conflict, told with great skill. It begins with the concept of longitude, why knowing ones longitude was critical and how it was treated prior to the seventeenth century. The key to longitude was time, or rather knowing the time of an event, at your location and at a reference location. The two primary methods to do this are via the use of a very accurate timepiece or through the use of a knowledge of the position of the moon relative to the sun and stars, both of which I discuss this in a bit more detail at the end of this review.
The problem of an accurate longitude measurement was so critical that the British Government created a £20,000 prize for the solution to this problem. Most of this discussion is about John Harrison, a carpenter and self taught clockmaker, who developed a timepiece that was accurate enough to be used to measure longitude, and the British Royal Astronomers, primarily Nevil Maskelyne, who favored the method based on the position of the moon. The book discusses Harrison's creation of marine timepieces (chronometers) that were accurate enough to solve this problem and win him the prize, and the astronomers, primarily Maskelyne, who favored the moon position method and thus sought to discredit the clock approach and deny Harrison the prize.
My only reservation about the book is that there is very little technical information about exactly how Harrison's chronometers operated. There is one figure showing an escapement mechanism and a very brief discussion of how he solved lubrication and temperature problems, but I did not feel that this was sufficient to really understand how his clocks worked. Thus, I feel that a serious student of clocks would likely to be disappointed in the technical aspects of the book. However, a more general reader like myself could overlook this deficiency and focus of the human aspects of the book. I was able to find enough technical information about the operation of watches from the Internet to satisfy my needs, so the lack of this level of detail did not cause me to downrate the book from 5-stars.
THE MEASUREMENT OF LONGITUDE -
The simplest event to use to determine longitude is high noon, the time when you sun reached its highest point in the sky. If you had a watch set to 12noon at a reference location you could know your longitude based on the time, on this watch, that you locally observed high noon. For instance, if this watch was set so that it registered noon at Greenwich England (the location of the Royal Observatory), and you saw the sun reach it highest point at 1 o'clock, then you knew that you were one hour west of Greenwich. Since a day (one complete earth rotation) is divided into 24 hours and a circle is divided into 360 degrees, each hour of difference corresponds to 360/24 or 15 degrees of longitude, or about 1000 miles at the equator. Unfortunately, in the early 18th century there was no clock that could operate on a ship that was accurate enough to yield time measurements that could be used to accurately perform this task. If the clock ran to fast or too slow, say by only one minute per day, then in 10 days it would be off by 10 minutes or 1/6 of an hour or 2.5 degrees of longitude. At the equator this corresponds to about 1000/6 or 166.7 miles, which was clearly unacceptable. Even a much more accurate clock, say one that was off by only 10 seconds per day, would be unacceptable for a long voyage. Such a clock would be off by 300 seconds in 30 days or 5 minutes, yielding an error of 1.25 degrees, or about 83 miles at the equator. In practice, two chronometers are used, one set to the reference time and one continually adjusted to 12 O'clock at local high noon. Since the local clock was continually being adjusted it did not have to be as accurate as the reference clock that was not adjusted. Using a locally adjusted clock allowed one to determine the longitude at any time of day, instead of just at high noon.
Another approach was to chart the position of the moon relative to specific stars, or the sun. One could then determine the longitude by using an almanac showing the time at Greenwich when the moon was in a particular position and when it occurred locally. The local time was determined by observing high noon. This method also had limitations as it required many laborious calculations, which were subject to error, and it could not be used when the moon could not be observed. A still earlier method used the eclipse of the moons of Jupiter as the reference, but this required a very accurate telescopic measurement that was very difficult to do on a swaying ship.
Enjoy fast, free delivery, exclusive deals, and award-winning movies & TV shows with Prime
Try Prime
and start saving today with fast, free delivery
Loading your book clubs
There was a problem loading your book clubs. Please try again.
Not in a club? Learn more
Join or create book clubs
Choose books together
Track your books
Bring your club to Amazon Book Clubs, start a new book club and invite your friends to join, or find a club that’s right for you for free.
Download the free Kindle app and start reading Kindle books instantly on your smartphone, tablet, or computer - no Kindle device required.
Read instantly on your browser with Kindle for Web.
Using your mobile phone camera - scan the code below and download the Kindle app.
Something went wrong. Please try your request again later.
Great on Kindle
Great Experience. Great Value.
Putting our best book forward
Each Great on Kindle book offers a great reading experience, at a better value than print to keep your wallet happy.
Explore your book, then jump right back to where you left off with Page Flip.
View high quality images that let you zoom in to take a closer look.
Enjoy features only possible in digital – start reading right away, carry your library with you, adjust the font, create shareable notes and highlights, and more.
Discover additional details about the events, people, and places in your book, with Wikipedia integration.
Get the free Kindle app: 
Enjoy a great reading experience when you buy the Kindle edition of this book. Learn more about Great on Kindle, available in select categories.
Book recommendations, author interviews, editors' picks, and more. Read it now.
Frequently bought together
$14.78
Get it as soon as Thursday, May 16
Only 1 left in stock - order soon.
Total price:
To see our price, add these items to your cart.
Choose items to buy together.
Similar items that may deliver to you quickly
Page 1 of 1 Start overPage 1 of 1
About the author
Follow authors to get new release updates, plus improved recommendations.
Reviews with images
Fantastic!!!!!
Simply wonderful description of the legacy left by John Harrison. Things that we take for granted surrounded by all the gadgetry. The author walks you through history making you read it forward. Maritime problem, hits and trials, disasters. Simple and engrossing read. Worth every dollar.
Top reviews from other countries
Fernando Velez.
Great book. Makes you understand both the importance and difficulty of Longitud.
Reviewed in Mexico on May 1, 2021
GIOVANNI TEDESCHINI
This tiny book is actually a masterpiece!
Reviewed in Italy on July 27, 2019