Richard MacKenzie - Collections, Maritime Museum of British Columbia, Discussing the challenges in measuring longitude
The real problem in navigation was knowing where longitude was. Looking at ancient maps, the distance between places north and south are all correct because they knew how to find latitude. But in the maps, things are very much elongated and that is because of the difficulty of finding longitude. This problem was eventually solved by a man of the name of Harrison in the late 1700's with the invention of the sea-going clock. The sea-going clock is basically a large pocket watch, or at least the first one was a large pocket-watch. Now they have clocks. This is what a chronometer looks like. The time would be set to Greenwich Meantime, a known time. Then, because the world is round, you can measure off the distance around it be they hours, minutes and seconds. Those will translate into lines of longitude. So, with my mariner's octant at noon I would take a series of sights – just before noon, noon and after noon and I would find the highest point and at that point I would know exactly when noon is where I am. Then I go down and I take a look at the chronometer and the chronometer says that it is 8:00 am in London, England. Knowing the difference between where I am, being noon and 8:00 in London, I know how many minutes, hours and degrees I am west.