Now, of course, GPS is the way to go on land and at sea as well. But what did a seaman do prior to such inventions? He used the latest technology available, of course. Seamen aren't dumb.
We have already discussed the cross-staff, which required a navigator to look directly into the sun (ouch!) and the back-staff, which took the pain away but was not the most accurate of navigational tools. Things really took a leap ahead in 1731 when John Hadley invented the octant, an example of which is pictured above. The octant essentially stabilized the process of taking sightings by improving on the human eye and hand with the use of two mirrors. This allowed the navigator to view the sun without looking directly into it, and then to view the horizon without having to change the position of his eye or hand. It was a major breakthrough in sailing technology.
Always trying to improve on a good thing, the seafaring scientist of The Enlightenment continued to struggle over the problem of getting a ship to its exact destination. Perhaps even more important was the need to document and map all the new places that (European) men were traveling to and accurate sightings were a must for accurate mapping. The real issue was not latitude, which could be calculated by measuring the sun in relation to the horizon, but longitude, which kept shifting with the Earth's orbit.
In 1757 John Hadley did it again and introduced the sextant (the one shown having belonged to Captain James Cook, whose first voyage was one of the initial long term trials of the invention). This instrument worked on the same two mirror principle as the octant but, with the addition of a wider arc and a telescope, it allowed the navigator to take sightings between stars, planets, the moon and the sun rather than just between a heavenly body and the horizon. As long as a ship knew the exact time of day when a sighting was taken (thus the sailors' obsession with "turning the glass and striking the bell" even aboard pirates and privateers), accuracy could be assured within one tenth of a degree. The sextant, like the octant, was a highly sensitive instrument, particularly to vibrations. Inaccurate readings continued to occur but, in skilled hands and until modern technology took over, the sextant was the best device for determining a ship's position at sea.
GPS is fine and all, and so is accuracy, but sometimes a piece of history is nice to hold. I kinda miss my sturdy, well worn Thomas Guide. And a shiny, brass sextant.