Overnight we passed the fishing port of Puerto Mont, Chile which we would have visited. According to a Lonely Planet travel guide “the town can be smelled before it can be seen”. This is its major attraction however. By sunrise we had reached 39° S 74° W and an announcement from the Captain said we would refuel and restock from 08:00 at Valparaiso.
We are navigating the Pacific using GPS navigation systems which keep us on track and show within metres our exact location. But having witnessed the vastness of the Atlantic we should reflect upon the experience of those explorers who went before us who were always unsure of their exact location.
The primary purpose of missions, such as the one of the Beagle led by Fitzroy (with Darwin on board), was to update charts of Cape Horn, Tierra del Fuego and the Pacific Ocean. Whaling vessels were down here regularly and came across new islands but they could not be or had not been accurately mapped. The problem was defining latitude and longitude. The former parallel the Equator and by using sextants to make celestial and solar observations captains could determine how far north or south of zero latitude (Equator) the vessel was. The lines of longitude were not measurable this way as they run north to south and converge at the poles, so degrees of longitude decrease as one travels north or south. At the time of these discovery missions there was no zero meridian i. e. no longitude equivalent to the Equator. The British, through the clockmaker John Harrison, had a clock on gimbals that would work on a ship. This solved the problem of longitude. In fact Captain Cook used one on his first voyage as a test.
Following meetings at very high levels, the zero meridian was set at Greenwich, London. Essentially, these clocks were set at 12 noon at Greenwich and for the whole of the voyage displayed London time. So a mariner had to use a sextant to find the maximum height of the sun (12 noon) and compare that noon to the clock. So you are either ahead (east) of London or behind (west) and also by how many hours (this is because the earth rotates eastwards). This position could then be converted to degrees of longitude.
The Earth takes 24 hours to rotate 360 degrees therefore one hour is equivalent to 15 degrees longitude. For example, plus 3 hours equals 45 degrees east: minus 5 hours equals 75 degrees west. Now mariners could determine where they were using latitude and longitude and with accurate speed measurements on their vessels could determine distances covered. Problems arise when one arrives at 180 degrees west or east because it is the same line of longitude, hence the International Dateline. But that is another chapter!!
At sunset we have 12 hours to go before docking.