{"product":{"product_id":"B003WUYE66","title":"Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time","price":"8.06","image_url":"https://m.media-amazon.com/images/I/41UeGCO8wjL._SL500_.jpg","url":"https://www.amazon.com/dp/B003WUYE66"},"comments":[{"body":"You may be confusing issues. (latitude, altitude, longitude)\r\n\r\nSailors wanted an accurate way to know the \"time at port\" to compare against local time (observable via stars or sun) so that they could compute their longitude (east or west location on the globe). (The difference in minutes would be equivalent to the relative difference in minutes of arc east/west.)\r\nGrandfather clocks (eg, pendulum clocks) wouldn't work for this, because the rocking of the ship would throw off the pendulum timing.\r\n\r\nIf I recall from the documentary based on Dava Sobel's book [Longitude(Amazon)] (http://www.amazon.com/Longitude-Dava-Sobel-ebook/dp/B003WUYE66/ref=sr_1_1?ie=UTF8\u0026amp;qid=1395258934\u0026amp;sr=8-1\u0026amp;keywords=longitude+dava+sobel) (looks like the documentary is available on [youtube](https://www.youtube.com/playlist?list=PLbFVv5C_WSedLIMG4tPlDnKlNFPoFGyPL)), the guy (John Harrison in 18th century) who rose to the challenge to make a clock that worked at sea had all sorts of problems with the pendulum motion, and somehow miraculously actually made a \"pocket watch\" to solve the problem.","subreddit_name":"todayilearned","author":"SailorDad"}]}