Navigation in the Eighteenth Century

In a world have our cell phones with us 24/7, it’s difficult to imagine a time when ships traversed the world’s oceans with only a basic understanding of where they were. Today “simple” triangulation via global positioning system (GPS) satellites gives even the most timid soul reliable information of their whereabouts to within a few feet. On a ship, in the eighteenth century, there was no such things as reliably knowing your location until you sighted land. The latitude of your position could be determined, but longitude remained guesswork until a reliable marine chronometer was developed around the end of the eighteenth and beginning of the nineteenth century.

Latitude and longitude are arbitrary lines on the globe that navigators use to fix their position: latitude being the horizontal lines climbing from the equator to the pole(s) and longitude being the vertical lines that each cross the equator at right angles, but meet at the poles. In the eighteenth century, navigators used a variety of instruments, such as the backstaff and octant, to measure the height of the sun above the horizon at noon. The observed angle, given the date the noon sight was taken, could then be used to determine the latitude. My own use of the sextant on a small boat was less than satisfactory; trying to get the sight on a moving boat is difficult.

The rest of the navigation was dependent upon marking the ship’s course and speed over time. Called dead reckoning, the course and speed of the vessel would be noted every half hour by the crew on watch. At the end of every four hour watch, the information would be marked on a chart, the line from the previous position extended to show their new estimate. At noon every day, the measured latitude by the navigator (the measure of which horizontal line you were on) would be checked against their estimated position on the chart and the navigator would determine how close they were.

In theory this worked well enough, a captain sailing from New York to the Caribbean would work his way far to the east of the islands, find the latitude for the island be wanted, then sail west until he made landfall. The obvious problem in practice was drift from an unknown current or leeway from sailing. Without a calculation for longitude to know precisely where you were, a current could set you further west than you intended and you would be unknowingly - and potentially dangerously - close to the islands. 

One of the founding fathers of our nation discovered this during a voyage in his youth. Sailing from Virginia for Barbados with his brother Lawrence, nineteen year old George Washington was woken one morning with the cry of “land ho!’” The ship was supposed to have been several hundred miles further to the east than they actually were, but had drifted westward on the trip south. Had they been faster, they might have run into the island at night and George Washington would be another nameless body washed up on the rocks of Barbados.

Octant

In 1767 Great Britain’s first nautical almanac was published, which provided tables of calculations related to celestial bodies. With a better octant or later with the sextant, a navigator could measure the distance at night between the moon and a visible star. Using the calculations in the nautical almanac, the navigator could determine their longitude. This measurement of lunar distance, called “lunars”, allowed navigators to determine longitude with reasonable accuracy until 1850, when the method was largely supplanted by the readily available marine chronometers.

One cool experiment we did once was on a sailing excursion to St. Vincent and the Grenadines. We planned to sail from Bequia to the Tobago Cays, a distance of some twenty nautical miles, so our friends took one watch and my wife and I the other. Our goal, in addition to some excellent sailing, was to use dead reckoning to measure our location on the chart, but then plot our GPS location to see how accurate dead reckoning was. It turned out that our dead reckoning, for twenty nautical miles, was fairly accurate. I’m not sure how accurate it would be over several hundred nautical miles.

REF:

Mount Vernon. Washington’s Journey to Barbados. https://www.mountvernon.org/george-washington/washingtons-youth/journey-to-barbados.

NOAA. What is Longitude. https://oceanservice.noaa.gov.